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Contents

Introduction

Hello World

Comments

Doc comments

Top-Level Doc Comments

Values

Primitive Types

Primitive Values

String Literals and Unicode Code Point Literals

Escape Sequences

Multiline String Literals

Assignment

undefined

Variables

Container Level Variables

Static Local Variables

Thread Local Variables

Local Variables

Integers

Integer Literals

Runtime Integer Values

Floats

Float Literals

Floating Point Operations

Operators

Table of Operators

Precedence

Arrays

Anonymous List Literals

Multidimensional Arrays

Sentinel-Terminated Arrays

Vectors

SIMD

Pointers

volatile

Alignment

allowzero

Sentinel-Terminated Pointers

Slices

Sentinel-Terminated Slices

struct

Default Field Values

extern struct

packed struct

Struct Naming

Anonymous Struct Literals

enum

extern enum

Enum Literals

Non-exhaustive enum

union

Tagged union

extern union

packed union

Anonymous Union Literals

opaque

blocks

Shadowing

switch

Exhaustive Switching

Switching with Enum Literals

while

Labeled while

while with Optionals

while with Error Unions

inline while

for

Labeled for

inline for

if

defer

unreachable

Basics

At Compile-Time

noreturn

Functions

Pass-by-value Parameters

Function Parameter Type Inference

Function Reflection

Errors

Error Set Type

The Global Error Set

Error Union Type

catch

try

errdefer

Merging Error Sets

Inferred Error Sets

Error Return Traces

Implementation Details

Optionals

Optional Type

null

Optional Pointers

Casting

Type Coercion

Type Coercion: Stricter Qualification

Type Coercion: Integer and Float Widening

Type Coercion: Coercion Float to Int

Type Coercion: Slices, Arrays and Pointers

Type Coercion: Optionals

Type Coercion: Error Unions

Type Coercion: Compile-Time Known Numbers

Type Coercion: unions and enums

Type Coercion: Zero Bit Types

Type Coercion: undefined

Explicit Casts

Peer Type Resolution

Zero Bit Types

void

Pointers to Zero Bit Types

Result Location Semantics

usingnamespace

comptime

Introducing the Compile-Time Concept

Compile-Time Parameters

Compile-Time Variables

Compile-Time Expressions

Generic Data Structures

Case Study: printf in Zig

Assembly

Output Constraints

Input Constraints

Clobbers

Global Assembly

Atomics

Async Functions

Suspend and Resume

Resuming from Suspend Blocks

Async and Await

Async Function Example

Builtin Functions

@addWithOverflow

@alignCast

@alignOf

@as

@asyncCall

@atomicLoad

@atomicRmw

@atomicStore

@bitCast

@bitOffsetOf

@boolToInt

@bitSizeOf

@breakpoint

@mulAdd

@byteSwap

@bitReverse

@byteOffsetOf

@call

@cDefine

@cImport

@cInclude

@clz

@cmpxchgStrong

@cmpxchgWeak

@compileError

@compileLog

@ctz

@cUndef

@divExact

@divFloor

@divTrunc

@embedFile

@enumToInt

@errorName

@errorReturnTrace

@errorToInt

@errSetCast

@export

@extern

@fence

@field

@fieldParentPtr

@floatCast

@floatToInt

@frame

@Frame

@frameAddress

@frameSize

@hasDecl

@hasField

@import

@intCast

@intToEnum

@intToError

@intToFloat

@intToPtr

@memcpy

@memset

@wasmMemorySize

@wasmMemoryGrow

@mod

@mulWithOverflow

@panic

@popCount

@ptrCast

@ptrToInt

@rem

@returnAddress

@setAlignStack

@setCold

@setEvalBranchQuota

@setFloatMode

@setRuntimeSafety

@shlExact

@shlWithOverflow

@shrExact

@shuffle

@sizeOf

@splat

@reduce

@src

@sqrt

@sin

@cos

@exp

@exp2

@log

@log2

@log10

@fabs

@floor

@ceil

@trunc

@round

@subWithOverflow

@tagName

@This

@truncate

@Type

@typeInfo

@typeName

@TypeOf

@unionInit

Build Mode

Debug

ReleaseFast

ReleaseSafe

ReleaseSmall

Single Threaded Builds

Undefined Behavior

Reaching Unreachable Code

Index out of Bounds

Cast Negative Number to Unsigned Integer

Cast Truncates Data

Integer Overflow

Default Operations

Standard Library Math Functions

Builtin Overflow Functions

Wrapping Operations

Exact Left Shift Overflow

Exact Right Shift Overflow

Division by Zero

Remainder Division by Zero

Exact Division Remainder

Attempt to Unwrap Null

Attempt to Unwrap Error

Invalid Error Code

Invalid Enum Cast

Invalid Error Set Cast

Incorrect Pointer Alignment

Wrong Union Field Access

Out of Bounds Float to Integer Cast

Pointer Cast Invalid Null

Memory

Choosing an Allocator

Where are the bytes?

Implementing an Allocator

Heap Allocation Failure

Recursion

Lifetime and Ownership

Compile Variables

Root Source File

Zig Test

Zig Build System

Building an Executable

Building a Library

Compiling C Source Code

C

C Type Primitives

Import from C Header File

C Pointers

Exporting a C Library

Mixing Object Files

WebAssembly

Freestanding

WASI

Targets

Style Guide

Whitespace

Names

Examples

Doc Comment Guidance

Source Encoding

Keyword Reference

Grammar

Zen

Introduction

Zig is a general-purpose programming language and toolchain for maintaining robust, optimal, and reusable software.

Robust - behavior is correct even for edge cases such as out of memory.

Optimal - write programs the best way they can behave and perform.

Reusable - the same code works in many environments which have different constraints.

Maintainable - precisely communicate intent to the compiler and other programmers. The language imposes a low overhead to reading code and is resilient to changing requirements and environments.

Often the most efficient way to learn something new is to see examples, so this documentation shows how to use each of Zig's features. It is all on one page so you can search with your browser's search tool.

The code samples in this document are compiled and tested as part of the main test suite of Zig.

This HTML document depends on no external files, so you can use it offline.

Where is the documentation for the Zig standard library?

Hello World

hello.zig

const std = @import("std");

pub fn main() !void {
    const stdout = std.io.getStdOut().writer();
    try stdout.print("Hello, {s}!\n", .{"world"});
}

$ zig build-exe hello.zig
$ ./hello
Hello, world!

The Zig code sample above demonstrates one way to create a program that will output Hello, world!.

The code sample shows the contents of a file named hello.zig. Files storing Zig source code are UTF-8 encoded text files. The files storing Zig source code are usually named with the .zig extension.

Following the hello.zig Zig code sample, the Zig Build System is used to build an executable program from the hello.zig source code. Then, the hello program is executed showing its output Hello, world!. The lines beginning with $ represent command line prompts and a command. Everything else is program output.

The code sample begins by adding Zig's Standard Library to the build using the @import builtin function. The @import("std") function call creates a structure to represent the Standard Library. The code then declares a constant identifier, named std, for easy access to Zig's standard library.

Next, a public function, pub fn, named main is declared. The main function is necessary because it tells the Zig compiler where the start of the program exists. Programs designed to be executed will need a pub fn main function. For more advanced use cases, Zig offers other features to inform the compiler where the start of the program exists. Libraries, on the other hand, do not need a main function because library code is usually called by other programs.

A function is a block of any number of statements and expressions that, as a whole, perform a task. Functions may or may not return data after they are done performing their task. If a function cannot perform its task, it might return an error. Zig makes all of this explicit.

In the hello.zig code sample, the main function is declared with the !void return type. This return type is known as an Error Union Type. This syntax tells the Zig compiler that the function will either return an error or a value. An error union type combines an Error Set Type and a Primitive Type. The full form of an error union type is <error set type>!<primitive type>. In the code sample, the error set type is not explicitly written on the left side of the ! operator. When written this way, the error set type is a special kind of error union type that has an inferred error set type. The void after the ! operator tells the compiler that the function will not return a value under normal circumstances (i.e. no errors occur).

Note to experienced programmers: Zig also has the boolean operator !a where a is a value of type bool. Error union types contain the name of the type in the syntax: !<primitive type>.

In Zig, a function's block of statements and expressions are surrounded by { and } curly-braces. Inside of the main function are expressions that perform the task of outputting Hello, world! to standard output.

First, a constant identifier, stdout, is initialized to represent standard output's writer. Then, the program tries to print the Hello, world! message to standard output.

Functions sometimes need information to perform their task. In Zig, information is passed to functions between open ( and close ) parenthesis placed after the function's name. This information is also known as arguments. When there are multiple arguments passed to a function, they are separated by commas ,.

The two arguments passed to the stdout.print() function, "Hello, {s}!\n" and .{"world"}, are evaluated at compile-time. The code sample is purposely written to show how to perform string substitution in the print function. The curly-braces inside of the first argument are substituted with the compile-time known value inside of the second argument (known as an anonymous struct literal). The \n inside of the double-quotes of the first argument is the escape sequence for the newline character. The try expression evaluates the result of stdout.print. If the result is an error, then the try expression will return from main with the error. Otherwise, the program will continue. In this case, there are no more statements or expressions left to execute in the main function, so the program exits.

In Zig, the standard output writer's print function is allowed to fail because it is actually a function defined as part of a generic Writer. Consider a generic Writer that represents writing data to a file. When the disk is full, a write to the file will fail. However, we typically do not expect writing text to the standard output to fail. To avoid having to handle the failure case of printing to standard output, you can use alternate functions: the functions in std.log for proper logging or the std.debug.print function. This documentation will use the latter option to print to standard error (stderr) and silently return on failure. The next code sample, hello_again.zig demonstrates the use of std.debug.print.

hello_again.zig

const print = @import("std").debug.print;

pub fn main() void {
    print("Hello, world!\n", .{});
}

$ zig build-exe hello_again.zig
$ ./hello_again
Hello, world!

Note that you can leave off the ! from the return type because std.debug.print cannot fail.

See also:

Values

@import

Errors

Root Source File

Source Encoding

Comments

comments.zig

const expect = @import("std").testing.expect;

test "comments" {
    // Comments in Zig start with "//" and end at the next LF byte (end of line).
    // The below line is a comment, and won't be executed.

    //expect(false);

    const x = true;  // another comment
    try expect(x);
}

$ zig test comments.zig
Test [1/1] test "comments"...

All 1 tests passed.

There are no multiline comments in Zig (e.g. like /* */ comments in C). This helps allow Zig to have the property that each line of code can be tokenized out of context.

Doc comments

A doc comment is one that begins with exactly three slashes (i.e. /// but not ////); multiple doc comments in a row are merged together to form a multiline doc comment. The doc comment documents whatever immediately follows it.

/// A structure for storing a timestamp, with nanosecond precision (this is a
/// multiline doc comment).
const Timestamp = struct {
    /// The number of seconds since the epoch (this is also a doc comment).
    seconds: i64,  // signed so we can represent pre-1970 (not a doc comment)
    /// The number of nanoseconds past the second (doc comment again).
    nanos: u32,

    /// Returns a `Timestamp` struct representing the Unix epoch; that is, the
    /// moment of 1970 Jan 1 00:00:00 UTC (this is a doc comment too).
    pub fn unixEpoch() Timestamp {
        return Timestamp{
            .seconds = 0,
            .nanos = 0,
        };
    }
};

Doc comments are only allowed in certain places; eventually, it will become a compile error to have a doc comment in an unexpected place, such as in the middle of an expression, or just before a non-doc comment.

Top-Level Doc Comments

User documentation that doesn't belong to whatever immediately follows it, like package-level documentation, goes in top-level doc comments. A top-level doc comment is one that begins with two slashes and an exclamation point: //!.

//! This module provides functions for retrieving the current date and
//! time with varying degrees of precision and accuracy. It does not
//! depend on libc, but will use functions from it if available.

Values

values.zig

// Top-level declarations are order-independent:
const print = std.debug.print;
const std = @import("std");
const os = std.os;
const assert = std.debug.assert;

pub fn main() void {
    // integers
    const one_plus_one: i32 = 1 + 1;
    print("1 + 1 = {}\n", .{one_plus_one});

    // floats
    const seven_div_three: f32 = 7.0 / 3.0;
    print("7.0 / 3.0 = {}\n", .{seven_div_three});

    // boolean
    print("{}\n{}\n{}\n", .{
        true and false,
        true or false,
        !true,
    });

    // optional
    var optional_value: ?[]const u8 = null;
    assert(optional_value == null);

    print("\noptional 1\ntype: {s}\nvalue: {s}\n", .{
        @typeName(@TypeOf(optional_value)),
        optional_value,
    });

    optional_value = "hi";
    assert(optional_value != null);

    print("\noptional 2\ntype: {s}\nvalue: {s}\n", .{
        @typeName(@TypeOf(optional_value)),
        optional_value,
    });

    // error union
    var number_or_error: anyerror!i32 = error.ArgNotFound;

    print("\nerror union 1\ntype: {s}\nvalue: {}\n", .{
        @typeName(@TypeOf(number_or_error)),
        number_or_error,
    });

    number_or_error = 1234;

    print("\nerror union 2\ntype: {s}\nvalue: {}\n", .{
        @typeName(@TypeOf(number_or_error)),
        number_or_error,
    });
}

$ zig build-exe values.zig
$ ./values
1 + 1 = 2
7.0 / 3.0 = 2.33333325e+00
false
true
false

optional 1
type: ?[]const u8
value: null

optional 2
type: ?[]const u8
value: hi

error union 1
type: anyerror!i32
value: error.ArgNotFound

error union 2
type: anyerror!i32
value: 1234

Primitive Types

Name | C Equivalent | Description
i8 | int8_t | signed 8-bit integer
u8 | uint8_t | unsigned 8-bit integer
i16 | int16_t | signed 16-bit integer
u16 | uint16_t | unsigned 16-bit integer
i32 | int32_t | signed 32-bit integer
u32 | uint32_t | unsigned 32-bit integer
i64 | int64_t | signed 64-bit integer
u64 | uint64_t | unsigned 64-bit integer
i128 | __int128 | signed 128-bit integer
u128 | unsigned __int128 | unsigned 128-bit integer
isize | intptr_t | signed pointer sized integer
usize | uintptr_t | unsigned pointer sized integer
c_short | short | for ABI compatibility with C
c_ushort | unsigned short | for ABI compatibility with C
c_int | int | for ABI compatibility with C
c_uint | unsigned int | for ABI compatibility with C
c_long | long | for ABI compatibility with C
c_ulong | unsigned long | for ABI compatibility with C
c_longlong | long long | for ABI compatibility with C
c_ulonglong | unsigned long long | for ABI compatibility with C
c_longdouble | long double | for ABI compatibility with C
c_void | void | for ABI compatibility with C
f16 | _Float16 | 16-bit floating point (10-bit mantissa) IEEE-754-2008 binary16
f32 | float | 32-bit floating point (23-bit mantissa) IEEE-754-2008 binary32
f64 | double | 64-bit floating point (52-bit mantissa) IEEE-754-2008 binary64
f128 | _Float128 | 128-bit floating point (112-bit mantissa) IEEE-754-2008 binary128
bool | bool | true or false
void | (none) | 0 bit type
noreturn | (none) | the type of break, continue, return, unreachable, and while (true) {}
type | (none) | the type of types
anyerror | (none) | an error code
comptime_int | (none) | Only allowed for comptime-known values. The type of integer literals.
comptime_float | (none) | Only allowed for comptime-known values. The type of float literals.

In addition to the integer types above, arbitrary bit-width integers can be referenced by using an identifier of i or u followed by digits. For example, the identifier i7 refers to a signed 7-bit integer. The maximum allowed bit-width of an integer type is 65535.

See also:

Integers

Floats

void

Errors

@Type

Primitive Values

Name | Description
true and false | bool values
null | used to set an optional type to null
undefined | used to leave a value unspecified

See also:

Optionals

undefined

String Literals and Unicode Code Point Literals

String literals are constant single-item Pointers to null-terminated byte arrays. The type of string literals encodes both the length, and the fact that they are null-terminated, and thus they can be coerced to both Slices and Null-Terminated Pointers. Dereferencing string literals converts them to Arrays.

The encoding of a string in Zig is de-facto assumed to be UTF-8. Because Zig source code is UTF-8 encoded, any non-ASCII bytes appearing within a string literal in source code carry their UTF-8 meaning into the content of the string in the Zig program; the bytes are not modified by the compiler. However, it is possible to embed non-UTF-8 bytes into a string literal using \xNN notation.

Unicode code point literals have type comptime_int, the same as Integer Literals. All Escape Sequences are valid in both string literals and Unicode code point literals.

In many other programming languages, a Unicode code point literal is called a "character literal". However, there is no precise technical definition of a "character" in recent versions of the Unicode specification (as of Unicode 13.0). In Zig, a Unicode code point literal corresponds to the Unicode definition of a code point.

test.zig

const expect = @import("std").testing.expect;
const mem = @import("std").mem;

test "string literals" {
    const bytes = "hello";
    try expect(@TypeOf(bytes) == *const [5:0]u8);
    try expect(bytes.len == 5);
    try expect(bytes[1] == 'e');
    try expect(bytes[5] == 0);
    try expect('e' == '\x65');
    try expect('\u{1f4a9}' == 128169);
    try expect('💯' == 128175);
    try expect(mem.eql(u8, "hello", "h\x65llo"));
    try expect("\xff"[0] == 0xff); // non-UTF-8 strings are possible with \xNN notation.
}

$ zig test test.zig
Test [1/1] test "string literals"...

All 1 tests passed.

See also:

Arrays

Zig Test

Source Encoding

Escape Sequences

Escape Sequence | Name
\n | Newline
\r | Carriage Return
\t | Tab
\\ | Backslash
\' | Single Quote
\" | Double Quote
\xNN | hexadecimal 8-bit byte value (2 digits)
\u{NNNNNN} | hexadecimal Unicode code point UTF-8 encoded (1 or more digits)

Note that the maximum valid Unicode point is 0x10ffff.

Multiline String Literals

Multiline string literals have no escapes and can span across multiple lines. To start a multiline string literal, use the \\ token. Just like a comment, the string literal goes until the end of the line. The end of the line is not included in the string literal. However, if the next line begins with \\ then a newline is appended and the string literal continues.

const hello_world_in_c =
    \\#include <stdio.h>
    \\
    \\int main(int argc, char **argv) {
    \\    printf("hello world\n");
    \\    return 0;
    \\}
;

See also:

@embedFile

Assignment

Use the const keyword to assign a value to an identifier:

test.zig

const x = 1234;

fn foo() void {
    // It works at file scope as well as inside functions.
    const y = 5678;

    // Once assigned, an identifier cannot be changed.
    y += 1;
}

test "assignment" {
    foo();
}

$ zig test test.zig
./docgen_tmp/test.zig:8:7: error: cannot assign to constant
    y += 1;
      ^

const applies to all of the bytes that the identifier immediately addresses. Pointers have their own const-ness.

If you need a variable that you can modify, use the var keyword:

test.zig

const expect = @import("std").testing.expect;

test "var" {
    var y: i32 = 5678;

    y += 1;

    try expect(y == 5679);
}

$ zig test test.zig
Test [1/1] test "var"...

All 1 tests passed.

Variables must be initialized:

test.zig

test "initialization" {
    var x: i32;

    x = 1;
}

$ zig test test.zig
./docgen_tmp/test.zig:2:5: error: variables must be initialized
    var x: i32;
    ^

undefined

Use undefined to leave variables uninitialized:

test.zig

const expect = @import("std").testing.expect;

test "init with undefined" {
    var x: i32 = undefined;
    x = 1;
    try expect(x == 1);
}

$ zig test test.zig
Test [1/1] test "init with undefined"...

All 1 tests passed.

undefined can be coerced to any type. Once this happens, it is no longer possible to detect that the value is undefined. undefined means the value could be anything, even something that is nonsense according to the type. Translated into English, undefined means "Not a meaningful value. Using this value would be a bug. The value will be unused, or overwritten before being used."

In Debug mode, Zig writes 0xaa bytes to undefined memory. This is to catch bugs early, and to help detect use of undefined memory in a debugger.

Variables

A variable is a unit of Memory storage.

Variables are never allowed to shadow identifiers from an outer scope.

It is generally preferable to use const rather than var when declaring a variable. This causes less work for both humans and computers to do when reading code, and creates more optimization opportunities.

Container Level Variables

Container level variables have static lifetime and are order-independent and lazily analyzed. The initialization value of container level variables is implicitly comptime. If a container level variable is const then its value is comptime-known, otherwise it is runtime-known.

container_level_variables.zig

var y: i32 = add(10, x);
const x: i32 = add(12, 34);

test "container level variables" {
    try expect(x == 46);
    try expect(y == 56);
}

fn add(a: i32, b: i32) i32 {
    return a + b;
}

const std = @import("std");
const expect = std.testing.expect;

$ zig test container_level_variables.zig
Test [1/1] test "container level variables"...

All 1 tests passed.

Container level variables may be declared inside a struct, union, or enum:

namespaced_container_level_variable.zig

const std = @import("std");
const expect = std.testing.expect;

test "namespaced container level variable" {
    try expect(foo() == 1235);
    try expect(foo() == 1236);
}

const S = struct {
    var x: i32 = 1234;
};

fn foo() i32 {
    S.x += 1;
    return S.x;
}

$ zig test namespaced_container_level_variable.zig
Test [1/1] test "namespaced container level variable"...

All 1 tests passed.

Static Local Variables

It is also possible to have local variables with static lifetime by using containers inside functions.

static_local_variable.zig

const std = @import("std");
  const expect = std.testing.expect;

  test "static local variable" {
      try expect(foo() == 1235);
      try expect(foo() == 1236);
  }

  fn foo() i32 {
      const S = struct {
          var x: i32 = 1234;
      };
      S.x += 1;
      return S.x;
  }

$ zig test static_local_variable.zig
Test [1/1] test "static local variable"...

All 1 tests passed.

The extern keyword or @extern builtin function can be used to link against a variable that is exported from another object. The export keyword or @export builtin function can be used to make a variable available to other objects at link time. In both cases, the type of the variable must be C ABI compatible.

See also:

Exporting a C Library

Thread Local Variables

A variable may be specified to be a thread-local variable using the threadlocal keyword:

tls.zig

const std = @import("std");
const assert = std.debug.assert;

threadlocal var x: i32 = 1234;

test "thread local storage" {
    const thread1 = try std.Thread.spawn(testTls, {});
    const thread2 = try std.Thread.spawn(testTls, {});
    testTls({});
    thread1.wait();
    thread2.wait();
}

fn testTls(context: void) void {
    assert(x == 1234);
    x += 1;
    assert(x == 1235);
}

$ zig test tls.zig
Test [1/1] test "thread local storage"...

All 1 tests passed.

For Single Threaded Builds, all thread local variables are treated as regular Container Level Variables.

Thread local variables may not be const.

Local Variables

Local variables occur inside Functions, comptime blocks, and @cImport blocks.

When a local variable is const, it means that after initialization, the variable's value will not change. If the initialization value of a const variable is comptime-known, then the variable is also comptime-known.

A local variable may be qualified with the comptime keyword. This causes the variable's value to be comptime-known, and all loads and stores of the variable to happen during semantic analysis of the program, rather than at runtime. All variables declared in a comptime expression are implicitly comptime variables.

comptime_vars.zig

const std = @import("std");
const expect = std.testing.expect;

test "comptime vars" {
    var x: i32 = 1;
    comptime var y: i32 = 1;

    x += 1;
    y += 1;

    try expect(x == 2);
    try expect(y == 2);

    if (y != 2) {
        // This compile error never triggers because y is a comptime variable,
        // and so `y != 2` is a comptime value, and this if is statically evaluated.
        @compileError("wrong y value");
    }
}

$ zig test comptime_vars.zig
Test [1/1] test "comptime vars"...

All 1 tests passed.

Integers

Integer Literals

const decimal_int = 98222;
const hex_int = 0xff;
const another_hex_int = 0xFF;
const octal_int = 0o755;
const binary_int = 0b11110000;

// underscores may be placed between two digits as a visual separator
const one_billion = 1_000_000_000;
const binary_mask = 0b1_1111_1111;
const permissions = 0o7_5_5;
const big_address = 0xFF80_0000_0000_0000;

Runtime Integer Values

Integer literals have no size limitation, and if any undefined behavior occurs, the compiler catches it.

However, once an integer value is no longer known at compile-time, it must have a known size, and is vulnerable to undefined behavior.

fn divide(a: i32, b: i32) i32 {
    return a / b;
}

In this function, values a and b are known only at runtime, and thus this division operation is vulnerable to both Integer Overflow and Division by Zero.

Operators such as + and - cause undefined behavior on integer overflow. Also available are operations such as +% and -% which are defined to have wrapping arithmetic on all targets.

Zig supports arbitrary bit-width integers, referenced by using an identifier of i or u followed by digits. For example, the identifier i7 refers to a signed 7-bit integer. The maximum allowed bit-width of an integer type is 65535.

See also:

Wrapping Operations

Floats

Zig has the following floating point types:

f16 - IEEE-754-2008 binary16

f32 - IEEE-754-2008 binary32

f64 - IEEE-754-2008 binary64

f128 - IEEE-754-2008 binary128

c_longdouble - matches long double for the target C ABI

Float Literals

Float literals have type comptime_float which is guaranteed to have the same precision and operations of the largest other floating point type, which is f128.

Float literals coerce to any floating point type, and to any integer type when there is no fractional component.

const floating_point = 123.0E+77;
const another_float = 123.0;
const yet_another = 123.0e+77;

const hex_floating_point = 0x103.70p-5;
const another_hex_float = 0x103.70;
const yet_another_hex_float = 0x103.70P-5;

// underscores may be placed between two digits as a visual separator
const lightspeed = 299_792_458.000_000;
const nanosecond = 0.000_000_001;
const more_hex = 0x1234_5678.9ABC_CDEFp-10;

There is no syntax for NaN, infinity, or negative infinity. For these special values, one must use the standard library:

const std = @import("std");

const inf = std.math.inf(f32);
const negative_inf = -std.math.inf(f64);
const nan = std.math.nan(f128);

Floating Point Operations

By default floating point operations use Strict mode, but you can switch to Optimized mode on a per-block basis:

foo.zig

const std = @import("std");
const big = @as(f64, 1 << 40);

export fn foo_strict(x: f64) f64 {
    return x + big - big;
}

export fn foo_optimized(x: f64) f64 {
    @setFloatMode(.Optimized);
    return x + big - big;
}

$ zig build-obj foo.zig -O ReleaseFast

For this test we have to separate code into two object files - otherwise the optimizer figures out all the values at compile-time, which operates in strict mode.

float_mode.zig

const print = @import("std").debug.print;

extern fn foo_strict(x: f64) f64;
extern fn foo_optimized(x: f64) f64;

pub fn main() void {
    const x = 0.001;
    print("optimized = {}\n", .{foo_optimized(x)});
    print("strict = {}\n", .{foo_strict(x)});
}

$ zig build-exe float_mode.zig foo.o
$ ./float_mode
optimized = 1.0e-03
strict = 9.765625e-04

See also:

@setFloatMode

Division by Zero

Operators

There is no operator overloading. When you see an operator in Zig, you know that it is doing something from this table, and nothing else.

Table of Operators

Syntax | Relevant Types | Description | Example

a + b
a += b

Integers

Floats

Addition.

Can cause overflow for integers.

Invokes Peer Type Resolution for the operands.

See also @addWithOverflow.

2 + 5 == 7

a +% b
a +%= b

Integers

Wrapping Addition.

Guaranteed to have twos-complement wrapping behavior.

Invokes Peer Type Resolution for the operands.

See also @addWithOverflow.

@as(u32, std.math.maxInt(u32)) +% 1 == 0

a - b
a -= b

Integers

Floats

Subtraction.

Can cause overflow for integers.

Invokes Peer Type Resolution for the operands.

See also @subWithOverflow.

2 - 5 == -3

a -% b
a -%= b

Integers

Wrapping Subtraction.

Guaranteed to have twos-complement wrapping behavior.

Invokes Peer Type Resolution for the operands.

See also @subWithOverflow.

@as(u32, 0) -% 1 == std.math.maxInt(u32)

-a

Integers

Floats

Negation.

Can cause overflow for integers.

-1 == 0 - 1

-%a

Integers

Wrapping Negation.

Guaranteed to have twos-complement wrapping behavior.

-%@as(i32, std.math.minInt(i32)) == std.math.minInt(i32)

a * b
a *= b

Integers

Floats

Multiplication.

Can cause overflow for integers.

Invokes Peer Type Resolution for the operands.

See also @mulWithOverflow.

2 * 5 == 10

a *% b
a *%= b

Integers

Wrapping Multiplication.

Guaranteed to have twos-complement wrapping behavior.

Invokes Peer Type Resolution for the operands.

See also @mulWithOverflow.

@as(u8, 200) *% 2 == 144

a / b
a /= b

Integers

Floats

Division.

Can cause overflow for integers.

Can cause Division by Zero for integers.

Can cause Division by Zero for floats in FloatMode.Optimized Mode.

Signed integer operands must be comptime-known and positive. In other cases, use @divTrunc, @divFloor, or @divExact instead.

Invokes Peer Type Resolution for the operands.

10 / 5 == 2

a % b
a %= b

Integers

Floats

Remainder Division.

Can cause Division by Zero for integers.

Can cause Division by Zero for floats in FloatMode.Optimized Mode.

Signed or floating-point operands must be comptime-known and positive. In other cases, use @rem or @mod instead.

Invokes Peer Type Resolution for the operands.

10 % 3 == 1

a << b
a <<= b

Integers

Bit Shift Left.

b must be comptime-known or have a type with log2 number of bits as a.

See also @shlExact.

See also @shlWithOverflow.

1 << 8 == 256

a >> b
a >>= b

Integers

Bit Shift Right.

b must be comptime-known or have a type with log2 number of bits as a.

See also @shrExact.

10 >> 1 == 5

a & b
a &= b

Integers

Bitwise AND.

Invokes Peer Type Resolution for the operands.

0b011 & 0b101 == 0b001

a | b
a |= b

Integers

Bitwise OR.

Invokes Peer Type Resolution for the operands.

0b010 | 0b100 == 0b110

a ^ b
a ^= b

Integers

Bitwise XOR.

Invokes Peer Type Resolution for the operands.

0b011 ^ 0b101 == 0b110

~a

Integers

Bitwise NOT.

~@as(u8, 0b10101111) == 0b01010000

a orelse b

Optionals

If a is null, returns b ("default value"), otherwise returns the unwrapped value of a. Note that b may be a value of type noreturn.

const value: ?u32 = null;
const unwrapped = value orelse 1234;
unwrapped == 1234

a.?

Optionals

Equivalent to:

a orelse unreachable

const value: ?u32 = 5678;
value.? == 5678

a catch b
a catch |err| b

Error Unions

If a is an error, returns b ("default value"), otherwise returns the unwrapped value of a. Note that b may be a value of type noreturn. err is the error and is in scope of the expression b.

const value: anyerror!u32 = error.Broken;
const unwrapped = value catch 1234;
unwrapped == 1234

a and b

bool

If a is false, returns false without evaluating b. Otherwise, returns b.

(false and true) == false

a or b

bool

If a is true, returns true without evaluating b. Otherwise, returns b.

false or true == true

!a

bool

Boolean NOT.

!false == true

a == b

Integers

Floats

bool

type

Returns true if a and b are equal, otherwise returns false. Invokes Peer Type Resolution for the operands.

(1 == 1) == true

a == null

Optionals

Returns true if a is null, otherwise returns false.

const value: ?u32 = null;
value == null

a != b

Integers

Floats

bool

type

Returns false if a and b are equal, otherwise returns true. Invokes Peer Type Resolution for the operands.

(1 != 1) == false

a > b

Integers

Floats

Returns true if a is greater than b, otherwise returns false. Invokes Peer Type Resolution for the operands.

(2 > 1) == true

a >= b

Integers

Floats

Returns true if a is greater than or equal to b, otherwise returns false. Invokes Peer Type Resolution for the operands.

(2 >= 1) == true

a < b

Integers

Floats

Returns true if a is less than b, otherwise returns false. Invokes Peer Type Resolution for the operands.

(1 < 2) == true>

a <= b

Integers

Floats

Returns true if a is less than or equal to b, otherwise returns false. Invokes Peer Type Resolution for the operands.

(1 <= 2) == true

a ++ b

Arrays

Array concatenation.

Only available when a and b are compile-time known.

const mem = @import("std").mem;
const array1 = [_]u32{1,2};
const array2 = [_]u32{3,4};
const together = array1 ++ array2;
mem.eql(u32, &together, &[_]u32{1,2,3,4})

a ** b

Arrays

Array multiplication.

Only available when a and b are compile-time known.

const mem = @import("std").mem;
const pattern = "ab" ** 3;
mem.eql(u8, pattern, "ababab")

a.*

Pointers

Pointer dereference.

const x: u32 = 1234;
const ptr = &x;
ptr.* == 1234

&a

All types
Address of.

const x: u32 = 1234;
const ptr = &x;
ptr.* == 1234

a || b

Error Set Type

Merging Error Sets

const A = error{One};
const B = error{Two};
(A || B) == error{One, Two}

Precedence

x() x[] x.y x.* x.?
a!b
x{}
!x -x -%x ~x &x ?x
* / % ** *% ||
+ - ++ +% -%
<< >>
& ^ | orelse catch
== != < > <= >=
and
or
= *= /= %= += -= <<= >>= &= ^= |=

Arrays

arrays.zig

const expect = @import("std").testing.expect;
const assert = @import("std").debug.assert;
const mem = @import("std").mem;

// array literal
const message = [_]u8{ 'h', 'e', 'l', 'l', 'o' };

// get the size of an array
comptime {
    assert(message.len == 5);
}

// A string literal is a single-item pointer to an array literal.
const same_message = "hello";

comptime {
    assert(mem.eql(u8, &message, same_message));
}

test "iterate over an array" {
    var sum: usize = 0;
    for (message) |byte| {
        sum += byte;
    }
    try expect(sum == 'h' + 'e' + 'l' * 2 + 'o');
}

// modifiable array
var some_integers: [100]i32 = undefined;

test "modify an array" {
    for (some_integers) |*item, i| {
        item.* = @intCast(i32, i);
    }
    try expect(some_integers[10] == 10);
    try expect(some_integers[99] == 99);
}

// array concatenation works if the values are known
// at compile time
const part_one = [_]i32{ 1, 2, 3, 4 };
const part_two = [_]i32{ 5, 6, 7, 8 };
const all_of_it = part_one ++ part_two;
comptime {
    assert(mem.eql(i32, &all_of_it, &[_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }));
}

// remember that string literals are arrays
const hello = "hello";
const world = "world";
const hello_world = hello ++ " " ++ world;
comptime {
    assert(mem.eql(u8, hello_world, "hello world"));
}

// ** does repeating patterns
const pattern = "ab" ** 3;
comptime {
    assert(mem.eql(u8, pattern, "ababab"));
}

// initialize an array to zero
const all_zero = [_]u16{0} ** 10;

comptime {
    assert(all_zero.len == 10);
    assert(all_zero[5] == 0);
}

// use compile-time code to initialize an array
var fancy_array = init: {
    var initial_value: [10]Point = undefined;
    for (initial_value) |*pt, i| {
        pt.* = Point{
            .x = @intCast(i32, i),
            .y = @intCast(i32, i) * 2,
        };
    }
    break :init initial_value;
};
const Point = struct {
    x: i32,
    y: i32,
};

test "compile-time array initialization" {
    try expect(fancy_array[4].x == 4);
    try expect(fancy_array[4].y == 8);
}

// call a function to initialize an array
var more_points = [_]Point{makePoint(3)} ** 10;
fn makePoint(x: i32) Point {
    return Point{
        .x = x,
        .y = x * 2,
    };
}
test "array initialization with function calls" {
    try expect(more_points[4].x == 3);
    try expect(more_points[4].y == 6);
    try expect(more_points.len == 10);
}

$ zig test arrays.zig
Test [1/4] test "iterate over an array"...
Test [2/4] test "modify an array"...
Test [3/4] test "compile-time array initialization"...
Test [4/4] test "array initialization with function calls"...

All 4 tests passed.

See also:

for

Slices

Anonymous List Literals

Similar to Enum Literals and Anonymous Struct Literals the type can be omitted from array literals:

anon_list.zig

const std = @import("std");
const expect = std.testing.expect;

test "anonymous list literal syntax" {
    var array: [4]u8 = .{11, 22, 33, 44};
    try expect(array[0] == 11);
    try expect(array[1] == 22);
    try expect(array[2] == 33);
    try expect(array[3] == 44);
}

$ zig test anon_list.zig
Test [1/1] test "anonymous list literal syntax"...

All 1 tests passed.

If there is no type in the result location then an anonymous list literal actually turns into a struct with numbered field names:

infer_list_literal.zig

const std = @import("std");
const expect = std.testing.expect;

test "fully anonymous list literal" {
    try dump(.{ @as(u32, 1234), @as(f64, 12.34), true, "hi"});
}

fn dump(args: anytype) !void {
    try expect(args.@"0" == 1234);
    try expect(args.@"1" == 12.34);
    try expect(args.@"2");
    try expect(args.@"3"[0] == 'h');
    try expect(args.@"3"[1] == 'i');
}

$ zig test infer_list_literal.zig
Test [1/1] test "fully anonymous list literal"...

All 1 tests passed.

Multidimensional Arrays

Mutlidimensional arrays can be created by nesting arrays:

multidimensional.zig

const std = @import("std");
const expect = std.testing.expect;

const mat4x4 = [4][4]f32{
    [_]f32{ 1.0, 0.0, 0.0, 0.0 },
    [_]f32{ 0.0, 1.0, 0.0, 1.0 },
    [_]f32{ 0.0, 0.0, 1.0, 0.0 },
    [_]f32{ 0.0, 0.0, 0.0, 1.0 },
};
test "multidimensional arrays" {
    // Access the 2D array by indexing the outer array, and then the inner array.
    try expect(mat4x4[1][1] == 1.0);

    // Here we iterate with for loops.
    for (mat4x4) |row, row_index| {
        for (row) |cell, column_index| {
            if (row_index == column_index) {
                try expect(cell == 1.0);
            }
        }
    }
}

$ zig test multidimensional.zig
Test [1/1] test "multidimensional arrays"...

All 1 tests passed.

Sentinel-Terminated Arrays

The syntax [N:x]T describes an array which has a sentinel element of value x at the index corresponding to len.

null_terminated_array.zig

const std = @import("std");
const expect = std.testing.expect;

test "null terminated array" {
    const array = [_:0]u8 {1, 2, 3, 4};

    try expect(@TypeOf(array) == [4:0]u8);
    try expect(array.len == 4);
    try expect(array[4] == 0);
}

$ zig test null_terminated_array.zig
Test [1/1] test "null terminated array"...

All 1 tests passed.

See also:

Sentinel-Terminated Pointers

Sentinel-Terminated Slices

Vectors

A vector is a group of booleans, Integers, Floats, or Pointers which are operated on in parallel using a single instruction (SIMD). Vector types are created with the builtin function @Type, or using the shorthand as std.meta.Vector.

TODO talk about C ABI interop

SIMD

TODO Zig's SIMD abilities are just beginning to be fleshed out. Here are some talking points to update the docs with: * What kind of operations can you do? All the operations on integers and floats? What about mixing scalar and vector? * How to convert to/from vectors/arrays * How to access individual elements from vectors, how to loop over the elements * "shuffle" * Advice on writing high perf software, how to abstract the best way

Pointers

Zig has two kinds of pointers: single-item and many-item.

*T - single-item pointer to exactly one item.

Supports deref syntax: ptr.*

[*]T - many-item pointer to unknown number of items.

Supports index syntax: ptr[i]

Supports slice syntax: ptr[start..end]

Supports pointer arithmetic: ptr + x, ptr - x

T must have a known size, which means that it cannot be c_void or any other opaque type.

These types are closely related to Arrays and Slices:

*[N]T - pointer to N items, same as single-item pointer to an array.

Supports index syntax: array_ptr[i]

Supports slice syntax: array_ptr[start..end]

Supports len property: array_ptr.len

[]T - pointer to runtime-known number of items.

Supports index syntax: slice[i]

Supports slice syntax: slice[start..end]

Supports len property: slice.len

Use &x to obtain a single-item pointer:

test.zig

const expect = @import("std").testing.expect;

test "address of syntax" {
    // Get the address of a variable:
    const x: i32 = 1234;
    const x_ptr = &x;

    // Dereference a pointer:
    try expect(x_ptr.* == 1234);

    // When you get the address of a const variable, you get a const single-item pointer.
    try expect(@TypeOf(x_ptr) == *const i32);

    // If you want to mutate the value, you'd need an address of a mutable variable:
    var y: i32 = 5678;
    const y_ptr = &y;
    try expect(@TypeOf(y_ptr) == *i32);
    y_ptr.* += 1;
    try expect(y_ptr.* == 5679);
}

test "pointer array access" {
    // Taking an address of an individual element gives a
    // single-item pointer. This kind of pointer
    // does not support pointer arithmetic.
    var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    const ptr = &array[2];
    try expect(@TypeOf(ptr) == *u8);

    try expect(array[2] == 3);
    ptr.* += 1;
    try expect(array[2] == 4);
}

$ zig test test.zig
Test [1/2] test "address of syntax"...
Test [2/2] test "pointer array access"...

All 2 tests passed.

In Zig, we generally prefer Slices rather than Sentinel-Terminated Pointers. You can turn an array or pointer into a slice using slice syntax.

Slices have bounds checking and are therefore protected against this kind of undefined behavior. This is one reason we prefer slices to pointers.

test.zig

const expect = @import("std").testing.expect;

test "pointer slicing" {
    var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    const slice = array[2..4];
    try expect(slice.len == 2);

    try expect(array[3] == 4);
    slice[1] += 1;
    try expect(array[3] == 5);
}

$ zig test test.zig
Test [1/1] test "pointer slicing"...

All 1 tests passed.

Pointers work at compile-time too, as long as the code does not depend on an undefined memory layout:

test.zig

const expect = @import("std").testing.expect;

test "comptime pointers" {
    comptime {
        var x: i32 = 1;
        const ptr = &x;
        ptr.* += 1;
        x += 1;
        try expect(ptr.* == 3);
    }
}

$ zig test test.zig
Test [1/1] test "comptime pointers"...

All 1 tests passed.

To convert an integer address into a pointer, use @intToPtr. To convert a pointer to an integer, use @ptrToInt:

test.zig

const expect = @import("std").testing.expect;

test "@ptrToInt and @intToPtr" {
    const ptr = @intToPtr(*i32, 0xdeadbee0);
    const addr = @ptrToInt(ptr);
    try expect(@TypeOf(addr) == usize);
    try expect(addr == 0xdeadbee0);
}

$ zig test test.zig
Test [1/1] test "@ptrToInt and @intToPtr"...

All 1 tests passed.

Zig is able to preserve memory addresses in comptime code, as long as the pointer is never dereferenced:

test.zig

const expect = @import("std").testing.expect;

test "comptime @intToPtr" {
    comptime {
        // Zig is able to do this at compile-time, as long as
        // ptr is never dereferenced.
        const ptr = @intToPtr(*i32, 0xdeadbee0);
        const addr = @ptrToInt(ptr);
        try expect(@TypeOf(addr) == usize);
        try expect(addr == 0xdeadbee0);
    }
}

$ zig test test.zig
Test [1/1] test "comptime @intToPtr"...

All 1 tests passed.

See also:

Optional Pointers

@intToPtr

@ptrToInt

C Pointers

Pointers to Zero Bit Types

volatile

Loads and stores are assumed to not have side effects. If a given load or store should have side effects, such as Memory Mapped Input/Output (MMIO), use volatile. In the following code, loads and stores with mmio_ptr are guaranteed to all happen and in the same order as in source code:

test.zig

const expect = @import("std").testing.expect;

test "volatile" {
    const mmio_ptr = @intToPtr(*volatile u8, 0x12345678);
    try expect(@TypeOf(mmio_ptr) == *volatile u8);
}

$ zig test test.zig
Test [1/1] test "volatile"...

All 1 tests passed.

Note that volatile is unrelated to concurrency and Atomics. If you see code that is using volatile for something other than Memory Mapped Input/Output, it is probably a bug.

To convert one pointer type to another, use @ptrCast. This is an unsafe operation that Zig cannot protect you against. Use @ptrCast only when other conversions are not possible.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "pointer casting" {
    const bytes align(@alignOf(u32)) = [_]u8{ 0x12, 0x12, 0x12, 0x12 };
    const u32_ptr = @ptrCast(*const u32, &bytes);
    try expect(u32_ptr.* == 0x12121212);

    // Even this example is contrived - there are better ways to do the above than
    // pointer casting. For example, using a slice narrowing cast:
    const u32_value = std.mem.bytesAsSlice(u32, bytes[0..])[0];
    try expect(u32_value == 0x12121212);

    // And even another way, the most straightforward way to do it:
    try expect(@bitCast(u32, bytes) == 0x12121212);
}

test "pointer child type" {
    // pointer types have a `child` field which tells you the type they point to.
    try expect(@typeInfo(*u32).Pointer.child == u32);
}

$ zig test test.zig
Test [1/2] test "pointer casting"...
Test [2/2] test "pointer child type"...

All 2 tests passed.

Alignment

Each type has an alignment - a number of bytes such that, when a value of the type is loaded from or stored to memory, the memory address must be evenly divisible by this number. You can use @alignOf to find out this value for any type.

Alignment depends on the CPU architecture, but is always a power of two, and less than 1 << 29.

In Zig, a pointer type has an alignment value. If the value is equal to the alignment of the underlying type, it can be omitted from the type:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "variable alignment" {
    var x: i32 = 1234;
    const align_of_i32 = @alignOf(@TypeOf(x));
    try expect(@TypeOf(&x) == *i32);
    try expect(*i32 == *align(align_of_i32) i32);
    if (std.Target.current.cpu.arch == .x86_64) {
        try expect(@typeInfo(*i32).Pointer.alignment == 4);
    }
}

$ zig test test.zig
Test [1/1] test "variable alignment"...

All 1 tests passed.

In the same way that a *i32 can be coerced to a *const i32, a pointer with a larger alignment can be implicitly cast to a pointer with a smaller alignment, but not vice versa.

You can specify alignment on variables and functions. If you do this, then pointers to them get the specified alignment:

test.zig

const expect = @import("std").testing.expect;

var foo: u8 align(4) = 100;

test "global variable alignment" {
    try expect(@typeInfo(@TypeOf(&foo)).Pointer.alignment == 4);
    try expect(@TypeOf(&foo) == *align(4) u8);
    const as_pointer_to_array: *[1]u8 = &foo;
    const as_slice: []u8 = as_pointer_to_array;
    try expect(@TypeOf(as_slice) == []align(4) u8);
}

fn derp() align(@sizeOf(usize) * 2) i32 { return 1234; }
fn noop1() align(1) void {}
fn noop4() align(4) void {}

test "function alignment" {
    try expect(derp() == 1234);
    try expect(@TypeOf(noop1) == fn() align(1) void);
    try expect(@TypeOf(noop4) == fn() align(4) void);
    noop1();
    noop4();
}

$ zig test test.zig
Test [1/2] test "global variable alignment"...
Test [2/2] test "function alignment"...

All 2 tests passed.

If you have a pointer or a slice that has a small alignment, but you know that it actually has a bigger alignment, use @alignCast to change the pointer into a more aligned pointer. This is a no-op at runtime, but inserts a safety check:

test.zig

const std = @import("std");

test "pointer alignment safety" {
    var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
    const bytes = std.mem.sliceAsBytes(array[0..]);
    try std.testing.expect(foo(bytes) == 0x11111111);
}
fn foo(bytes: []u8) u32 {
    const slice4 = bytes[1..5];
    const int_slice = std.mem.bytesAsSlice(u32, @alignCast(4, slice4));
    return int_slice[0];
}

$ zig test test.zig
Test [1/1] test "pointer alignment safety"...
thread 10521 panic: incorrect alignment
/home/andy/Downloads/zig/docgen_tmp/test.zig:10:63: 0x2081bc in foo (test)
    const int_slice = std.mem.bytesAsSlice(u32, @alignCast(4, slice4));
                                                              ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:31: 0x206799 in test "pointer alignment safety" (test)
    try std.testing.expect(foo(bytes) == 0x11111111);
                              ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22d2da in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x22583a in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20970e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2086a6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x2084b2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

allowzero

This pointer attribute allows a pointer to have address zero. This is only ever needed on the freestanding OS target, where the address zero is mappable. If you want to represent null pointers, use Optional Pointers instead. Optional Pointers with allowzero are not the same size as pointers. In this code example, if the pointer did not have the allowzero attribute, this would be a Pointer Cast Invalid Null panic:

allowzero.zig

const std = @import("std");
const expect = std.testing.expect;

test "allowzero" {
    var zero: usize = 0;
    var ptr = @intToPtr(*allowzero i32, zero);
    try expect(@ptrToInt(ptr) == 0);
}

$ zig test allowzero.zig
Test [1/1] test "allowzero"...

All 1 tests passed.

Sentinel-Terminated Pointers

The syntax [*:x]T describes a pointer that has a length determined by a sentinel value. This provides protection against buffer overflow and overreads.

test.zig

const std = @import("std");

// This is also available as `std.c.printf`.
pub extern "c" fn printf(format: [*:0]const u8, ...) c_int;

pub fn main() anyerror!void {
    _ = printf("Hello, world!\n"); // OK

    const msg = "Hello, world!\n";
    const non_null_terminated_msg: [msg.len]u8 = msg.*;
    _ = printf(&non_null_terminated_msg);
}

$ zig build-exe test.zig -lc
./docgen_tmp/test.zig:11:17: error: expected type '[*:0]const u8', found '*const [14]u8'
    _ = printf(&non_null_terminated_msg);
                ^
./docgen_tmp/test.zig:11:17: note: destination pointer requires a terminating '0' sentinel
    _ = printf(&non_null_terminated_msg);
                ^

See also:

Sentinel-Terminated Slices

Sentinel-Terminated Arrays

Slices

test.zig

const expect = @import("std").testing.expect;

test "basic slices" {
    var array = [_]i32{ 1, 2, 3, 4 };
    // A slice is a pointer and a length. The difference between an array and
    // a slice is that the array's length is part of the type and known at
    // compile-time, whereas the slice's length is known at runtime.
    // Both can be accessed with the `len` field.
    var known_at_runtime_zero: usize = 0;
    const slice = array[known_at_runtime_zero..array.len];
    try expect(&slice[0] == &array[0]);
    try expect(slice.len == array.len);

    // Using the address-of operator on a slice gives a single-item pointer,
    // while using the `ptr` field gives a many-item pointer.
    try expect(@TypeOf(slice.ptr) == [*]i32);
    try expect(@TypeOf(&slice[0]) == *i32);
    try expect(@ptrToInt(slice.ptr) == @ptrToInt(&slice[0]));

    // Slices have array bounds checking. If you try to access something out
    // of bounds, you'll get a safety check failure:
    slice[10] += 1;

    // Note that `slice.ptr` does not invoke safety checking, while `&slice[0]`
    // asserts that the slice has len >= 1.
}

$ zig test test.zig
Test [1/1] test "basic slices"...
thread 10827 panic: index out of bounds
/home/andy/Downloads/zig/docgen_tmp/test.zig:22:10: 0x2069b3 in test "basic slices" (test)
    slice[10] += 1;
         ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22d2da in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x22583a in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20970e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x208736 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x208542 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

This is one reason we prefer slices to pointers.

slices.zig

const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;
const fmt = std.fmt;

test "using slices for strings" {
    // Zig has no concept of strings. String literals are const pointers
    // to null-terminated arrays of u8, and by convention parameters
    // that are "strings" are expected to be UTF-8 encoded slices of u8.
    // Here we coerce *const [5:0]u8 and *const [6:0]u8 to []const u8
    const hello: []const u8 = "hello";
    const world: []const u8 = "世界";

    var all_together: [100]u8 = undefined;
    // You can use slice syntax on an array to convert an array into a slice.
    const all_together_slice = all_together[0..];
    // String concatenation example.
    const hello_world = try fmt.bufPrint(all_together_slice, "{s} {s}", .{ hello, world });

    // Generally, you can use UTF-8 and not worry about whether something is a
    // string. If you don't need to deal with individual characters, no need
    // to decode.
    try expect(mem.eql(u8, hello_world, "hello 世界"));
}

test "slice pointer" {
    var array: [10]u8 = undefined;
    const ptr = &array;

    // You can use slicing syntax to convert a pointer into a slice:
    const slice = ptr[0..5];
    slice[2] = 3;
    try expect(slice[2] == 3);
    // The slice is mutable because we sliced a mutable pointer.
    // Furthermore, it is actually a pointer to an array, since the start
    // and end indexes were both comptime-known.
    try expect(@TypeOf(slice) == *[5]u8);

    // You can also slice a slice:
    const slice2 = slice[2..3];
    try expect(slice2.len == 1);
    try expect(slice2[0] == 3);
}

$ zig test slices.zig
Test [1/2] test "using slices for strings"...
Test [2/2] test "slice pointer"...

All 2 tests passed.

See also:

Pointers

for

Arrays

Sentinel-Terminated Slices

The syntax [:x]T is a slice which has a runtime known length and also guarantees a sentinel value at the element indexed by the length. The type does not guarantee that there are no sentinel elements before that. Sentinel-terminated slices allow element access to the len index.

null_terminated_slice.zig

const std = @import("std");
const expect = std.testing.expect;

test "null terminated slice" {
    const slice: [:0]const u8 = "hello";

    try expect(slice.len == 5);
    try expect(slice[5] == 0);
}

$ zig test null_terminated_slice.zig
Test [1/1] test "null terminated slice"...

All 1 tests passed.

See also:

Sentinel-Terminated Pointers

Sentinel-Terminated Arrays

struct

structs.zig

// Declare a struct.
// Zig gives no guarantees about the order of fields and the size of
// the struct but the fields are guaranteed to be ABI-aligned.
const Point = struct {
    x: f32,
    y: f32,
};

// Maybe we want to pass it to OpenGL so we want to be particular about
// how the bytes are arranged.
const Point2 = packed struct {
    x: f32,
    y: f32,
};

// Declare an instance of a struct.
const p = Point {
    .x = 0.12,
    .y = 0.34,
};

// Maybe we're not ready to fill out some of the fields.
var p2 = Point {
    .x = 0.12,
    .y = undefined,
};

// Structs can have methods
// Struct methods are not special, they are only namespaced
// functions that you can call with dot syntax.
const Vec3 = struct {
    x: f32,
    y: f32,
    z: f32,

    pub fn init(x: f32, y: f32, z: f32) Vec3 {
        return Vec3 {
            .x = x,
            .y = y,
            .z = z,
        };
    }

    pub fn dot(self: Vec3, other: Vec3) f32 {
        return self.x * other.x + self.y * other.y + self.z * other.z;
    }
};

const expect = @import("std").testing.expect;
test "dot product" {
    const v1 = Vec3.init(1.0, 0.0, 0.0);
    const v2 = Vec3.init(0.0, 1.0, 0.0);
    try expect(v1.dot(v2) == 0.0);

    // Other than being available to call with dot syntax, struct methods are
    // not special. You can reference them as any other declaration inside
    // the struct:
    try expect(Vec3.dot(v1, v2) == 0.0);
}

// Structs can have declarations.
// Structs can have 0 fields.
const Empty = struct {
    pub const PI = 3.14;
};
test "struct namespaced variable" {
    try expect(Empty.PI == 3.14);
    try expect(@sizeOf(Empty) == 0);

    // you can still instantiate an empty struct
    const does_nothing = Empty {};
}

// struct field order is determined by the compiler for optimal performance.
// however, you can still calculate a struct base pointer given a field pointer:
fn setYBasedOnX(x: *f32, y: f32) void {
    const point = @fieldParentPtr(Point, "x", x);
    point.y = y;
}
test "field parent pointer" {
    var point = Point {
        .x = 0.1234,
        .y = 0.5678,
    };
    setYBasedOnX(&point.x, 0.9);
    try expect(point.y == 0.9);
}

// You can return a struct from a function. This is how we do generics
// in Zig:
fn LinkedList(comptime T: type) type {
    return struct {
        pub const Node = struct {
            prev: ?*Node,
            next: ?*Node,
            data: T,
        };

        first: ?*Node,
        last:  ?*Node,
        len:   usize,
    };
}

test "linked list" {
    // Functions called at compile-time are memoized. This means you can
    // do this:
    try expect(LinkedList(i32) == LinkedList(i32));

    var list = LinkedList(i32) {
        .first = null,
        .last = null,
        .len = 0,
    };
    try expect(list.len == 0);

    // Since types are first class values you can instantiate the type
    // by assigning it to a variable:
    const ListOfInts = LinkedList(i32);
    try expect(ListOfInts == LinkedList(i32));

    var node = ListOfInts.Node {
        .prev = null,
        .next = null,
        .data = 1234,
    };
    var list2 = LinkedList(i32) {
        .first = &node,
        .last = &node,
        .len = 1,
    };
    try expect(list2.first.?.data == 1234);
}

$ zig test structs.zig
Test [1/4] test "dot product"...
Test [2/4] test "struct namespaced variable"...
Test [3/4] test "field parent pointer"...
Test [4/4] test "linked list"...

All 4 tests passed.

Default Field Values

Each struct field may have an expression indicating the default field value. Such expressions are executed at comptime, and allow the field to be omitted in a struct literal expression:

test.zig

const Foo = struct {
    a: i32 = 1234,
    b: i32,
};

test "default struct initialization fields" {
    const x = Foo{
        .b = 5,
    };
    if (x.a + x.b != 1239) {
        @compileError("it's even comptime known!");
    }
}

$ zig test test.zig
Test [1/1] test "default struct initialization fields"...

All 1 tests passed.

extern struct

An extern struct has in-memory layout guaranteed to match the C ABI for the target.

This kind of struct should only be used for compatibility with the C ABI. Every other use case should be solved with packed struct or normal struct.

See also:

extern union

extern enum

packed struct

Unlike normal structs, packed structs have guaranteed in-memory layout:

Fields remain in the order declared.

There is no padding between fields.

Zig supports arbitrary width Integers and although normally, integers with fewer than 8 bits will still use 1 byte of memory, in packed structs, they use exactly their bit width.

bool fields use exactly 1 bit.

An enum field uses exactly the bit width of its integer tag type.

A packed union field uses exactly the bit width of the union field with the largest bit width.

Non-ABI-aligned fields are packed into the smallest possible ABI-aligned integers in accordance with the target endianness.

This means that a packed struct can participate in a @bitCast or a @ptrCast to reinterpret memory. This even works at comptime:

test.zig

const std = @import("std");
const native_endian = @import("builtin").target.cpu.arch.endian();
const expect = std.testing.expect;

const Full = packed struct {
    number: u16,
};
const Divided = packed struct {
    half1: u8,
    quarter3: u4,
    quarter4: u4,
};

test "@bitCast between packed structs" {
    try doTheTest();
    comptime try doTheTest();
}

fn doTheTest() !void {
    try expect(@sizeOf(Full) == 2);
    try expect(@sizeOf(Divided) == 2);
    var full = Full{ .number = 0x1234 };
    var divided = @bitCast(Divided, full);
    switch (native_endian) {
        .Big => {
            try expect(divided.half1 == 0x12);
            try expect(divided.quarter3 == 0x3);
            try expect(divided.quarter4 == 0x4);
        },
        .Little => {
            try expect(divided.half1 == 0x34);
            try expect(divided.quarter3 == 0x2);
            try expect(divided.quarter4 == 0x1);
        },
    }
}

$ zig test test.zig
Test [1/1] test "@bitCast between packed structs"...

All 1 tests passed.

Zig allows the address to be taken of a non-byte-aligned field:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};

var foo = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};

test "pointer to non-byte-aligned field" {
    const ptr = &foo.b;
    try expect(ptr.* == 2);
}

$ zig test test.zig
Test [1/1] test "pointer to non-byte-aligned field"...

All 1 tests passed.

However, the pointer to a non-byte-aligned field has special properties and cannot be passed when a normal pointer is expected:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};

var bit_field = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};

test "pointer to non-bit-aligned field" {
    try expect(bar(&bit_field.b) == 2);
}

fn bar(x: *const u3) u3 {
    return x.*;
}

$ zig test test.zig
./docgen_tmp/test.zig:17:30: error: expected type '*const u3', found '*align(:3:1) u3'
    try expect(bar(&bit_field.b) == 2);
                             ^

In this case, the function bar cannot be called becuse the pointer to the non-ABI-aligned field mentions the bit offset, but the function expects an ABI-aligned pointer.

Pointers to non-ABI-aligned fields share the same address as the other fields within their host integer:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};

var bit_field = BitField{
    .a = 1,
    .b = 2,
    .c = 3,
};

test "pointer to non-bit-aligned field" {
    try expect(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.b));
    try expect(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.c));
}

$ zig test test.zig
Test [1/1] test "pointer to non-bit-aligned field"...

All 1 tests passed.

This can be observed with @bitOffsetOf and byteOffsetOf:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const BitField = packed struct {
    a: u3,
    b: u3,
    c: u2,
};

test "pointer to non-bit-aligned field" {
    comptime {
        try expect(@bitOffsetOf(BitField, "a") == 0);
        try expect(@bitOffsetOf(BitField, "b") == 3);
        try expect(@bitOffsetOf(BitField, "c") == 6);

        try expect(@byteOffsetOf(BitField, "a") == 0);
        try expect(@byteOffsetOf(BitField, "b") == 0);
        try expect(@byteOffsetOf(BitField, "c") == 0);
    }
}

$ zig test test.zig
Test [1/1] test "pointer to non-bit-aligned field"...

All 1 tests passed.

Packed structs have 1-byte alignment. However if you have an overaligned pointer to a packed struct, Zig should correctly understand the alignment of fields. However there is a bug:

test.zig

const S = packed struct {
    a: u32,
    b: u32,
};
test "overaligned pointer to packed struct" {
    var foo: S align(4) = undefined;
    const ptr: *align(4) S = &foo;
    const ptr_to_b: *u32 = &ptr.b;
}

$ zig test test.zig
./docgen_tmp/test.zig:8:32: error: expected type '*u32', found '*align(1) u32'
    const ptr_to_b: *u32 = &ptr.b;
                               ^

When this bug is fixed, the above test in the documentation will unexpectedly pass, which will cause the test suite to fail, notifying the bug fixer to update these docs.

It's also possible to set alignment of struct fields:

test.zig

const std = @import("std");
const expectEqual = std.testing.expectEqual;

test "aligned struct fields" {
    const S = struct {
        a: u32 align(2),
        b: u32 align(64),
    };
    var foo = S{ .a = 1, .b = 2 };

    try expectEqual(64, @alignOf(S));
    try expectEqual(*align(2) u32, @TypeOf(&foo.a));
    try expectEqual(*align(64) u32, @TypeOf(&foo.b));
}

$ zig test test.zig
Test [1/1] test "aligned struct fields"...

All 1 tests passed.

Using packed structs with volatile is problematic, and may be a compile error in the future. For details on this subscribe to this issue. TODO update these docs with a recommendation on how to use packed structs with MMIO (the use case for volatile packed structs) once this issue is resolved. Don't worry, there will be a good solution for this use case in zig.

Struct Naming

Since all structs are anonymous, Zig infers the type name based on a few rules.

If the struct is in the initialization expression of a variable, it gets named after that variable.

If the struct is in the return expression, it gets named after the function it is returning from, with the parameter values serialized.

Otherwise, the struct gets a name such as (anonymous struct at file.zig:7:38).

struct_name.zig

const std = @import("std");

pub fn main() void {
    const Foo = struct {};
    std.debug.print("variable: {s}\n", .{@typeName(Foo)});
    std.debug.print("anonymous: {s}\n", .{@typeName(struct {})});
    std.debug.print("function: {s}\n", .{@typeName(List(i32))});
}

fn List(comptime T: type) type {
    return struct {
        x: T,
    };
}

$ zig build-exe struct_name.zig
$ ./struct_name
variable: Foo
anonymous: struct:6:53
function: List(i32)

Anonymous Struct Literals

Zig allows omitting the struct type of a literal. When the result is coerced, the struct literal will directly instantiate the result location, with no copy:

struct_result.zig

const std = @import("std");
const expect = std.testing.expect;

const Point = struct {x: i32, y: i32};

test "anonymous struct literal" {
    var pt: Point = .{
        .x = 13,
        .y = 67,
    };
    try expect(pt.x == 13);
    try expect(pt.y == 67);
}

$ zig test struct_result.zig
Test [1/1] test "anonymous struct literal"...

All 1 tests passed.

The struct type can be inferred. Here the result location does not include a type, and so Zig infers the type:

struct_anon.zig

const std = @import("std");
const expect = std.testing.expect;

test "fully anonymous struct" {
    try dump(.{
        .int = @as(u32, 1234),
        .float = @as(f64, 12.34),
        .b = true,
        .s = "hi",
    });
}

fn dump(args: anytype) !void {
    try expect(args.int == 1234);
    try expect(args.float == 12.34);
    try expect(args.b);
    try expect(args.s[0] == 'h');
    try expect(args.s[1] == 'i');
}

$ zig test struct_anon.zig
Test [1/1] test "fully anonymous struct"...

All 1 tests passed.

Anonymous structs can be created without specifying field names, and are referred to as "tuples".

The fields are implicitly named using numbers starting from 0. Because their names are integers, the @"0" syntax must be used to access them. Names inside @"" are always recognised as identifiers.

Like arrays, tuples have a .len field, can be indexed and work with the ++ and ** operators. They can also be iterated over with inline for.

tuple.zig

const std = @import("std");
const expect = std.testing.expect;

test "tuple" {
    const values = .{
        @as(u32, 1234),
        @as(f64, 12.34),
        true,
        "hi",
    } ++ .{false} ** 2;
    try expect(values[0] == 1234);
    try expect(values[4] == false);
    inline for (values) |v, i| {
        if (i != 2) continue;
        try expect(v);
    }
    try expect(values.len == 6);
    try expect(values.@"3"[0] == 'h');
}

$ zig test tuple.zig
Test [1/1] test "tuple"...

All 1 tests passed.

See also:

comptime

@fieldParentPtr

enum

enums.zig

const expect = @import("std").testing.expect;
const mem = @import("std").mem;

// Declare an enum.
const Type = enum {
    ok,
    not_ok,
};

// Declare a specific instance of the enum variant.
const c = Type.ok;

// If you want access to the ordinal value of an enum, you
// can specify the tag type.
const Value = enum(u2) {
    zero,
    one,
    two,
};

// Now you can cast between u2 and Value.
// The ordinal value starts from 0, counting up for each member.
test "enum ordinal value" {
    try expect(@enumToInt(Value.zero) == 0);
    try expect(@enumToInt(Value.one) == 1);
    try expect(@enumToInt(Value.two) == 2);
}

// You can override the ordinal value for an enum.
const Value2 = enum(u32) {
    hundred = 100,
    thousand = 1000,
    million = 1000000,
};
test "set enum ordinal value" {
    try expect(@enumToInt(Value2.hundred) == 100);
    try expect(@enumToInt(Value2.thousand) == 1000);
    try expect(@enumToInt(Value2.million) == 1000000);
}

// Enums can have methods, the same as structs and unions.
// Enum methods are not special, they are only namespaced
// functions that you can call with dot syntax.
const Suit = enum {
    clubs,
    spades,
    diamonds,
    hearts,

    pub fn isClubs(self: Suit) bool {
        return self == Suit.clubs;
    }
};
test "enum method" {
    const p = Suit.spades;
    try expect(!p.isClubs());
}

// An enum variant of different types can be switched upon.
const Foo = enum {
    string,
    number,
    none,
};
test "enum variant switch" {
    const p = Foo.number;
    const what_is_it = switch (p) {
        Foo.string => "this is a string",
        Foo.number => "this is a number",
        Foo.none => "this is a none",
    };
    try expect(mem.eql(u8, what_is_it, "this is a number"));
}

// @typeInfo can be used to access the integer tag type of an enum.
const Small = enum {
    one,
    two,
    three,
    four,
};
test "std.meta.Tag" {
    try expect(@typeInfo(Small).Enum.tag_type == u2);
}

// @typeInfo tells us the field count and the fields names:
test "@typeInfo" {
    try expect(@typeInfo(Small).Enum.fields.len == 4);
    try expect(mem.eql(u8, @typeInfo(Small).Enum.fields[1].name, "two"));
}

// @tagName gives a []const u8 representation of an enum value:
test "@tagName" {
    try expect(mem.eql(u8, @tagName(Small.three), "three"));
}

$ zig test enums.zig
Test [1/7] test "enum ordinal value"...
Test [2/7] test "set enum ordinal value"...
Test [3/7] test "enum method"...
Test [4/7] test "enum variant switch"...
Test [5/7] test "std.meta.Tag"...
Test [6/7] test "@typeInfo"...
Test [7/7] test "@tagName"...

All 7 tests passed.

See also:

@typeInfo

@tagName

@sizeOf

extern enum

By default, enums are not guaranteed to be compatible with the C ABI:

test.zig

const Foo = enum { a, b, c };
export fn entry(foo: Foo) void { }

$ zig build-obj test.zig
./docgen_tmp/test.zig:2:22: error: parameter of type 'Foo' not allowed in function with calling convention 'C'
export fn entry(foo: Foo) void { }
                     ^

For a C-ABI-compatible enum, use extern enum:

test.zig

const Foo = extern enum { a, b, c };
export fn entry(foo: Foo) void { }

$ zig build-obj test.zig

Enum Literals

Enum literals allow specifying the name of an enum field without specifying the enum type:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Color = enum {
    auto,
    off,
    on,
};

test "enum literals" {
    const color1: Color = .auto;
    const color2 = Color.auto;
    try expect(color1 == color2);
}

test "switch using enum literals" {
    const color = Color.on;
    const result = switch (color) {
        .auto => false,
        .on => true,
        .off => false,
    };
    try expect(result);
}

$ zig test test.zig
Test [1/2] test "enum literals"...
Test [2/2] test "switch using enum literals"...

All 2 tests passed.

Non-exhaustive enum

A Non-exhaustive enum can be created by adding a trailing '_' field. It must specify a tag type and cannot consume every enumeration value.

@intToEnum on a non-exhaustive enum cannot fail.

A switch on a non-exhaustive enum can include a '_' prong as an alternative to an else prong with the difference being that it makes it a compile error if all the known tag names are not handled by the switch.

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Number = enum(u8) {
    one,
    two,
    three,
    _,
};

test "switch on non-exhaustive enum" {
    const number = Number.one;
    const result = switch (number) {
        .one => true,
        .two,
        .three => false,
        _ => false,
    };
    try expect(result);
    const is_one = switch (number) {
        .one => true,
        else => false,
    };
    try expect(is_one);
}

$ zig test test.zig
Test [1/1] test "switch on non-exhaustive enum"...

All 1 tests passed.

union

A bare union defines a set of possible types that a value can be as a list of fields. Only one field can be active at a time. The in-memory representation of bare unions is not guaranteed. Bare unions cannot be used to reinterpret memory. For that, use @ptrCast, or use an extern union or a packed union which have guaranteed in-memory layout. Accessing the non-active field is safety-checked Undefined Behavior:

test.zig

const Payload = union {
    int: i64,
    float: f64,
    boolean: bool,
};
test "simple union" {
    var payload = Payload{ .int = 1234 };
    payload.float = 12.34;
}

$ zig test test.zig
Test [1/1] test "simple union"...
thread 11537 panic: access of inactive union field
/home/andy/Downloads/zig/docgen_tmp/test.zig:8:12: 0x20677b in test "simple union" (test)
    payload.float = 12.34;
           ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22d05a in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x2255ba in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20943e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x208466 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x208272 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

You can activate another field by assigning the entire union:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Payload = union {
    int: i64,
    float: f64,
    boolean: bool,
};
test "simple union" {
    var payload = Payload{ .int = 1234 };
    try expect(payload.int == 1234);
    payload = Payload{ .float = 12.34 };
    try expect(payload.float == 12.34);
}

$ zig test test.zig
Test [1/1] test "simple union"...

All 1 tests passed.

In order to use switch with a union, it must be a Tagged union.

To initialize a union when the tag is a comptime-known name, see @unionInit.

Tagged union

Unions can be declared with an enum tag type. This turns the union into a tagged union, which makes it eligible to use with switch expressions. Tagged unions coerce to their tag type: Type Coercion: unions and enums.

test.zig

const std = @import("std");
const expect = std.testing.expect;

const ComplexTypeTag = enum {
    ok,
    not_ok,
};
const ComplexType = union(ComplexTypeTag) {
    ok: u8,
    not_ok: void,
};

test "switch on tagged union" {
    const c = ComplexType{ .ok = 42 };
    try expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);

    switch (c) {
        ComplexTypeTag.ok => |value| try expect(value == 42),
        ComplexTypeTag.not_ok => unreachable,
    }
}

test "get tag type" {
    try expect(std.meta.Tag(ComplexType) == ComplexTypeTag);
}

test "coerce to enum" {
    const c1 = ComplexType{ .ok = 42 };
    const c2 = ComplexType.not_ok;

    try expect(c1 == .ok);
    try expect(c2 == .not_ok);
}

$ zig test test.zig
Test [1/3] test "switch on tagged union"...
Test [2/3] test "get tag type"...
Test [3/3] test "coerce to enum"...

All 3 tests passed.

In order to modify the payload of a tagged union in a switch expression, place a * before the variable name to make it a pointer:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const ComplexTypeTag = enum {
    ok,
    not_ok,
};
const ComplexType = union(ComplexTypeTag) {
    ok: u8,
    not_ok: void,
};

test "modify tagged union in switch" {
    var c = ComplexType{ .ok = 42 };
    try expect(@as(ComplexTypeTag, c) == ComplexTypeTag.ok);

    switch (c) {
        ComplexTypeTag.ok => |*value| value.* += 1,
        ComplexTypeTag.not_ok => unreachable,
    }

    try expect(c.ok == 43);
}

$ zig test test.zig
Test [1/1] test "modify tagged union in switch"...

All 1 tests passed.

Unions can be made to infer the enum tag type. Further, unions can have methods just like structs and enums.

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Variant = union(enum) {
    int: i32,
    boolean: bool,

    // void can be omitted when inferring enum tag type.
    none,

    fn truthy(self: Variant) bool {
        return switch (self) {
            Variant.int => |x_int| x_int != 0,
            Variant.boolean => |x_bool| x_bool,
            Variant.none => false,
        };
    }
};

test "union method" {
    var v1 = Variant{ .int = 1 };
    var v2 = Variant{ .boolean = false };

    try expect(v1.truthy());
    try expect(!v2.truthy());
}

$ zig test test.zig
Test [1/1] test "union method"...

All 1 tests passed.

@tagName can be used to return a comptime []const u8 value representing the field name:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Small2 = union(enum) {
    a: i32,
    b: bool,
    c: u8,
};
test "@tagName" {
    try expect(std.mem.eql(u8, @tagName(Small2.a), "a"));
}

$ zig test test.zig
Test [1/1] test "@tagName"...

All 1 tests passed.

extern union

An extern union has memory layout guaranteed to be compatible with the target C ABI.

See also:

extern struct

packed union

A packed union has well-defined in-memory layout and is eligible to be in a packed struct.

Anonymous Union Literals

Anonymous Struct Literals syntax can be used to initialize unions without specifying the type:

anon_union.zig

const std = @import("std");
const expect = std.testing.expect;

const Number = union {
    int: i32,
    float: f64,
};

test "anonymous union literal syntax" {
    var i: Number = .{.int = 42};
    var f = makeNumber();
    try expect(i.int == 42);
    try expect(f.float == 12.34);
}

fn makeNumber() Number {
    return .{.float = 12.34};
}

$ zig test anon_union.zig
Test [1/1] test "anonymous union literal syntax"...

All 1 tests passed.

opaque

opaque {} declares a new type with an unknown (but non-zero) size and alignment. It can contain declarations the same as structs, unions, and enums.

This is typically used for type safety when interacting with C code that does not expose struct details. Example:

test.zig

const Derp = opaque {};
const Wat = opaque {};

extern fn bar(d: *Derp) void;
fn foo(w: *Wat) callconv(.C) void {
    bar(w);
}

test "call foo" {
    foo(undefined);
}

$ zig test test.zig
./docgen_tmp/test.zig:6:9: error: expected type '*Derp', found '*Wat'
    bar(w);
        ^
./docgen_tmp/test.zig:6:9: note: pointer type child 'Wat' cannot cast into pointer type child 'Derp'
    bar(w);
        ^

blocks

Blocks are used to limit the scope of variable declarations:

test.zig

test "access variable after block scope" {
    {
        var x: i32 = 1;
    }
    x += 1;
}

$ zig test test.zig
./docgen_tmp/test.zig:5:5: error: use of undeclared identifier 'x'
    x += 1;
    ^

Blocks are expressions. When labeled, break can be used to return a value from the block:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "labeled break from labeled block expression" {
    var y: i32 = 123;

    const x = blk: {
        y += 1;
        break :blk y;
    };
    try expect(x == 124);
    try expect(y == 124);
}

$ zig test test.zig
Test [1/1] test "labeled break from labeled block expression"...

All 1 tests passed.

Here, blk can be any name.

See also:

Labeled while

Labeled for

Shadowing

It is never allowed for an identifier to "hide" another one by using the same name:

test.zig

const pi = 3.14;

test "inside test block" {
    // Let's even go inside another block
    {
        var pi: i32 = 1234;
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:6:9: error: redefinition of 'pi'
        var pi: i32 = 1234;
        ^
./docgen_tmp/test.zig:1:1: note: previous definition is here
const pi = 3.14;
^

Because of this, when you read Zig code you can rely on an identifier always meaning the same thing, within the scope it is defined. Note that you can, however use the same name if the scopes are separate:

test.zig

test "separate scopes" {
    {
        const pi = 3.14;
    }
    {
        var pi: bool = true;
    }
}

$ zig test test.zig
Test [1/1] test "separate scopes"...

All 1 tests passed.

switch

switch.zig

const std = @import("std");
const expect = std.testing.expect;

test "switch simple" {
    const a: u64 = 10;
    const zz: u64 = 103;

    // All branches of a switch expression must be able to be coerced to a
    // common type.
    //
    // Branches cannot fallthrough. If fallthrough behavior is desired, combine
    // the cases and use an if.
    const b = switch (a) {
        // Multiple cases can be combined via a ','
        1, 2, 3 => 0,

        // Ranges can be specified using the ... syntax. These are inclusive
        // both ends.
        5...100 => 1,

        // Branches can be arbitrarily complex.
        101 => blk: {
            const c: u64 = 5;
            break :blk c * 2 + 1;
        },

        // Switching on arbitrary expressions is allowed as long as the
        // expression is known at compile-time.
        zz => zz,
        comptime blk: {
            const d: u32 = 5;
            const e: u32 = 100;
            break :blk d + e;
        } => 107,

        // The else branch catches everything not already captured.
        // Else branches are mandatory unless the entire range of values
        // is handled.
        else => 9,
    };

    try expect(b == 1);
}

// Switch expressions can be used outside a function:
const os_msg = switch (std.Target.current.os.tag) {
    .linux => "we found a linux user",
    else => "not a linux user",
};

// Inside a function, switch statements implicitly are compile-time
// evaluated if the target expression is compile-time known.
test "switch inside function" {
    switch (std.Target.current.os.tag) {
        .fuchsia => {
            // On an OS other than fuchsia, block is not even analyzed,
            // so this compile error is not triggered.
            // On fuchsia this compile error would be triggered.
            @compileError("fuchsia not supported");
        },
        else => {},
    }
}

$ zig test switch.zig
Test [1/2] test "switch simple"...
Test [2/2] test "switch inside function"...

All 2 tests passed.

switch can be used to capture the field values of a Tagged union. Modifications to the field values can be done by placing a * before the capture variable name, turning it into a pointer.

test.zig

const expect = @import("std").testing.expect;

test "switch on tagged union" {
    const Point = struct {
        x: u8,
        y: u8,
    };
    const Item = union(enum) {
        a: u32,
        c: Point,
        d,
        e: u32,
    };

    var a = Item{ .c = Point{ .x = 1, .y = 2 } };

    // Switching on more complex enums is allowed.
    const b = switch (a) {
        // A capture group is allowed on a match, and will return the enum
        // value matched. If the payload types of both cases are the same
        // they can be put into the same switch prong.
        Item.a, Item.e => |item| item,

        // A reference to the matched value can be obtained using `*` syntax.
        Item.c => |*item| blk: {
            item.*.x += 1;
            break :blk 6;
        },

        // No else is required if the types cases was exhaustively handled
        Item.d => 8,
    };

    try expect(b == 6);
    try expect(a.c.x == 2);
}

$ zig test test.zig
Test [1/1] test "switch on tagged union"...

All 1 tests passed.

See also:

comptime

enum

@compileError

Compile Variables

Exhaustive Switching

When a switch expression does not have an else clause, it must exhaustively list all the possible values. Failure to do so is a compile error:

test.zig

const Color = enum {
    auto,
    off,
    on,
};

test "exhaustive switching" {
    const color = Color.off;
    switch (color) {
        Color.auto => {},
        Color.on => {},
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:9:5: error: enumeration value 'Color.off' not handled in switch
    switch (color) {
    ^
./docgen_tmp/test.zig:7:29: note: referenced here
test "exhaustive switching" {
                            ^

Switching with Enum Literals

Enum Literals can be useful to use with switch to avoid repetitively specifying enum or union types:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Color = enum {
    auto,
    off,
    on,
};

test "enum literals with switch" {
    const color = Color.off;
    const result = switch (color) {
        .auto => false,
        .on => false,
        .off => true,
    };
    try expect(result);
}

$ zig test test.zig
Test [1/1] test "enum literals with switch"...

All 1 tests passed.

while

A while loop is used to repeatedly execute an expression until some condition is no longer true.

while.zig

const expect = @import("std").testing.expect;

test "while basic" {
    var i: usize = 0;
    while (i < 10) {
        i += 1;
    }
    try expect(i == 10);
}

$ zig test while.zig
Test [1/1] test "while basic"...

All 1 tests passed.

Use break to exit a while loop early.

while.zig

const expect = @import("std").testing.expect;

test "while break" {
    var i: usize = 0;
    while (true) {
        if (i == 10)
            break;
        i += 1;
    }
    try expect(i == 10);
}

$ zig test while.zig
Test [1/1] test "while break"...

All 1 tests passed.

Use continue to jump back to the beginning of the loop.

while.zig

const expect = @import("std").testing.expect;

test "while continue" {
    var i: usize = 0;
    while (true) {
        i += 1;
        if (i < 10)
            continue;
        break;
    }
    try expect(i == 10);
}

$ zig test while.zig
Test [1/1] test "while continue"...

All 1 tests passed.

While loops support a continue expression which is executed when the loop is continued. The continue keyword respects this expression.

while.zig

const expect = @import("std").testing.expect;

test "while loop continue expression" {
    var i: usize = 0;
    while (i < 10) : (i += 1) {}
    try expect(i == 10);
}

test "while loop continue expression, more complicated" {
    var i: usize = 1;
    var j: usize = 1;
    while (i * j < 2000) : ({ i *= 2; j *= 3; }) {
        const my_ij = i * j;
        try expect(my_ij < 2000);
    }
}

$ zig test while.zig
Test [1/2] test "while loop continue expression"...
Test [2/2] test "while loop continue expression, more complicated"...

All 2 tests passed.

While loops are expressions. The result of the expression is the result of the else clause of a while loop, which is executed when the condition of the while loop is tested as false.

break, like return, accepts a value parameter. This is the result of the while expression. When you break from a while loop, the else branch is not evaluated.

while.zig

const expect = @import("std").testing.expect;

test "while else" {
    try expect(rangeHasNumber(0, 10, 5));
    try expect(!rangeHasNumber(0, 10, 15));
}

fn rangeHasNumber(begin: usize, end: usize, number: usize) bool {
    var i = begin;
    return while (i < end) : (i += 1) {
        if (i == number) {
            break true;
        }
    } else false;
}

$ zig test while.zig
Test [1/1] test "while else"...

All 1 tests passed.

Labeled while

When a while loop is labeled, it can be referenced from a break or continue from within a nested loop:

test.zig

test "nested break" {
    outer: while (true) {
        while (true) {
            break :outer;
        }
    }
}

test "nested continue" {
    var i: usize = 0;
    outer: while (i < 10) : (i += 1) {
        while (true) {
            continue :outer;
        }
    }
}

$ zig test test.zig
Test [1/2] test "nested break"...
Test [2/2] test "nested continue"...

All 2 tests passed.

while with Optionals

Just like if expressions, while loops can take an optional as the condition and capture the payload. When null is encountered the loop exits.

When the |x| syntax is present on a while expression, the while condition must have an Optional Type.

The else branch is allowed on optional iteration. In this case, it will be executed on the first null value encountered.

while.zig

const expect = @import("std").testing.expect;

test "while null capture" {
    var sum1: u32 = 0;
    numbers_left = 3;
    while (eventuallyNullSequence()) |value| {
        sum1 += value;
    }
    try expect(sum1 == 3);

    var sum2: u32 = 0;
    numbers_left = 3;
    while (eventuallyNullSequence()) |value| {
        sum2 += value;
    } else {
        try expect(sum2 == 3);
    }
}

var numbers_left: u32 = undefined;
fn eventuallyNullSequence() ?u32 {
    return if (numbers_left == 0) null else blk: {
        numbers_left -= 1;
        break :blk numbers_left;
    };
}

$ zig test while.zig
Test [1/1] test "while null capture"...

All 1 tests passed.

while with Error Unions

Just like if expressions, while loops can take an error union as the condition and capture the payload or the error code. When the condition results in an error code the else branch is evaluated and the loop is finished.

When the else |x| syntax is present on a while expression, the while condition must have an Error Union Type.

while.zig

const expect = @import("std").testing.expect;

test "while error union capture" {
    var sum1: u32 = 0;
    numbers_left = 3;
    while (eventuallyErrorSequence()) |value| {
        sum1 += value;
    } else |err| {
        try expect(err == error.ReachedZero);
    }
}

var numbers_left: u32 = undefined;

fn eventuallyErrorSequence() anyerror!u32 {
    return if (numbers_left == 0) error.ReachedZero else blk: {
        numbers_left -= 1;
        break :blk numbers_left;
    };
}

$ zig test while.zig
Test [1/1] test "while error union capture"...

All 1 tests passed.

inline while

While loops can be inlined. This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values.

test.zig

const expect = @import("std").testing.expect;

test "inline while loop" {
    comptime var i = 0;
    var sum: usize = 0;
    inline while (i < 3) : (i += 1) {
        const T = switch (i) {
            0 => f32,
            1 => i8,
            2 => bool,
            else => unreachable,
        };
        sum += typeNameLength(T);
    }
    try expect(sum == 9);
}

fn typeNameLength(comptime T: type) usize {
    return @typeName(T).len;
}

$ zig test test.zig
Test [1/1] test "inline while loop"...

All 1 tests passed.

It is recommended to use inline loops only for one of these reasons:

You need the loop to execute at comptime for the semantics to work.

You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.

See also:

if

Optionals

Errors

comptime

unreachable

for

for.zig

const expect = @import("std").testing.expect;

test "for basics" {
    const items = [_]i32 { 4, 5, 3, 4, 0 };
    var sum: i32 = 0;

    // For loops iterate over slices and arrays.
    for (items) |value| {
        // Break and continue are supported.
        if (value == 0) {
            continue;
        }
        sum += value;
    }
    try expect(sum == 16);

    // To iterate over a portion of a slice, reslice.
    for (items[0..1]) |value| {
        sum += value;
    }
    try expect(sum == 20);

    // To access the index of iteration, specify a second capture value.
    // This is zero-indexed.
    var sum2: i32 = 0;
    for (items) |value, i| {
        try expect(@TypeOf(i) == usize);
        sum2 += @intCast(i32, i);
    }
    try expect(sum2 == 10);
}

test "for reference" {
    var items = [_]i32 { 3, 4, 2 };

    // Iterate over the slice by reference by
    // specifying that the capture value is a pointer.
    for (items) |*value| {
        value.* += 1;
    }

    try expect(items[0] == 4);
    try expect(items[1] == 5);
    try expect(items[2] == 3);
}

test "for else" {
    // For allows an else attached to it, the same as a while loop.
    var items = [_]?i32 { 3, 4, null, 5 };

    // For loops can also be used as expressions.
    // Similar to while loops, when you break from a for loop, the else branch is not evaluated.
    var sum: i32 = 0;
    const result = for (items) |value| {
        if (value != null) {
            sum += value.?;
        }
    } else blk: {
        try expect(sum == 12);
        break :blk sum;
    };
    try expect(result == 12);
}

$ zig test for.zig
Test [1/3] test "for basics"...
Test [2/3] test "for reference"...
Test [3/3] test "for else"...

All 3 tests passed.

Labeled for

When a for loop is labeled, it can be referenced from a break or continue from within a nested loop:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "nested break" {
    var count: usize = 0;
    outer: for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
        for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
            count += 1;
            break :outer;
        }
    }
    try expect(count == 1);
}

test "nested continue" {
    var count: usize = 0;
    outer: for ([_]i32{ 1, 2, 3, 4, 5, 6, 7, 8 }) |_| {
        for ([_]i32{ 1, 2, 3, 4, 5 }) |_| {
            count += 1;
            continue :outer;
        }
    }

    try expect(count == 8);
}

$ zig test test.zig
Test [1/2] test "nested break"...
Test [2/2] test "nested continue"...

All 2 tests passed.

inline for

For loops can be inlined. This causes the loop to be unrolled, which allows the code to do some things which only work at compile time, such as use types as first class values. The capture value and iterator value of inlined for loops are compile-time known.

test.zig

const expect = @import("std").testing.expect;

test "inline for loop" {
    const nums = [_]i32{2, 4, 6};
    var sum: usize = 0;
    inline for (nums) |i| {
        const T = switch (i) {
            2 => f32,
            4 => i8,
            6 => bool,
            else => unreachable,
        };
        sum += typeNameLength(T);
    }
    try expect(sum == 9);
}

fn typeNameLength(comptime T: type) usize {
    return @typeName(T).len;
}

$ zig test test.zig
Test [1/1] test "inline for loop"...

All 1 tests passed.

It is recommended to use inline loops only for one of these reasons:

You need the loop to execute at comptime for the semantics to work.

You have a benchmark to prove that forcibly unrolling the loop in this way is measurably faster.

See also:

while

comptime

Arrays

Slices

if

if.zig

// If expressions have three uses, corresponding to the three types:
// * bool
// * ?T
// * anyerror!T

const expect = @import("std").testing.expect;

test "if expression" {
    // If expressions are used instead of a ternary expression.
    const a: u32 = 5;
    const b: u32 = 4;
    const result = if (a != b) 47 else 3089;
    try expect(result == 47);
}

test "if boolean" {
    // If expressions test boolean conditions.
    const a: u32 = 5;
    const b: u32 = 4;
    if (a != b) {
        try expect(true);
    } else if (a == 9) {
        unreachable;
    } else {
        unreachable;
    }
}

test "if optional" {
    // If expressions test for null.

    const a: ?u32 = 0;
    if (a) |value| {
        try expect(value == 0);
    } else {
        unreachable;
    }

    const b: ?u32 = null;
    if (b) |value| {
        unreachable;
    } else {
        try expect(true);
    }

    // The else is not required.
    if (a) |value| {
        try expect(value == 0);
    }

    // To test against null only, use the binary equality operator.
    if (b == null) {
        try expect(true);
    }

    // Access the value by reference using a pointer capture.
    var c: ?u32 = 3;
    if (c) |*value| {
        value.* = 2;
    }

    if (c) |value| {
        try expect(value == 2);
    } else {
        unreachable;
    }
}

test "if error union" {
    // If expressions test for errors.
    // Note the |err| capture on the else.

    const a: anyerror!u32 = 0;
    if (a) |value| {
        try expect(value == 0);
    } else |err| {
        unreachable;
    }

    const b: anyerror!u32 = error.BadValue;
    if (b) |value| {
        unreachable;
    } else |err| {
        try expect(err == error.BadValue);
    }

    // The else and |err| capture is strictly required.
    if (a) |value| {
        try expect(value == 0);
    } else |_| {}

    // To check only the error value, use an empty block expression.
    if (b) |_| {} else |err| {
        try expect(err == error.BadValue);
    }

    // Access the value by reference using a pointer capture.
    var c: anyerror!u32 = 3;
    if (c) |*value| {
        value.* = 9;
    } else |err| {
        unreachable;
    }

    if (c) |value| {
        try expect(value == 9);
    } else |err| {
        unreachable;
    }
}

test "if error union with optional" {
    // If expressions test for errors before unwrapping optionals.
    // The |optional_value| capture's type is ?u32.

    const a: anyerror!?u32 = 0;
    if (a) |optional_value| {
        try expect(optional_value.? == 0);
    } else |err| {
        unreachable;
    }

    const b: anyerror!?u32 = null;
    if (b) |optional_value| {
        try expect(optional_value == null);
    } else |err| {
        unreachable;
    }

    const c: anyerror!?u32 = error.BadValue;
    if (c) |optional_value| {
        unreachable;
    } else |err| {
        try expect(err == error.BadValue);
    }

    // Access the value by reference by using a pointer capture each time.
    var d: anyerror!?u32 = 3;
    if (d) |*optional_value| {
        if (optional_value.*) |*value| {
            value.* = 9;
        }
    } else |err| {
        unreachable;
    }

    if (d) |optional_value| {
        try expect(optional_value.? == 9);
    } else |err| {
        unreachable;
    }
}

$ zig test if.zig
Test [1/5] test "if expression"...
Test [2/5] test "if boolean"...
Test [3/5] test "if optional"...
Test [4/5] test "if error union"...
Test [5/5] test "if error union with optional"...

All 5 tests passed.

See also:

Optionals

Errors

defer

defer.zig

const std = @import("std");
const expect = std.testing.expect;
const print = std.debug.print;

// defer will execute an expression at the end of the current scope.
fn deferExample() !usize {
    var a: usize = 1;

    {
        defer a = 2;
        a = 1;
    }
    try expect(a == 2);

    a = 5;
    return a;
}

test "defer basics" {
    try expect((try deferExample()) == 5);
}

// If multiple defer statements are specified, they will be executed in
// the reverse order they were run.
fn deferUnwindExample() void {
    print("\n", .{});

    defer {
        print("1 ", .{});
    }
    defer {
        print("2 ", .{});
    }
    if (false) {
        // defers are not run if they are never executed.
        defer {
            print("3 ", .{});
        }
    }
}

test "defer unwinding" {
    deferUnwindExample();
}

// The errdefer keyword is similar to defer, but will only execute if the
// scope returns with an error.
//
// This is especially useful in allowing a function to clean up properly
// on error, and replaces goto error handling tactics as seen in c.
fn deferErrorExample(is_error: bool) !void {
    print("\nstart of function\n", .{});

    // This will always be executed on exit
    defer {
        print("end of function\n", .{});
    }

    errdefer {
        print("encountered an error!\n", .{});
    }

    if (is_error) {
        return error.DeferError;
    }
}

test "errdefer unwinding" {
    deferErrorExample(false) catch {};
    deferErrorExample(true) catch {};
}

$ zig test defer.zig
Test [1/3] test "defer basics"...
Test [2/3] test "defer unwinding"...

2 1 Test [3/3] test "errdefer unwinding"...

start of function
end of function

start of function
encountered an error!
end of function

All 3 tests passed.

See also:

Errors

unreachable

In Debug and ReleaseSafe mode, and when using zig test, unreachable emits a call to panic with the message reached unreachable code.

In ReleaseFast mode, the optimizer uses the assumption that unreachable code will never be hit to perform optimizations. However, zig test even in ReleaseFast mode still emits unreachable as calls to panic.

Basics

test.zig

// unreachable is used to assert that control flow will never happen upon a
// particular location:
test "basic math" {
    const x = 1;
    const y = 2;
    if (x + y != 3) {
        unreachable;
    }
}

$ zig test test.zig
Test [1/1] test "basic math"...

All 1 tests passed.

In fact, this is how std.debug.assert is implemented:

test.zig

// This is how std.debug.assert is implemented
fn assert(ok: bool) void {
    if (!ok) unreachable; // assertion failure
}

// This test will fail because we hit unreachable.
test "this will fail" {
    assert(false);
}

$ zig test test.zig
Test [1/1] test "this will fail"...
thread 12592 panic: reached unreachable code
/home/andy/Downloads/zig/docgen_tmp/test.zig:3:14: 0x207feb in assert (test)
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:8:11: 0x2066de in test "this will fail" (test)
    assert(false);
          ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22d00a in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x22556a in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2093ee in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x208416 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x208222 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

At Compile-Time

test.zig

const assert = @import("std").debug.assert;

test "type of unreachable" {
    comptime {
        // The type of unreachable is noreturn.

        // However this assertion will still fail because
        // evaluating unreachable at compile-time is a compile error.

        assert(@TypeOf(unreachable) == noreturn);
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:10:16: error: unreachable code
        assert(@TypeOf(unreachable) == noreturn);
               ^
./docgen_tmp/test.zig:3:28: note: referenced here
test "type of unreachable" {
                           ^

See also:

Zig Test

Build Mode

comptime

noreturn

noreturn is the type of:

break

continue

return

unreachable

while (true) {}

When resolving types together, such as if clauses or switch prongs, the noreturn type is compatible with every other type. Consider:

test.zig

fn foo(condition: bool, b: u32) void {
    const a = if (condition) b else return;
    @panic("do something with a");
}
test "noreturn" {
    foo(false, 1);
}

$ zig test test.zig
Test [1/1] test "noreturn"...

All 1 tests passed.

Another use case for noreturn is the exit function:

test.zig

pub extern "kernel32" fn ExitProcess(exit_code: c_uint) callconv(if (@import("builtin").target.cpu.arch == .i386) .Stdcall else .C) noreturn;

test "foo" {
    const value = bar() catch ExitProcess(1);
    try expect(value == 1234);
}

fn bar() anyerror!u32 {
    return 1234;
}

const expect = @import("std").testing.expect;

$ zig test test.zig -target x86_64-windows
warning: created docgen_tmp/zig-cache/o/c42fd17f6f3b779d958fa42345e9399d/test.exe but skipping execution because it is non-native

Functions

functions.zig

const expect = @import("std").testing.expect;

// Functions are declared like this
fn add(a: i8, b: i8) i8 {
    if (a == 0) {
        return b;
    }

    return a + b;
}

// The export specifier makes a function externally visible in the generated
// object file, and makes it use the C ABI.
export fn sub(a: i8, b: i8) i8 { return a - b; }

// The extern specifier is used to declare a function that will be resolved
// at link time, when linking statically, or at runtime, when linking
// dynamically.
// The callconv specifier changes the calling convention of the function.
extern "kernel32" fn ExitProcess(exit_code: u32) callconv(if (@import("builtin").target.cpu.arch == .i386) .Stdcall else .C) noreturn;
extern "c" fn atan2(a: f64, b: f64) f64;

// The @setCold builtin tells the optimizer that a function is rarely called.
fn abort() noreturn {
    @setCold(true);
    while (true) {}
}

// The naked calling convention makes a function not have any function prologue or epilogue.
// This can be useful when integrating with assembly.
fn _start() callconv(.Naked) noreturn {
    abort();
}

// The inline calling convention forces a function to be inlined at all call sites.
// If the function cannot be inlined, it is a compile-time error.
fn shiftLeftOne(a: u32) callconv(.Inline) u32 {
    return a << 1;
}

// The pub specifier allows the function to be visible when importing.
// Another file can use @import and call sub2
pub fn sub2(a: i8, b: i8) i8 { return a - b; }

// Functions can be used as values and are equivalent to pointers.
const call2_op = fn (a: i8, b: i8) i8;
fn do_op(fn_call: call2_op, op1: i8, op2: i8) i8 {
    return fn_call(op1, op2);
}

test "function" {
    try expect(do_op(add, 5, 6) == 11);
    try expect(do_op(sub2, 5, 6) == -1);
}

$ zig test functions.zig
Test [1/1] test "function"...

All 1 tests passed.

Function values are like pointers:

test.zig

const assert = @import("std").debug.assert;

comptime {
    assert(@TypeOf(foo) == fn()void);
    assert(@sizeOf(fn()void) == @sizeOf(?fn()void));
}

fn foo() void { }

$ zig build-obj test.zig

Pass-by-value Parameters

Primitive types such as Integers and Floats passed as parameters are copied, and then the copy is available in the function body. This is called "passing by value". Copying a primitive type is essentially free and typically involves nothing more than setting a register.

Structs, unions, and arrays can sometimes be more efficiently passed as a reference, since a copy could be arbitrarily expensive depending on the size. When these types are passed as parameters, Zig may choose to copy and pass by value, or pass by reference, whichever way Zig decides will be faster. This is made possible, in part, by the fact that parameters are immutable.

test.zig

const Point = struct {
    x: i32,
    y: i32,
};

fn foo(point: Point) i32 {
    // Here, `point` could be a reference, or a copy. The function body
    // can ignore the difference and treat it as a value. Be very careful
    // taking the address of the parameter - it should be treated as if
    // the address will become invalid when the function returns.
    return point.x + point.y;
}

const expect = @import("std").testing.expect;

test "pass struct to function" {
    try expect(foo(Point{ .x = 1, .y = 2 }) == 3);
}

$ zig test test.zig
Test [1/1] test "pass struct to function"...

All 1 tests passed.

For extern functions, Zig follows the C ABI for passing structs and unions by value.

Function Parameter Type Inference

Function parameters can be declared with anytype in place of the type. In this case the parameter types will be inferred when the function is called. Use @TypeOf and @typeInfo to get information about the inferred type.

test.zig

const expect = @import("std").testing.expect;

fn addFortyTwo(x: anytype) @TypeOf(x) {
    return x + 42;
}

test "fn type inference" {
    try expect(addFortyTwo(1) == 43);
    try expect(@TypeOf(addFortyTwo(1)) == comptime_int);
    var y: i64 = 2;
    try expect(addFortyTwo(y) == 44);
    try expect(@TypeOf(addFortyTwo(y)) == i64);
}

$ zig test test.zig
Test [1/1] test "fn type inference"...

All 1 tests passed.

Function Reflection

test.zig

const expect = @import("std").testing.expect;

test "fn reflection" {
    try expect(@typeInfo(@TypeOf(expect)).Fn.args[0].arg_type.? == bool);
    try expect(@typeInfo(@TypeOf(expect)).Fn.is_var_args == false);
}

$ zig test test.zig
Test [1/1] test "fn reflection"...

All 1 tests passed.

Errors

Error Set Type

An error set is like an enum. However, each error name across the entire compilation gets assigned an unsigned integer greater than 0. You are allowed to declare the same error name more than once, and if you do, it gets assigned the same integer value.

The number of unique error values across the entire compilation should determine the size of the error set type. However right now it is hard coded to be a u16. See #768.

You can coerce an error from a subset to a superset:

test.zig

const std = @import("std");

const FileOpenError = error {
    AccessDenied,
    OutOfMemory,
    FileNotFound,
};

const AllocationError = error {
    OutOfMemory,
};

test "coerce subset to superset" {
    const err = foo(AllocationError.OutOfMemory);
    try std.testing.expect(err == FileOpenError.OutOfMemory);
}

fn foo(err: AllocationError) FileOpenError {
    return err;
}

$ zig test test.zig
Test [1/1] test "coerce subset to superset"...

All 1 tests passed.

But you cannot coerce an error from a superset to a subset:

test.zig

const FileOpenError = error {
    AccessDenied,
    OutOfMemory,
    FileNotFound,
};

const AllocationError = error {
    OutOfMemory,
};

test "coerce superset to subset" {
    foo(FileOpenError.OutOfMemory) catch {};
}

fn foo(err: FileOpenError) AllocationError {
    return err;
}

$ zig test test.zig
./docgen_tmp/test.zig:16:12: error: expected type 'AllocationError', found 'FileOpenError'
    return err;
           ^
./docgen_tmp/test.zig:2:5: note: 'error.AccessDenied' not a member of destination error set
    AccessDenied,
    ^
./docgen_tmp/test.zig:4:5: note: 'error.FileNotFound' not a member of destination error set
    FileNotFound,
    ^

There is a shortcut for declaring an error set with only 1 value, and then getting that value:

const err = error.FileNotFound;

This is equivalent to:

const err = (error {FileNotFound}).FileNotFound;

This becomes useful when using Inferred Error Sets.

The Global Error Set

anyerror refers to the global error set. This is the error set that contains all errors in the entire compilation unit. It is a superset of all other error sets and a subset of none of them.

You can coerce any error set to the global one, and you can explicitly cast an error of the global error set to a non-global one. This inserts a language-level assert to make sure the error value is in fact in the destination error set.

The global error set should generally be avoided because it prevents the compiler from knowing what errors are possible at compile-time. Knowing the error set at compile-time is better for generated documentation and helpful error messages, such as forgetting a possible error value in a switch.

Error Union Type

An error set type and normal type can be combined with the ! binary operator to form an error union type. You are likely to use an error union type more often than an error set type by itself.

Here is a function to parse a string into a 64-bit integer:

test.zig

const std = @import("std");
const maxInt = std.math.maxInt;

pub fn parseU64(buf: []const u8, radix: u8) !u64 {
    var x: u64 = 0;

    for (buf) |c| {
        const digit = charToDigit(c);

        if (digit >= radix) {
            return error.InvalidChar;
        }

        // x *= radix
        if (@mulWithOverflow(u64, x, radix, &x)) {
            return error.Overflow;
        }

        // x += digit
        if (@addWithOverflow(u64, x, digit, &x)) {
            return error.Overflow;
        }
    }

    return x;
}

fn charToDigit(c: u8) u8 {
    return switch (c) {
        '0' ... '9' => c - '0',
        'A' ... 'Z' => c - 'A' + 10,
        'a' ... 'z' => c - 'a' + 10,
        else => maxInt(u8),
    };
}

test "parse u64" {
    const result = try parseU64("1234", 10);
    try std.testing.expect(result == 1234);
}

$ zig test test.zig
Test [1/1] test "parse u64"...

All 1 tests passed.

Notice the return type is !u64. This means that the function either returns an unsigned 64 bit integer, or an error. We left off the error set to the left of the !, so the error set is inferred.

Within the function definition, you can see some return statements that return an error, and at the bottom a return statement that returns a u64. Both types coerce to anyerror!u64.

What it looks like to use this function varies depending on what you're trying to do. One of the following:

You want to provide a default value if it returned an error.

If it returned an error then you want to return the same error.

You know with complete certainty it will not return an error, so want to unconditionally unwrap it.

You want to take a different action for each possible error.

catch

If you want to provide a default value, you can use the catch binary operator:

fn doAThing(str: []u8) void {
    const number = parseU64(str, 10) catch 13;
    // ...
}

In this code, number will be equal to the successfully parsed string, or a default value of 13. The type of the right hand side of the binary catch operator must match the unwrapped error union type, or be of type noreturn.

try

Let's say you wanted to return the error if you got one, otherwise continue with the function logic:

fn doAThing(str: []u8) !void {
    const number = parseU64(str, 10) catch |err| return err;
    // ...
}

There is a shortcut for this. The try expression:

fn doAThing(str: []u8) !void {
    const number = try parseU64(str, 10);
    // ...
}

try evaluates an error union expression. If it is an error, it returns from the current function with the same error. Otherwise, the expression results in the unwrapped value.

Maybe you know with complete certainty that an expression will never be an error. In this case you can do this:

const number = parseU64("1234", 10) catch unreachable;

Here we know for sure that "1234" will parse successfully. So we put the unreachable value on the right hand side. unreachable generates a panic in Debug and ReleaseSafe modes and undefined behavior in ReleaseFast mode. So, while we're debugging the application, if there was a surprise error here, the application would crash appropriately.

Finally, you may want to take a different action for every situation. For that, we combine the if and switch expression:

fn doAThing(str: []u8) void {
    if (parseU64(str, 10)) |number| {
        doSomethingWithNumber(number);
    } else |err| switch (err) {
        error.Overflow => {
            // handle overflow...
        },
        // we promise that InvalidChar won't happen (or crash in debug mode if it does)
        error.InvalidChar => unreachable,
    }
}

errdefer

The other component to error handling is defer statements. In addition to an unconditional defer, Zig has errdefer, which evaluates the deferred expression on block exit path if and only if the function returned with an error from the block.

Example:

fn createFoo(param: i32) !Foo {
    const foo = try tryToAllocateFoo();
    // now we have allocated foo. we need to free it if the function fails.
    // but we want to return it if the function succeeds.
    errdefer deallocateFoo(foo);

    const tmp_buf = allocateTmpBuffer() orelse return error.OutOfMemory;
    // tmp_buf is truly a temporary resource, and we for sure want to clean it up
    // before this block leaves scope
    defer deallocateTmpBuffer(tmp_buf);

    if (param > 1337) return error.InvalidParam;

    // here the errdefer will not run since we're returning success from the function.
    // but the defer will run!
    return foo;
}

The neat thing about this is that you get robust error handling without the verbosity and cognitive overhead of trying to make sure every exit path is covered. The deallocation code is always directly following the allocation code.

A couple of other tidbits about error handling:

These primitives give enough expressiveness that it's completely practical to have failing to check for an error be a compile error. If you really want to ignore the error, you can add catch unreachable and get the added benefit of crashing in Debug and ReleaseSafe modes if your assumption was wrong.

Since Zig understands error types, it can pre-weight branches in favor of errors not occurring. Just a small optimization benefit that is not available in other languages.

See also:

defer

if

switch

An error union is created with the ! binary operator. You can use compile-time reflection to access the child type of an error union:

test.zig

const expect = @import("std").testing.expect;

test "error union" {
    var foo: anyerror!i32 = undefined;

    // Coerce from child type of an error union:
    foo = 1234;

    // Coerce from an error set:
    foo = error.SomeError;

    // Use compile-time reflection to access the payload type of an error union:
    comptime try expect(@typeInfo(@TypeOf(foo)).ErrorUnion.payload == i32);

    // Use compile-time reflection to access the error set type of an error union:
    comptime try expect(@typeInfo(@TypeOf(foo)).ErrorUnion.error_set == anyerror);
}

$ zig test test.zig
Test [1/1] test "error union"...

All 1 tests passed.

Merging Error Sets

Use the || operator to merge two error sets together. The resulting error set contains the errors of both error sets. Doc comments from the left-hand side override doc comments from the right-hand side. In this example, the doc comments for C.PathNotFound is A doc comment.

This is especially useful for functions which return different error sets depending on comptime branches. For example, the Zig standard library uses LinuxFileOpenError || WindowsFileOpenError for the error set of opening files.

test.zig

const A = error{
    NotDir,

    /// A doc comment
    PathNotFound,
};
const B = error{
    OutOfMemory,

    /// B doc comment
    PathNotFound,
};

const C = A || B;

fn foo() C!void {
    return error.NotDir;
}

test "merge error sets" {
    if (foo()) {
        @panic("unexpected");
    } else |err| switch (err) {
        error.OutOfMemory => @panic("unexpected"),
        error.PathNotFound => @panic("unexpected"),
        error.NotDir => {},
    }
}

$ zig test test.zig
Test [1/1] test "merge error sets"...

All 1 tests passed.

Inferred Error Sets

Because many functions in Zig return a possible error, Zig supports inferring the error set. To infer the error set for a function, use this syntax:

test.zig

// With an inferred error set
pub fn add_inferred(comptime T: type, a: T, b: T) !T {
    var answer: T = undefined;
    return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
}

// With an explicit error set
pub fn add_explicit(comptime T: type, a: T, b: T) Error!T {
    var answer: T = undefined;
    return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
}

const Error = error {
    Overflow,
};

const std = @import("std");

test "inferred error set" {
    if (add_inferred(u8, 255, 1)) |_| unreachable else |err| switch (err) {
        error.Overflow => {}, // ok
    }
}

$ zig test test.zig
Test [1/1] test "inferred error set"...

All 1 tests passed.

When a function has an inferred error set, that function becomes generic and thus it becomes trickier to do certain things with it, such as obtain a function pointer, or have an error set that is consistent across different build targets. Additionally, inferred error sets are incompatible with recursion.

In these situations, it is recommended to use an explicit error set. You can generally start with an empty error set and let compile errors guide you toward completing the set.

These limitations may be overcome in a future version of Zig.

Error Return Traces

Error Return Traces show all the points in the code that an error was returned to the calling function. This makes it practical to use try everywhere and then still be able to know what happened if an error ends up bubbling all the way out of your application.

test.zig

pub fn main() !void {
    try foo(12);
}

fn foo(x: i32) !void {
    if (x >= 5) {
        try bar();
    } else {
        try bang2();
    }
}

fn bar() !void {
    if (baz()) {
        try quux();
    } else |err| switch (err) {
        error.FileNotFound => try hello(),
        else => try another(),
    }
}

fn baz() !void {
    try bang1();
}

fn quux() !void {
    try bang2();
}

fn hello() !void {
    try bang2();
}

fn another() !void {
    try bang1();
}

fn bang1() !void {
    return error.FileNotFound;
}

fn bang2() !void {
    return error.PermissionDenied;
}

$ zig build-exe test.zig
$ ./test
error: PermissionDenied
/home/andy/Downloads/zig/docgen_tmp/test.zig:39:5: 0x231632 in bang1 (test)
    return error.FileNotFound;
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:23:5: 0x23150f in baz (test)
    try bang1();
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:43:5: 0x2314d2 in bang2 (test)
    return error.PermissionDenied;
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:31:5: 0x2315ff in hello (test)
    try bang2();
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:17:31: 0x23149e in bar (test)
        error.FileNotFound => try hello(),
                              ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:7:9: 0x23138c in foo (test)
        try bar();
        ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:2:5: 0x22a0c4 in main (test)
    try foo(12);
    ^

Look closely at this example. This is no stack trace.

You can see that the final error bubbled up was PermissionDenied, but the original error that started this whole thing was FileNotFound. In the bar function, the code handles the original error code, and then returns another one, from the switch statement. Error Return Traces make this clear, whereas a stack trace would look like this:

test.zig

pub fn main() void {
    foo(12);
}

fn foo(x: i32) void {
    if (x >= 5) {
        bar();
    } else {
        bang2();
    }
}

fn bar() void {
    if (baz()) {
        quux();
    } else {
        hello();
    }
}

fn baz() bool {
    return bang1();
}

fn quux() void {
    bang2();
}

fn hello() void {
    bang2();
}

fn bang1() bool {
    return false;
}

fn bang2() void {
    @panic("PermissionDenied");
}

$ zig build-exe test.zig
$ ./test
thread 13421 panic: PermissionDenied
/home/andy/Downloads/zig/docgen_tmp/test.zig:38:5: 0x232a66 in bang2 (test)
    @panic("PermissionDenied");
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:30:10: 0x2331a8 in hello (test)
    bang2();
         ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:17:14: 0x232a4a in bar (test)
        hello();
             ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:7:12: 0x231155 in foo (test)
        bar();
           ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:2:8: 0x229efd in main (test)
    foo(12);
       ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x2222dc in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20616e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205196 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fa2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Here, the stack trace does not explain how the control flow in bar got to the hello() call. One would have to open a debugger or further instrument the application in order to find out. The error return trace, on the other hand, shows exactly how the error bubbled up.

This debugging feature makes it easier to iterate quickly on code that robustly handles all error conditions. This means that Zig developers will naturally find themselves writing correct, robust code in order to increase their development pace.

Error Return Traces are enabled by default in Debug and ReleaseSafe builds and disabled by default in ReleaseFast and ReleaseSmall builds.

There are a few ways to activate this error return tracing feature:

Return an error from main

An error makes its way to catch unreachable and you have not overridden the default panic handler

Use errorReturnTrace to access the current return trace. You can use std.debug.dumpStackTrace to print it. This function returns comptime-known null when building without error return tracing support.

Implementation Details

To analyze performance cost, there are two cases:

when no errors are returned

when returning errors

For the case when no errors are returned, the cost is a single memory write operation, only in the first non-failable function in the call graph that calls a failable function, i.e. when a function returning void calls a function returning error. This is to initialize this struct in the stack memory:

pub const StackTrace = struct {
    index: usize,
    instruction_addresses: [N]usize,
};

Here, N is the maximum function call depth as determined by call graph analysis. Recursion is ignored and counts for 2.

A pointer to StackTrace is passed as a secret parameter to every function that can return an error, but it's always the first parameter, so it can likely sit in a register and stay there.

That's it for the path when no errors occur. It's practically free in terms of performance.

When generating the code for a function that returns an error, just before the return statement (only for the return statements that return errors), Zig generates a call to this function:

// marked as "no-inline" in LLVM IR
fn __zig_return_error(stack_trace: *StackTrace) void {
    stack_trace.instruction_addresses[stack_trace.index] = @returnAddress();
    stack_trace.index = (stack_trace.index + 1) % N;
}

The cost is 2 math operations plus some memory reads and writes. The memory accessed is constrained and should remain cached for the duration of the error return bubbling.

As for code size cost, 1 function call before a return statement is no big deal. Even so, I have a plan to make the call to __zig_return_error a tail call, which brings the code size cost down to actually zero. What is a return statement in code without error return tracing can become a jump instruction in code with error return tracing.

Optionals

One area that Zig provides safety without compromising efficiency or readability is with the optional type.

The question mark symbolizes the optional type. You can convert a type to an optional type by putting a question mark in front of it, like this:

// normal integer
const normal_int: i32 = 1234;

// optional integer
const optional_int: ?i32 = 5678;

Now the variable optional_int could be an i32, or null.

Instead of integers, let's talk about pointers. Null references are the source of many runtime exceptions, and even stand accused of being the worst mistake of computer science.

Zig does not have them.

Instead, you can use an optional pointer. This secretly compiles down to a normal pointer, since we know we can use 0 as the null value for the optional type. But the compiler can check your work and make sure you don't assign null to something that can't be null.

Typically the downside of not having null is that it makes the code more verbose to write. But, let's compare some equivalent C code and Zig code.

Task: call malloc, if the result is null, return null.

C code

// malloc prototype included for reference
void *malloc(size_t size);

struct Foo *do_a_thing(void) {
    char *ptr = malloc(1234);
    if (!ptr) return NULL;
    // ...
}

Zig code

// malloc prototype included for reference
extern fn malloc(size: size_t) ?*u8;

fn doAThing() ?*Foo {
    const ptr = malloc(1234) orelse return null;
    // ...
}

Here, Zig is at least as convenient, if not more, than C. And, the type of "ptr" is *u8 not ?*u8. The orelse keyword unwrapped the optional type and therefore ptr is guaranteed to be non-null everywhere it is used in the function.

The other form of checking against NULL you might see looks like this:

void do_a_thing(struct Foo *foo) {
    // do some stuff

    if (foo) {
        do_something_with_foo(foo);
    }

    // do some stuff
}

In Zig you can accomplish the same thing:

fn doAThing(optional_foo: ?*Foo) void {
    // do some stuff

    if (optional_foo) |foo| {
      doSomethingWithFoo(foo);
    }

    // do some stuff
}

Once again, the notable thing here is that inside the if block, foo is no longer an optional pointer, it is a pointer, which cannot be null.

One benefit to this is that functions which take pointers as arguments can be annotated with the "nonnull" attribute - __attribute__((nonnull)) in GCC. The optimizer can sometimes make better decisions knowing that pointer arguments cannot be null.

Optional Type

An optional is created by putting ? in front of a type. You can use compile-time reflection to access the child type of an optional:

test.zig

const expect = @import("std").testing.expect;

test "optional type" {
    // Declare an optional and coerce from null:
    var foo: ?i32 = null;

    // Coerce from child type of an optional
    foo = 1234;

    // Use compile-time reflection to access the child type of the optional:
    comptime try expect(@typeInfo(@TypeOf(foo)).Optional.child == i32);
}

$ zig test test.zig
Test [1/1] test "optional type"...

All 1 tests passed.

null

Just like undefined, null has its own type, and the only way to use it is to cast it to a different type:

const optional_value: ?i32 = null;

Optional Pointers

An optional pointer is guaranteed to be the same size as a pointer. The null of the optional is guaranteed to be address 0.

test.zig

const expect = @import("std").testing.expect;

test "optional pointers" {
    // Pointers cannot be null. If you want a null pointer, use the optional
    // prefix `?` to make the pointer type optional.
    var ptr: ?*i32 = null;

    var x: i32 = 1;
    ptr = &x;

    try expect(ptr.?.* == 1);

    // Optional pointers are the same size as normal pointers, because pointer
    // value 0 is used as the null value.
    try expect(@sizeOf(?*i32) == @sizeOf(*i32));
}

$ zig test test.zig
Test [1/1] test "optional pointers"...

All 1 tests passed.

Casting

A type cast converts a value of one type to another. Zig has Type Coercion for conversions that are known to be completely safe and unambiguous, and Explicit Casts for conversions that one would not want to happen on accident. There is also a third kind of type conversion called Peer Type Resolution for the case when a result type must be decided given multiple operand types.

Type Coercion

Type coercion occurs when one type is expected, but different type is provided:

test.zig

test "type coercion - variable declaration" {
    var a: u8 = 1;
    var b: u16 = a;
}

test "type coercion - function call" {
    var a: u8 = 1;
    foo(a);
}

fn foo(b: u16) void {}

test "type coercion - @as builtin" {
    var a: u8 = 1;
    var b = @as(u16, a);
}

$ zig test test.zig
Test [1/3] test "type coercion - variable declaration"...
Test [2/3] test "type coercion - function call"...
Test [3/3] test "type coercion - @as builtin"...

All 3 tests passed.

Type coercions are only allowed when it is completely unambiguous how to get from one type to another, and the transformation is guaranteed to be safe. There is one exception, which is C Pointers.

Type Coercion: Stricter Qualification

Values which have the same representation at runtime can be cast to increase the strictness of the qualifiers, no matter how nested the qualifiers are:

const - non-const to const is allowed

volatile - non-volatile to volatile is allowed

align - bigger to smaller alignment is allowed

error sets to supersets is allowed

These casts are no-ops at runtime since the value representation does not change.

test.zig

test "type coercion - const qualification" {
    var a: i32 = 1;
    var b: *i32 = &a;
    foo(b);
}

fn foo(a: *const i32) void {}

$ zig test test.zig
Test [1/1] test "type coercion - const qualification"...

All 1 tests passed.

In addition, pointers coerce to const optional pointers:

test.zig

const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;

test "cast *[1][*]const u8 to [*]const ?[*]const u8" {
    const window_name = [1][*]const u8{"window name"};
    const x: [*]const ?[*]const u8 = &window_name;
    try expect(mem.eql(u8, std.mem.spanZ(@ptrCast([*:0]const u8, x[0].?)), "window name"));
}

$ zig test test.zig
Test [1/1] test "cast *[1][*]const u8 to [*]const ?[*]const u8"...

All 1 tests passed.

Type Coercion: Integer and Float Widening

Integers coerce to integer types which can represent every value of the old type, and likewise Floats coerce to float types which can represent every value of the old type.

test.zig

const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;

test "integer widening" {
    var a: u8 = 250;
    var b: u16 = a;
    var c: u32 = b;
    var d: u64 = c;
    var e: u64 = d;
    var f: u128 = e;
    try expect(f == a);
}

test "implicit unsigned integer to signed integer" {
    var a: u8 = 250;
    var b: i16 = a;
    try expect(b == 250);
}

test "float widening" {
    // Note: there is an open issue preventing this from working on aarch64:
    // https://github.com/ziglang/zig/issues/3282
    if (std.Target.current.cpu.arch == .aarch64) return error.SkipZigTest;

    var a: f16 = 12.34;
    var b: f32 = a;
    var c: f64 = b;
    var d: f128 = c;
    try expect(d == a);
}

$ zig test test.zig
Test [1/3] test "integer widening"...
Test [2/3] test "implicit unsigned integer to signed integer"...
Test [3/3] test "float widening"...

All 3 tests passed.

Type Coercion: Coercion Float to Int

A compiler error is appropriate because this ambiguous expression leaves the compiler two choices about the coercion.

Cast 54.0 to comptime_int resulting in @as(comptime_int, 10), which is casted to @as(f32, 10)

Cast 5 to comptime_float resulting in @as(comptime_float, 10.8), which is casted to @as(f32, 10.8)

test.zig

// Compile time coercion of float to int
test "implicit cast to comptime_int" {
    var f: f32 = 54.0 / 5;
}

$ zig test test.zig
./docgen_tmp/test.zig:3:18: error: float value 54.000000 cannot be coerced to type 'comptime_int'
    var f: f32 = 54.0 / 5;
                 ^
./docgen_tmp/test.zig:3:23: note: referenced here
    var f: f32 = 54.0 / 5;
                      ^

Type Coercion: Slices, Arrays and Pointers

coerce__slices_arrays_and_ptrs.zig

const std = @import("std");
const expect = std.testing.expect;

// You can assign constant pointers to arrays to a slice with
// const modifier on the element type. Useful in particular for
// String literals.
test "*const [N]T to []const T" {
    var x1: []const u8 = "hello";
    var x2: []const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
    try expect(std.mem.eql(u8, x1, x2));

    var y: []const f32 = &[2]f32{ 1.2, 3.4 };
    try expect(y[0] == 1.2);
}

// Likewise, it works when the destination type is an error union.
test "*const [N]T to E![]const T" {
    var x1: anyerror![]const u8 = "hello";
    var x2: anyerror![]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
    try expect(std.mem.eql(u8, try x1, try x2));

    var y: anyerror![]const f32 = &[2]f32{ 1.2, 3.4 };
    try expect((try y)[0] == 1.2);
}

// Likewise, it works when the destination type is an optional.
test "*const [N]T to ?[]const T" {
    var x1: ?[]const u8 = "hello";
    var x2: ?[]const u8 = &[5]u8{ 'h', 'e', 'l', 'l', 111 };
    try expect(std.mem.eql(u8, x1.?, x2.?));

    var y: ?[]const f32 = &[2]f32{ 1.2, 3.4 };
    try expect(y.?[0] == 1.2);
}

// In this cast, the array length becomes the slice length.
test "*[N]T to []T" {
    var buf: [5]u8 = "hello".*;
    const x: []u8 = &buf;
    try expect(std.mem.eql(u8, x, "hello"));

    const buf2 = [2]f32{ 1.2, 3.4 };
    const x2: []const f32 = &buf2;
    try expect(std.mem.eql(f32, x2, &[2]f32{ 1.2, 3.4 }));
}

// Single-item pointers to arrays can be coerced to many-item pointers.
test "*[N]T to [*]T" {
    var buf: [5]u8 = "hello".*;
    const x: [*]u8 = &buf;
    try expect(x[4] == 'o');
    // x[5] would be an uncaught out of bounds pointer dereference!
}

// Likewise, it works when the destination type is an optional.
test "*[N]T to ?[*]T" {
    var buf: [5]u8 = "hello".*;
    const x: ?[*]u8 = &buf;
    try expect(x.?[4] == 'o');
}

// Single-item pointers can be cast to len-1 single-item arrays.
test "*T to *[1]T" {
    var x: i32 = 1234;
    const y: *[1]i32 = &x;
    const z: [*]i32 = y;
    try expect(z[0] == 1234);
}

$ zig test coerce__slices_arrays_and_ptrs.zig
Test [1/7] test "*const [N]T to []const T"...
Test [2/7] test "*const [N]T to E![]const T"...
Test [3/7] test "*const [N]T to ?[]const T"...
Test [4/7] test "*[N]T to []T"...
Test [5/7] test "*[N]T to [*]T"...
Test [6/7] test "*[N]T to ?[*]T"...
Test [7/7] test "*T to *[1]T"...

All 7 tests passed.

See also:

C Pointers

Type Coercion: Optionals

The payload type of Optionals, as well as null, coerce to the optional type.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "coerce to optionals" {
    const x: ?i32 = 1234;
    const y: ?i32 = null;

    try expect(x.? == 1234);
    try expect(y == null);
}

$ zig test test.zig
Test [1/1] test "coerce to optionals"...

All 1 tests passed.

It works nested inside the Error Union Type, too:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "coerce to optionals wrapped in error union" {
    const x: anyerror!?i32 = 1234;
    const y: anyerror!?i32 = null;

    try expect((try x).? == 1234);
    try expect((try y) == null);
}

$ zig test test.zig
Test [1/1] test "coerce to optionals wrapped in error union"...

All 1 tests passed.

Type Coercion: Error Unions

The payload type of an Error Union Type as well as the Error Set Type coerce to the error union type:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "coercion to error unions" {
    const x: anyerror!i32 = 1234;
    const y: anyerror!i32 = error.Failure;

    try expect((try x) == 1234);
    try std.testing.expectError(error.Failure, y);
}

$ zig test test.zig
Test [1/1] test "coercion to error unions"...

All 1 tests passed.

Type Coercion: Compile-Time Known Numbers

When a number is comptime-known to be representable in the destination type, it may be coerced:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "coercing large integer type to smaller one when value is comptime known to fit" {
    const x: u64 = 255;
    const y: u8 = x;
    try expect(y == 255);
}

$ zig test test.zig
Test [1/1] test "coercing large integer type to smaller one when value is comptime known ...

All 1 tests passed.

Type Coercion: unions and enums

Tagged unions can be coerced to enums, and enums can be coerced to tagged unions when they are comptime-known to be a field of the union that has only one possible value, such as void:

test.zig

const std = @import("std");
const expect = std.testing.expect;

const E = enum {
    one,
    two,
    three,
};

const U = union(E) {
    one: i32,
    two: f32,
    three,
};

test "coercion between unions and enums" {
    var u = U{ .two = 12.34 };
    var e: E = u;
    try expect(e == E.two);

    const three = E.three;
    var another_u: U = three;
    try expect(another_u == E.three);
}

$ zig test test.zig
Test [1/1] test "coercion between unions and enums"...

All 1 tests passed.

See also:

union

enum

Type Coercion: Zero Bit Types

Zero Bit Types may be coerced to single-item Pointers, regardless of const.

TODO document the reasoning for this

TODO document whether vice versa should work and why

test.zig

test "coercion of zero bit types" {
    var x: void = {};
    var y: *void = x;
    //var z: void = y; // TODO
}

$ zig test test.zig
Test [1/1] test "coercion of zero bit types"...

All 1 tests passed.

Type Coercion: undefined

undefined can be cast to any type.

Explicit Casts

Explicit casts are performed via Builtin Functions. Some explicit casts are safe; some are not. Some explicit casts perform language-level assertions; some do not. Some explicit casts are no-ops at runtime; some are not.

@bitCast - change type but maintain bit representation

@alignCast - make a pointer have more alignment

@boolToInt - convert true to 1 and false to 0

@enumToInt - obtain the integer tag value of an enum or tagged union

@errSetCast - convert to a smaller error set

@errorToInt - obtain the integer value of an error code

@floatCast - convert a larger float to a smaller float

@floatToInt - obtain the integer part of a float value

@intCast - convert between integer types

@intToEnum - obtain an enum value based on its integer tag value

@intToError - obtain an error code based on its integer value

@intToFloat - convert an integer to a float value

@intToPtr - convert an address to a pointer

@ptrCast - convert between pointer types

@ptrToInt - obtain the address of a pointer

@truncate - convert between integer types, chopping off bits

Peer Type Resolution

Peer Type Resolution occurs in these places:

switch expressions

if expressions

while expressions

for expressions

Multiple break statements in a block

Some binary operations

This kind of type resolution chooses a type that all peer types can coerce into. Here are some examples:

peer_type_resolution.zig

const std = @import("std");
const expect = std.testing.expect;
const mem = std.mem;

test "peer resolve int widening" {
    var a: i8 = 12;
    var b: i16 = 34;
    var c = a + b;
    try expect(c == 46);
    try expect(@TypeOf(c) == i16);
}

test "peer resolve arrays of different size to const slice" {
    try expect(mem.eql(u8, boolToStr(true), "true"));
    try expect(mem.eql(u8, boolToStr(false), "false"));
    comptime try expect(mem.eql(u8, boolToStr(true), "true"));
    comptime try expect(mem.eql(u8, boolToStr(false), "false"));
}
fn boolToStr(b: bool) []const u8 {
    return if (b) "true" else "false";
}

test "peer resolve array and const slice" {
    try testPeerResolveArrayConstSlice(true);
    comptime try testPeerResolveArrayConstSlice(true);
}
fn testPeerResolveArrayConstSlice(b: bool) !void {
    const value1 = if (b) "aoeu" else @as([]const u8, "zz");
    const value2 = if (b) @as([]const u8, "zz") else "aoeu";
    try expect(mem.eql(u8, value1, "aoeu"));
    try expect(mem.eql(u8, value2, "zz"));
}

test "peer type resolution: ?T and T" {
    try expect(peerTypeTAndOptionalT(true, false).? == 0);
    try expect(peerTypeTAndOptionalT(false, false).? == 3);
    comptime {
        try expect(peerTypeTAndOptionalT(true, false).? == 0);
        try expect(peerTypeTAndOptionalT(false, false).? == 3);
    }
}
fn peerTypeTAndOptionalT(c: bool, b: bool) ?usize {
    if (c) {
        return if (b) null else @as(usize, 0);
    }

    return @as(usize, 3);
}

test "peer type resolution: *[0]u8 and []const u8" {
    try expect(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
    try expect(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
    comptime {
        try expect(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
        try expect(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
    }
}
fn peerTypeEmptyArrayAndSlice(a: bool, slice: []const u8) []const u8 {
    if (a) {
        return &[_]u8{};
    }

    return slice[0..1];
}
test "peer type resolution: *[0]u8, []const u8, and anyerror![]u8" {
    {
        var data = "hi".*;
        const slice = data[0..];
        try expect((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
        try expect((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
    }
    comptime {
        var data = "hi".*;
        const slice = data[0..];
        try expect((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
        try expect((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
    }
}
fn peerTypeEmptyArrayAndSliceAndError(a: bool, slice: []u8) anyerror![]u8 {
    if (a) {
        return &[_]u8{};
    }

    return slice[0..1];
}

test "peer type resolution: *const T and ?*T" {
    const a = @intToPtr(*const usize, 0x123456780);
    const b = @intToPtr(?*usize, 0x123456780);
    try expect(a == b);
    try expect(b == a);
}

$ zig test peer_type_resolution.zig
Test [1/7] test "peer resolve int widening"...
Test [2/7] test "peer resolve arrays of different size to const slice"...
Test [3/7] test "peer resolve array and const slice"...
Test [4/7] test "peer type resolution: ?T and T"...
Test [5/7] test "peer type resolution: *[0]u8 and []const u8"...
Test [6/7] test "peer type resolution: *[0]u8, []const u8, and anyerror![]u8"...
Test [7/7] test "peer type resolution: *const T and ?*T"...

All 7 tests passed.

Zero Bit Types

For some types, @sizeOf is 0:

void

The Integers u0 and i0.

Arrays and Vectors with len 0, or with an element type that is a zero bit type.

An enum with only 1 tag.

A struct with all fields being zero bit types.

A union with only 1 field which is a zero bit type.

Pointers to Zero Bit Types are themselves zero bit types.

These types can only ever have one possible value, and thus require 0 bits to represent. Code that makes use of these types is not included in the final generated code:

export fn entry() void {
    var x: void = {};
    var y: void = {};
    x = y;
}

When this turns into machine code, there is no code generated in the body of entry, even in Debug mode. For example, on x86_64:

0000000000000010 <entry>:
  10:	55                   	push   %rbp
  11:	48 89 e5             	mov    %rsp,%rbp
  14:	5d                   	pop    %rbp
  15:	c3                   	retq

These assembly instructions do not have any code associated with the void values - they only perform the function call prologue and epilog.

void

void can be useful for instantiating generic types. For example, given a Map(Key, Value), one can pass void for the Value type to make it into a Set:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "turn HashMap into a set with void" {
    var map = std.AutoHashMap(i32, void).init(std.testing.allocator);
    defer map.deinit();

    try map.put(1, {});
    try map.put(2, {});

    try expect(map.contains(2));
    try expect(!map.contains(3));

    _ = map.remove(2);
    try expect(!map.contains(2));
}

$ zig test test.zig
Test [1/1] test "turn HashMap into a set with void"...

All 1 tests passed.

Note that this is different from using a dummy value for the hash map value. By using void as the type of the value, the hash map entry type has no value field, and thus the hash map takes up less space. Further, all the code that deals with storing and loading the value is deleted, as seen above.

void is distinct from c_void. void has a known size of 0 bytes, and c_void has an unknown, but non-zero, size.

Expressions of type void are the only ones whose value can be ignored. For example:

test.zig

test "ignoring expression value" {
    foo();
}

fn foo() i32 {
    return 1234;
}

$ zig test test.zig
./docgen_tmp/test.zig:2:8: error: expression value is ignored
    foo();
       ^
./docgen_tmp/test.zig:1:34: note: referenced here
test "ignoring expression value" {
                                 ^

However, if the expression has type void, there will be no error. Function return values can also be explicitly ignored by assigning them to _.

test.zig

test "void is ignored" {
    returnsVoid();
}

test "explicitly ignoring expression value" {
    _ = foo();
}

fn returnsVoid() void {}

fn foo() i32 {
    return 1234;
}

$ zig test test.zig
Test [1/2] test "void is ignored"...
Test [2/2] test "explicitly ignoring expression value"...

All 2 tests passed.

Pointers to Zero Bit Types

Pointers to zero bit types also have zero bits. They always compare equal to each other:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "pointer to empty struct" {
    const Empty = struct {};
    var a = Empty{};
    var b = Empty{};
    var ptr_a = &a;
    var ptr_b = &b;
    comptime try expect(ptr_a == ptr_b);
}

$ zig test test.zig
Test [1/1] test "pointer to empty struct"...

All 1 tests passed.

The type being pointed to can only ever be one value; therefore loads and stores are never generated. ptrToInt and intToPtr are not allowed:

test.zig

const Empty = struct {};

test "@ptrToInt for pointer to zero bit type" {
    var a = Empty{};
    _ = @ptrToInt(&a);
}

test "@intToPtr for pointer to zero bit type" {
    _ = @intToPtr(*Empty, 0x1);
}

$ zig test test.zig
./docgen_tmp/test.zig:4:5: error: pointer to size 0 type has no address
    var a = Empty{};
    ^
./docgen_tmp/test.zig:5:9: note: referenced here
    _ = @ptrToInt(&a);
        ^
./docgen_tmp/test.zig:9:19: error: type '*Empty' has 0 bits and cannot store information
    _ = @intToPtr(*Empty, 0x1);
                  ^
./docgen_tmp/test.zig:9:9: note: referenced here
    _ = @intToPtr(*Empty, 0x1);
        ^

Result Location Semantics

TODO add documentation for this

usingnamespace

usingnamespace is a declaration that imports all the public declarations of the operand, which must be a struct, union, or enum, into the current scope:

usingnamespace.zig

usingnamespace @import("std");

test "using std namespace" {
    try testing.expect(true);
}

$ zig test usingnamespace.zig
Test [1/1] test "using std namespace"...

All 1 tests passed.

usingnamespace can also be used in containers:

usingnamespace_inside_struct.zig

test "using namespace inside struct" {
    const L = struct {
        usingnamespace struct {
            pub fn f() void {}
        };
    };
    L.f();
}

$ zig test usingnamespace_inside_struct.zig
Test [1/1] test "using namespace inside struct"...

All 1 tests passed.

Instead of the above pattern, it is generally recommended to explicitly alias individual declarations. However, usingnamespace has an important use case when organizing the public API of a file or package. For example, one might have c.zig with all of the C imports:

pub usingnamespace @cImport({
    @cInclude("epoxy/gl.h");
    @cInclude("GLFW/glfw3.h");
    @cDefine("STBI_ONLY_PNG", "");
    @cDefine("STBI_NO_STDIO", "");
    @cInclude("stb_image.h");
});

The above example demonstrates using pub to qualify the usingnamespace additionally makes the imported declarations pub. This can be used to forward declarations, giving precise control over what declarations a given file exposes.

comptime

Zig places importance on the concept of whether an expression is known at compile-time. There are a few different places this concept is used, and these building blocks are used to keep the language small, readable, and powerful.

Introducing the Compile-Time Concept

Compile-Time Parameters

Compile-time parameters is how Zig implements generics. It is compile-time duck typing.

fn max(comptime T: type, a: T, b: T) T {
    return if (a > b) a else b;
}
fn gimmeTheBiggerFloat(a: f32, b: f32) f32 {
    return max(f32, a, b);
}
fn gimmeTheBiggerInteger(a: u64, b: u64) u64 {
    return max(u64, a, b);
}

In Zig, types are first-class citizens. They can be assigned to variables, passed as parameters to functions, and returned from functions. However, they can only be used in expressions which are known at compile-time, which is why the parameter T in the above snippet must be marked with comptime.

A comptime parameter means that:

At the callsite, the value must be known at compile-time, or it is a compile error.

In the function definition, the value is known at compile-time.

For example, if we were to introduce another function to the above snippet:

test.zig

fn max(comptime T: type, a: T, b: T) T {
    return if (a > b) a else b;
}
test "try to pass a runtime type" {
    foo(false);
}
fn foo(condition: bool) void {
    const result = max(
        if (condition) f32 else u64,
        1234,
        5678);
}

$ zig test test.zig
./docgen_tmp/test.zig:9:9: error: values of type 'type' must be comptime known
        if (condition) f32 else u64,
        ^

This is an error because the programmer attempted to pass a value only known at run-time to a function which expects a value known at compile-time.

Another way to get an error is if we pass a type that violates the type checker when the function is analyzed. This is what it means to have compile-time duck typing.

For example:

test.zig

fn max(comptime T: type, a: T, b: T) T {
    return if (a > b) a else b;
}
test "try to compare bools" {
    _ = max(bool, true, false);
}

$ zig test test.zig
./docgen_tmp/test.zig:2:18: error: operator not allowed for type 'bool'
    return if (a > b) a else b;
                 ^
./docgen_tmp/test.zig:5:12: note: called from here
    _ = max(bool, true, false);
           ^
./docgen_tmp/test.zig:4:29: note: called from here
test "try to compare bools" {
                            ^

On the flip side, inside the function definition with the comptime parameter, the value is known at compile-time. This means that we actually could make this work for the bool type if we wanted to:

test.zig

fn max(comptime T: type, a: T, b: T) T {
    if (T == bool) {
        return a or b;
    } else if (a > b) {
        return a;
    } else {
        return b;
    }
}
test "try to compare bools" {
    try @import("std").testing.expect(max(bool, false, true) == true);
}

$ zig test test.zig
Test [1/1] test "try to compare bools"...

All 1 tests passed.

This works because Zig implicitly inlines if expressions when the condition is known at compile-time, and the compiler guarantees that it will skip analysis of the branch not taken.

This means that the actual function generated for max in this situation looks like this:

fn max(a: bool, b: bool) bool {
    return a or b;
}

All the code that dealt with compile-time known values is eliminated and we are left with only the necessary run-time code to accomplish the task.

This works the same way for switch expressions - they are implicitly inlined when the target expression is compile-time known.

Compile-Time Variables

In Zig, the programmer can label variables as comptime. This guarantees to the compiler that every load and store of the variable is performed at compile-time. Any violation of this results in a compile error.

This combined with the fact that we can inline loops allows us to write a function which is partially evaluated at compile-time and partially at run-time.

For example:

comptime_vars.zig

const expect = @import("std").testing.expect;

const CmdFn = struct {
    name: []const u8,
    func: fn(i32) i32,
};

const cmd_fns = [_]CmdFn{
    CmdFn {.name = "one", .func = one},
    CmdFn {.name = "two", .func = two},
    CmdFn {.name = "three", .func = three},
};
fn one(value: i32) i32 { return value + 1; }
fn two(value: i32) i32 { return value + 2; }
fn three(value: i32) i32 { return value + 3; }

fn performFn(comptime prefix_char: u8, start_value: i32) i32 {
    var result: i32 = start_value;
    comptime var i = 0;
    inline while (i < cmd_fns.len) : (i += 1) {
        if (cmd_fns[i].name[0] == prefix_char) {
            result = cmd_fns[i].func(result);
        }
    }
    return result;
}

test "perform fn" {
    try expect(performFn('t', 1) == 6);
    try expect(performFn('o', 0) == 1);
    try expect(performFn('w', 99) == 99);
}

$ zig test comptime_vars.zig
Test [1/1] test "perform fn"...

All 1 tests passed.

This example is a bit contrived, because the compile-time evaluation component is unnecessary; this code would work fine if it was all done at run-time. But it does end up generating different code. In this example, the function performFn is generated three different times, for the different values of prefix_char provided:

// From the line:
// expect(performFn('t', 1) == 6);
fn performFn(start_value: i32) i32 {
    var result: i32 = start_value;
    result = two(result);
    result = three(result);
    return result;
}

// From the line:
// expect(performFn('o', 0) == 1);
fn performFn(start_value: i32) i32 {
    var result: i32 = start_value;
    result = one(result);
    return result;
}

// From the line:
// expect(performFn('w', 99) == 99);
fn performFn(start_value: i32) i32 {
    var result: i32 = start_value;
    return result;
}

Note that this happens even in a debug build; in a release build these generated functions still pass through rigorous LLVM optimizations. The important thing to note, however, is not that this is a way to write more optimized code, but that it is a way to make sure that what should happen at compile-time, does happen at compile-time. This catches more errors and as demonstrated later in this article, allows expressiveness that in other languages requires using macros, generated code, or a preprocessor to accomplish.

Compile-Time Expressions

In Zig, it matters whether a given expression is known at compile-time or run-time. A programmer can use a comptime expression to guarantee that the expression will be evaluated at compile-time. If this cannot be accomplished, the compiler will emit an error. For example:

test.zig

extern fn exit() noreturn;

test "foo" {
    comptime {
        exit();
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:5:9: error: unable to evaluate constant expression
        exit();
        ^
./docgen_tmp/test.zig:5:13: note: referenced here
        exit();
            ^

It doesn't make sense that a program could call exit() (or any other external function) at compile-time, so this is a compile error. However, a comptime expression does much more than sometimes cause a compile error.

Within a comptime expression:

All variables are comptime variables.

All if, while, for, and switch expressions are evaluated at compile-time, or emit a compile error if this is not possible.

All function calls cause the compiler to interpret the function at compile-time, emitting a compile error if the function tries to do something that has global run-time side effects.

This means that a programmer can create a function which is called both at compile-time and run-time, with no modification to the function required.

Let's look at an example:

test.zig

const expect = @import("std").testing.expect;

fn fibonacci(index: u32) u32 {
    if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    // test fibonacci at run-time
    try expect(fibonacci(7) == 13);

    // test fibonacci at compile-time
    comptime {
        try expect(fibonacci(7) == 13);
    }
}

$ zig test test.zig
Test [1/1] test "fibonacci"...

All 1 tests passed.

Imagine if we had forgotten the base case of the recursive function and tried to run the tests:

test.zig

const expect = @import("std").testing.expect;

fn fibonacci(index: u32) u32 {
    //if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        try expect(fibonacci(7) == 13);
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:5:28: error: operation caused overflow
    return fibonacci(index - 1) + fibonacci(index - 2);
                           ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:10:29: note: called from here
        try expect(fibonacci(7) == 13);
                            ^
./docgen_tmp/test.zig:8:18: note: called from here
test "fibonacci" {
                 ^
./docgen_tmp/test.zig:5:21: note: referenced here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:10:29: note: referenced here
        try expect(fibonacci(7) == 13);
                            ^

The compiler produces an error which is a stack trace from trying to evaluate the function at compile-time.

Luckily, we used an unsigned integer, and so when we tried to subtract 1 from 0, it triggered undefined behavior, which is always a compile error if the compiler knows it happened. But what would have happened if we used a signed integer?

test.zig

const expect = @import("std").testing.expect;

fn fibonacci(index: i32) i32 {
    //if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        try expect(fibonacci(7) == 13);
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:5:21: error: evaluation exceeded 1000 backwards branches
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
./docgen_tmp/test.zig:10:29: note: referenced here
        try expect(fibonacci(7) == 13);
                            ^

The compiler noticed that evaluating this function at compile-time took a long time, and thus emitted a compile error and gave up. If the programmer wants to increase the budget for compile-time computation, they can use a built-in function called @setEvalBranchQuota to change the default number 1000 to something else.

What if we fix the base case, but put the wrong value in the expect line?

test.zig

const expect = @import("std").testing.expect;

fn fibonacci(index: i32) i32 {
    if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        try expect(fibonacci(7) == 99999);
    }
}

$ zig test test.zig
Test [1/1] test "fibonacci"...
Test [2/1]
test "fibonacci"... FAIL (TestUnexpectedResult)

0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

What happened is Zig started interpreting the expect function with the parameter ok set to false. When the interpreter hit @panic it emitted a compile error because a panic during compile causes a compile error if it is detected at compile-time.

At container level (outside of any function), all expressions are implicitly comptime expressions. This means that we can use functions to initialize complex static data. For example:

test.zig

const first_25_primes = firstNPrimes(25);
const sum_of_first_25_primes = sum(&first_25_primes);

fn firstNPrimes(comptime n: usize) [n]i32 {
    var prime_list: [n]i32 = undefined;
    var next_index: usize = 0;
    var test_number: i32 = 2;
    while (next_index < prime_list.len) : (test_number += 1) {
        var test_prime_index: usize = 0;
        var is_prime = true;
        while (test_prime_index < next_index) : (test_prime_index += 1) {
            if (test_number % prime_list[test_prime_index] == 0) {
                is_prime = false;
                break;
            }
        }
        if (is_prime) {
            prime_list[next_index] = test_number;
            next_index += 1;
        }
    }
    return prime_list;
}

fn sum(numbers: []const i32) i32 {
    var result: i32 = 0;
    for (numbers) |x| {
        result += x;
    }
    return result;
}

test "variable values" {
    try @import("std").testing.expect(sum_of_first_25_primes == 1060);
}

$ zig test test.zig
Test [1/1] test "variable values"...

All 1 tests passed.

When we compile this program, Zig generates the constants with the answer pre-computed. Here are the lines from the generated LLVM IR:

@0 = internal unnamed_addr constant [25 x i32] [i32 2, i32 3, i32 5, i32 7, i32 11, i32 13, i32 17, i32 19, i32 23, i32 29, i32 31, i32 37, i32 41, i32 43, i32 47, i32 53, i32 59, i32 61, i32 67, i32 71, i32 73, i32 79, i32 83, i32 89, i32 97]
@1 = internal unnamed_addr constant i32 1060

Note that we did not have to do anything special with the syntax of these functions. For example, we could call the sum function as is with a slice of numbers whose length and values were only known at run-time.

Generic Data Structures

Zig uses these capabilities to implement generic data structures without introducing any special-case syntax. If you followed along so far, you may already know how to create a generic data structure.

Here is an example of a generic List data structure, that we will instantiate with the type i32. In Zig we refer to the type as List(i32).

fn List(comptime T: type) type {
    return struct {
        items: []T,
        len: usize,
    };
}

That's it. It's a function that returns an anonymous struct. For the purposes of error messages and debugging, Zig infers the name "List(i32)" from the function name and parameters invoked when creating the anonymous struct.

To keep the language small and uniform, all aggregate types in Zig are anonymous. To give a type a name, we assign it to a constant:

const Node = struct {
    next: *Node,
    name: []u8,
};

This works because all top level declarations are order-independent, and as long as there isn't an actual infinite regression, values can refer to themselves, directly or indirectly. In this case, Node refers to itself as a pointer, which is not actually an infinite regression, so it works fine.

Case Study: printf in Zig

Putting all of this together, let's see how printf works in Zig.

printf.zig

const print = @import("std").debug.print;

const a_number: i32 = 1234;
const a_string = "foobar";

pub fn main() void {
    print("here is a string: '{s}' here is a number: {}\n", .{a_string, a_number});
}

$ zig build-exe printf.zig
$ ./printf
here is a string: 'foobar' here is a number: 1234

Let's crack open the implementation of this and see how it works:

/// Calls print and then flushes the buffer.
pub fn printf(self: *Writer, comptime format: []const u8, args: anytype) anyerror!void {
    const State = enum {
        start,
        open_brace,
        close_brace,
    };

    comptime var start_index: usize = 0;
    comptime var state = State.start;
    comptime var next_arg: usize = 0;

    inline for (format) |c, i| {
        switch (state) {
            State.start => switch (c) {
                '{' => {
                    if (start_index < i) try self.write(format[start_index..i]);
                    state = State.open_brace;
                },
                '}' => {
                    if (start_index < i) try self.write(format[start_index..i]);
                    state = State.close_brace;
                },
                else => {},
            },
            State.open_brace => switch (c) {
                '{' => {
                    state = State.start;
                    start_index = i;
                },
                '}' => {
                    try self.printValue(args[next_arg]);
                    next_arg += 1;
                    state = State.start;
                    start_index = i + 1;
                },
                else => @compileError("Unknown format character: " ++ c),
            },
            State.close_brace => switch (c) {
                '}' => {
                    state = State.start;
                    start_index = i;
                },
                else => @compileError("Single '}' encountered in format string"),
            },
        }
    }
    comptime {
        if (args.len != next_arg) {
            @compileError("Unused arguments");
        }
        if (state != State.Start) {
            @compileError("Incomplete format string: " ++ format);
        }
    }
    if (start_index < format.len) {
        try self.write(format[start_index..format.len]);
    }
    try self.flush();
}

This is a proof of concept implementation; the actual function in the standard library has more formatting capabilities.

Note that this is not hard-coded into the Zig compiler; this is userland code in the standard library.

When this function is analyzed from our example code above, Zig partially evaluates the function and emits a function that actually looks like this:

pub fn printf(self: *Writer, arg0: i32, arg1: []const u8) !void {
    try self.write("here is a string: '");
    try self.printValue(arg0);
    try self.write("' here is a number: ");
    try self.printValue(arg1);
    try self.write("\n");
    try self.flush();
}

printValue is a function that takes a parameter of any type, and does different things depending on the type:

pub fn printValue(self: *Writer, value: anytype) !void {
    switch (@typeInfo(@TypeOf(value))) {
        .Int => {
            return self.printInt(T, value);
        },
        .Float => {
            return self.printFloat(T, value);
        },
        else => {
            @compileError("Unable to print type '" ++ @typeName(T) ++ "'");
        },
    }
}

And now, what happens if we give too many arguments to printf?

test.zig

const print = @import("std").debug.print;

const a_number: i32 = 1234;
const a_string = "foobar";

test "printf too many arguments" {
    print("here is a string: '{s}' here is a number: {}\n", .{
        a_string,
        a_number,
        a_number,
    });
}

$ zig test test.zig
./lib/std/fmt.zig:362:9: error: Unused arguments
        @compileError("Unused arguments");
        ^
./lib/std/io/writer.zig:34:34: note: called from here
            return std.fmt.format(self, format, args);
                                 ^
./lib/std/debug.zig:68:27: note: called from here
    nosuspend stderr.print(fmt, args) catch return;
                          ^
./docgen_tmp/test.zig:7:10: note: called from here
    print("here is a string: '{s}' here is a number: {}\n", .{
         ^
./docgen_tmp/test.zig:6:34: note: called from here
test "printf too many arguments" {
                                 ^
./lib/std/io/writer.zig:34:34: error: expected type 'std.os.WriteError!void', found '@typeInfo(@typeInfo(@TypeOf(std.fmt.format)).Fn.return_type.?).ErrorUnion.error_set!void'
            return std.fmt.format(self, format, args);
                                 ^
./lib/std/debug.zig:68:27: note: called from here
    nosuspend stderr.print(fmt, args) catch return;
                          ^
./docgen_tmp/test.zig:7:10: note: called from here
    print("here is a string: '{s}' here is a number: {}\n", .{
         ^
./docgen_tmp/test.zig:6:34: note: called from here
test "printf too many arguments" {
                                 ^
./lib/std/io/writer.zig:34:34: note: error set '@typeInfo(@typeInfo(@TypeOf(std.fmt.format)).Fn.return_type.?).ErrorUnion.error_set' cannot cast into error set 'std.os.WriteError'
            return std.fmt.format(self, format, args);
                                 ^

Zig gives programmers the tools needed to protect themselves against their own mistakes.

Zig doesn't care whether the format argument is a string literal, only that it is a compile-time known value that can be coerced to a []const u8:

printf.zig

const print = @import("std").debug.print;

const a_number: i32 = 1234;
const a_string = "foobar";
const fmt = "here is a string: '{s}' here is a number: {}\n";

pub fn main() void {
    print(fmt, .{a_string, a_number});
}

$ zig build-exe printf.zig
$ ./printf
here is a string: 'foobar' here is a number: 1234

This works fine.

Zig does not special case string formatting in the compiler and instead exposes enough power to accomplish this task in userland. It does so without introducing another language on top of Zig, such as a macro language or a preprocessor language. It's Zig all the way down.

See also:

inline while

inline for

Assembly

For some use cases, it may be necessary to directly control the machine code generated by Zig programs, rather than relying on Zig's code generation. For these cases, one can use inline assembly. Here is an example of implementing Hello, World on x86_64 Linux using inline assembly:

test.zig

pub fn main() noreturn {
    const msg = "hello world\n";
    _ = syscall3(SYS_write, STDOUT_FILENO, @ptrToInt(msg), msg.len);
    _ = syscall1(SYS_exit, 0);
    unreachable;
}

pub const SYS_write = 1;
pub const SYS_exit = 60;

pub const STDOUT_FILENO = 1;

pub fn syscall1(number: usize, arg1: usize) usize {
    return asm volatile ("syscall"
        : [ret] "={rax}" (-> usize)
        : [number] "{rax}" (number),
          [arg1] "{rdi}" (arg1)
        : "rcx", "r11"
    );
}

pub fn syscall3(number: usize, arg1: usize, arg2: usize, arg3: usize) usize {
    return asm volatile ("syscall"
        : [ret] "={rax}" (-> usize)
        : [number] "{rax}" (number),
          [arg1] "{rdi}" (arg1),
          [arg2] "{rsi}" (arg2),
          [arg3] "{rdx}" (arg3)
        : "rcx", "r11"
    );
}

$ zig build-exe test.zig -target x86_64-linux
$ ./test
hello world

Dissecting the syntax:

// Inline assembly is an expression which returns a value.
// the `asm` keyword begins the expression.
_ = asm
// `volatile` is an optional modifier that tells Zig this
// inline assembly expression has side-effects. Without
// `volatile`, Zig is allowed to delete the inline assembly
// code if the result is unused.
volatile (
// Next is a comptime string which is the assembly code.
// Inside this string one may use `%[ret]`, `%[number]`,
// or `%[arg1]` where a register is expected, to specify
// the register that Zig uses for the argument or return value,
// if the register constraint strings are used. However in
// the below code, this is not used. A literal `%` can be
// obtained by escaping it with a double percent: `%%`.
// Often multiline string syntax comes in handy here.
    \\syscall
// Next is the output. It is possible in the future Zig will
// support multiple outputs, depending on how
// https://github.com/ziglang/zig/issues/215 is resolved.
// It is allowed for there to be no outputs, in which case
// this colon would be directly followed by the colon for the inputs.
    :
// This specifies the name to be used in `%[ret]` syntax in
// the above assembly string. This example does not use it,
// but the syntax is mandatory.
    [ret]
// Next is the output constraint string. This feature is still
// considered unstable in Zig, and so LLVM/GCC documentation
// must be used to understand the semantics.
// http://releases.llvm.org/10.0.0/docs/LangRef.html#inline-asm-constraint-string
// https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
// In this example, the constraint string means "the result value of
// this inline assembly instruction is whatever is in $rax".
    "={rax}"
// Next is either a value binding, or `->` and then a type. The
// type is the result type of the inline assembly expression.
// If it is a value binding, then `%[ret]` syntax would be used
// to refer to the register bound to the value.
    (-> usize)
// Next is the list of inputs.
// The constraint for these inputs means, "when the assembly code is
// executed, $rax shall have the value of `number` and $rdi shall have
// the value of `arg1`". Any number of input parameters is allowed,
// including none.
    : [number] "{rax}" (number),
        [arg1] "{rdi}" (arg1)
// Next is the list of clobbers. These declare a set of registers whose
// values will not be preserved by the execution of this assembly code.
// These do not include output or input registers. The special clobber
// value of "memory" means that the assembly writes to arbitrary undeclared
// memory locations - not only the memory pointed to by a declared indirect
// output. In this example we list $rcx and $r11 because it is known the
// kernel syscall does not preserve these registers.
    : "rcx", "r11"
);

For i386 and x86_64 targets, the syntax is AT&T syntax, rather than the more popular Intel syntax. This is due to technical constraints; assembly parsing is provided by LLVM and its support for Intel syntax is buggy and not well tested.

Some day Zig may have its own assembler. This would allow it to integrate more seamlessly into the language, as well as be compatible with the popular NASM syntax. This documentation section will be updated before 1.0.0 is released, with a conclusive statement about the status of AT&T vs Intel/NASM syntax.

Output Constraints

Output constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics.

Note that some breaking changes to output constraints are planned with issue #215.

Input Constraints

Input constraints are still considered to be unstable in Zig, and so LLVM documentation and GCC documentation must be used to understand the semantics.

Note that some breaking changes to input constraints are planned with issue #215.

Clobbers

Clobbers are the set of registers whose values will not be preserved by the execution of the assembly code. These do not include output or input registers. The special clobber value of "memory" means that the assembly causes writes to arbitrary undeclared memory locations - not only the memory pointed to by a declared indirect output.

Failure to declare the full set of clobbers for a given inline assembly expression is unchecked Undefined Behavior.

Global Assembly

When an assembly expression occurs in a container level comptime block, this is global assembly.

This kind of assembly has different rules than inline assembly. First, volatile is not valid because all global assembly is unconditionally included. Second, there are no inputs, outputs, or clobbers. All global assembly is concatenated verbatim into one long string and assembled together. There are no template substitution rules regarding % as there are in inline assembly expressions.

global-asm.zig

const std = @import("std");
const expect = std.testing.expect;

comptime {
    asm (
        \\.global my_func;
        \\.type my_func, @function;
        \\my_func:
        \\  lea (%rdi,%rsi,1),%eax
        \\  retq
    );
}

extern fn my_func(a: i32, b: i32) i32;

test "global assembly" {
    try expect(my_func(12, 34) == 46);
}

$ zig test global-asm.zig -target x86_64-linux
Test [1/1] test "global assembly"...

All 1 tests passed.

Atomics

TODO: @fence()

TODO: @atomic rmw

TODO: builtin atomic memory ordering enum

Async Functions

When a function is called, a frame is pushed to the stack, the function runs until it reaches a return statement, and then the frame is popped from the stack. At the callsite, the following code does not run until the function returns.

An async function is a function whose callsite is split into an async initiation, followed by an await completion. Its frame is provided explicitly by the caller, and it can be suspended and resumed any number of times.

Zig infers that a function is async when it observes that the function contains a suspension point. Async functions can be called the same as normal functions. A function call of an async function is a suspend point.

Suspend and Resume

At any point, a function may suspend itself. This causes control flow to return to the callsite (in the case of the first suspension), or resumer (in the case of subsequent suspensions).

test.zig

const std = @import("std");
const expect = std.testing.expect;

var x: i32 = 1;

test "suspend with no resume" {
    var frame = async func();
    try expect(x == 2);
}

fn func() void {
    x += 1;
    suspend {}
    // This line is never reached because the suspend has no matching resume.
    x += 1;
}

$ zig test test.zig
Test [1/1] test "suspend with no resume"...

All 1 tests passed.

In the same way that each allocation should have a corresponding free, Each suspend should have a corresponding resume. A suspend block allows a function to put a pointer to its own frame somewhere, for example into an event loop, even if that action will perform a resume operation on a different thread. @frame provides access to the async function frame pointer.

test.zig

const std = @import("std");
const expect = std.testing.expect;

var the_frame: anyframe = undefined;
var result = false;

test "async function suspend with block" {
    _ = async testSuspendBlock();
    try expect(!result);
    resume the_frame;
    try expect(result);
}

fn testSuspendBlock() void {
    suspend {
        comptime try expect(@TypeOf(@frame()) == *@Frame(testSuspendBlock));
        the_frame = @frame();
    }
    result = true;
}

$ zig test test.zig
Test [1/1] test "async function suspend with block"...

All 1 tests passed.

suspend causes a function to be async.

Resuming from Suspend Blocks

Upon entering a suspend block, the async function is already considered suspended, and can be resumed. For example, if you started another kernel thread, and had that thread call resume on the frame pointer provided by the @frame, the new thread would begin executing after the suspend block, while the old thread continued executing the suspend block.

However, the async function can be directly resumed from the suspend block, in which case it never returns to its resumer and continues executing.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "resume from suspend" {
    var my_result: i32 = 1;
    _ = async testResumeFromSuspend(&my_result);
    try std.testing.expect(my_result == 2);
}
fn testResumeFromSuspend(my_result: *i32) void {
    suspend {
        resume @frame();
    }
    my_result.* += 1;
    suspend {}
    my_result.* += 1;
}

$ zig test test.zig
Test [1/1] test "resume from suspend"...

All 1 tests passed.

This is guaranteed to tail call, and therefore will not cause a new stack frame.

Async and Await

In the same way that every suspend has a matching resume, every async has a matching await.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "async and await" {
    // The test block is not async and so cannot have a suspend
    // point in it. By using the nosuspend keyword, we promise that
    // the code in amain will finish executing without suspending
    // back to the test block.
    nosuspend amain();
}

fn amain() void {
    var frame = async func();
    comptime try expect(@TypeOf(frame) == @Frame(func));

    const ptr: anyframe->void = &frame;
    const any_ptr: anyframe = ptr;

    resume any_ptr;
    await ptr;
}

fn func() void {
    suspend {}
}

$ zig test test.zig
Test [1/1] test "async and await"...

All 1 tests passed.

The await keyword is used to coordinate with an async function's return statement.

await is a suspend point, and takes as an operand anything that coerces to anyframe->T.

There is a common misconception that await resumes the target function. It is the other way around: it suspends until the target function completes. In the event that the target function has already completed, await does not suspend; instead it copies the return value directly from the target function's frame.

test.zig

const std = @import("std");
const expect = std.testing.expect;

var the_frame: anyframe = undefined;
var final_result: i32 = 0;

test "async function await" {
    seq('a');
    _ = async amain();
    seq('f');
    resume the_frame;
    seq('i');
    try expect(final_result == 1234);
    try expect(std.mem.eql(u8, &seq_points, "abcdefghi"));
}
fn amain() void {
    seq('b');
    var f = async another();
    seq('e');
    final_result = await f;
    seq('h');
}
fn another() i32 {
    seq('c');
    suspend {
        seq('d');
        the_frame = @frame();
    }
    seq('g');
    return 1234;
}

var seq_points = [_]u8{0} ** "abcdefghi".len;
var seq_index: usize = 0;

fn seq(c: u8) void {
    seq_points[seq_index] = c;
    seq_index += 1;
}

$ zig test test.zig
Test [1/1] test "async function await"...

All 1 tests passed.

In general, suspend is lower level than await. Most application code will use only async and await, but event loop implementations will make use of suspend internally.

Async Function Example

Putting all of this together, here is an example of typical async/await usage:

async.zig

const std = @import("std");
const Allocator = std.mem.Allocator;

pub fn main() void {
    _ = async amainWrap();

    // Typically we would use an event loop to manage resuming async functions,
    // but in this example we hard code what the event loop would do,
    // to make things deterministic.
    resume global_file_frame;
    resume global_download_frame;
}

fn amainWrap() void {
    amain() catch |e| {
        std.debug.print("{}\n", .{e});
        if (@errorReturnTrace()) |trace| {
            std.debug.dumpStackTrace(trace.*);
        }
        std.process.exit(1);
    };
}

fn amain() !void {
    const allocator = std.heap.page_allocator;
    var download_frame = async fetchUrl(allocator, "https://example.com/");
    var awaited_download_frame = false;
    errdefer if (!awaited_download_frame) {
        if (await download_frame) |r| allocator.free(r) else |_| {}
    };

    var file_frame = async readFile(allocator, "something.txt");
    var awaited_file_frame = false;
    errdefer if (!awaited_file_frame) {
        if (await file_frame) |r| allocator.free(r) else |_| {}
    };

    awaited_file_frame = true;
    const file_text = try await file_frame;
    defer allocator.free(file_text);

    awaited_download_frame = true;
    const download_text = try await download_frame;
    defer allocator.free(download_text);

    std.debug.print("download_text: {s}\n", .{download_text});
    std.debug.print("file_text: {s}\n", .{file_text});
}

var global_download_frame: anyframe = undefined;
fn fetchUrl(allocator: *Allocator, url: []const u8) ![]u8 {
    const result = try std.mem.dupe(allocator, u8, "this is the downloaded url contents");
    errdefer allocator.free(result);
    suspend {
        global_download_frame = @frame();
    }
    std.debug.print("fetchUrl returning\n", .{});
    return result;
}

var global_file_frame: anyframe = undefined;
fn readFile(allocator: *Allocator, filename: []const u8) ![]u8 {
    const result = try std.mem.dupe(allocator, u8, "this is the file contents");
    errdefer allocator.free(result);
    suspend {
        global_file_frame = @frame();
    }
    std.debug.print("readFile returning\n", .{});
    return result;
}

$ zig build-exe async.zig
$ ./async
readFile returning
fetchUrl returning
download_text: this is the downloaded url contents
file_text: this is the file contents

Now we remove the suspend and resume code, and observe the same behavior, with one tiny difference:

blocking.zig

const std = @import("std");
const Allocator = std.mem.Allocator;

pub fn main() void {
    _ = async amainWrap();
}

fn amainWrap() void {
    amain() catch |e| {
        std.debug.print("{}\n", .{e});
        if (@errorReturnTrace()) |trace| {
            std.debug.dumpStackTrace(trace.*);
        }
        std.process.exit(1);
    };
}

fn amain() !void {
    const allocator = std.heap.page_allocator;
    var download_frame = async fetchUrl(allocator, "https://example.com/");
    var awaited_download_frame = false;
    errdefer if (!awaited_download_frame) {
        if (await download_frame) |r| allocator.free(r) else |_| {}
    };

    var file_frame = async readFile(allocator, "something.txt");
    var awaited_file_frame = false;
    errdefer if (!awaited_file_frame) {
        if (await file_frame) |r| allocator.free(r) else |_| {}
    };

    awaited_file_frame = true;
    const file_text = try await file_frame;
    defer allocator.free(file_text);

    awaited_download_frame = true;
    const download_text = try await download_frame;
    defer allocator.free(download_text);

    std.debug.print("download_text: {s}\n", .{download_text});
    std.debug.print("file_text: {s}\n", .{file_text});
}

fn fetchUrl(allocator: *Allocator, url: []const u8) ![]u8 {
    const result = try std.mem.dupe(allocator, u8, "this is the downloaded url contents");
    errdefer allocator.free(result);
    std.debug.print("fetchUrl returning\n", .{});
    return result;
}

fn readFile(allocator: *Allocator, filename: []const u8) ![]u8 {
    const result = try std.mem.dupe(allocator, u8, "this is the file contents");
    errdefer allocator.free(result);
    std.debug.print("readFile returning\n", .{});
    return result;
}

$ zig build-exe blocking.zig
$ ./blocking
fetchUrl returning
readFile returning
download_text: this is the downloaded url contents
file_text: this is the file contents

Previously, the fetchUrl and readFile functions suspended, and were resumed in an order determined by the main function. Now, since there are no suspend points, the order of the printed "... returning" messages is determined by the order of async callsites.

Builtin Functions

Builtin functions are provided by the compiler and are prefixed with @. The comptime keyword on a parameter means that the parameter must be known at compile time.

@addWithOverflow

@addWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs result.* = a + b. If overflow or underflow occurs, stores the overflowed bits in result and returns true. If no overflow or underflow occurs, returns false.

@alignCast

@alignCast(comptime alignment: u29, ptr: anytype) anytype

ptr can be *T, fn(), ?*T, ?fn(), or []T. It returns the same type as ptr except with the alignment adjusted to the new value.

A pointer alignment safety check is added to the generated code to make sure the pointer is aligned as promised.

@alignOf

@alignOf(comptime T: type) comptime_int

This function returns the number of bytes that this type should be aligned to for the current target to match the C ABI. When the child type of a pointer has this alignment, the alignment can be omitted from the type.

const expect = @import("std").debug.assert;
comptime {
    assert(*u32 == *align(@alignOf(u32)) u32);
}

The result is a target-specific compile time constant. It is guaranteed to be less than or equal to @sizeOf(T).

See also:

Alignment

@as

@as(comptime T: type, expression) T

Performs Type Coercion. This cast is allowed when the conversion is unambiguous and safe, and is the preferred way to convert between types, whenever possible.

@asyncCall

@asyncCall(frame_buffer: []align(@alignOf(@Frame(anyAsyncFunction))) u8, result_ptr, function_ptr, args: anytype) anyframe->T

@asyncCall performs an async call on a function pointer, which may or may not be an async function.

The provided frame_buffer must be large enough to fit the entire function frame. This size can be determined with @frameSize. To provide a too-small buffer invokes safety-checked Undefined Behavior.

result_ptr is optional (null may be provided). If provided, the function call will write its result directly to the result pointer, which will be available to read after await completes. Any result location provided to await will copy the result from result_ptr.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "async fn pointer in a struct field" {
    var data: i32 = 1;
    const Foo = struct {
        bar: fn (*i32) callconv(.Async) void,
    };
    var foo = Foo{ .bar = func };
    var bytes: [64]u8 align(@alignOf(@Frame(func))) = undefined;
    const f = @asyncCall(&bytes, {}, foo.bar, .{&data});
    try expect(data == 2);
    resume f;
    try expect(data == 4);
}

fn func(y: *i32) void {
    defer y.* += 2;
    y.* += 1;
    suspend {}
}

$ zig test test.zig
Test [1/1] test "async fn pointer in a struct field"...

All 1 tests passed.

@atomicLoad

@atomicLoad(comptime T: type, ptr: *const T, comptime ordering: builtin.AtomicOrder) T

This builtin function atomically dereferences a pointer and returns the value.

T must be a pointer, a bool, a float, an integer or an enum.

@atomicRmw

@atomicRmw(comptime T: type, ptr: *T, comptime op: builtin.AtomicRmwOp, operand: T, comptime ordering: builtin.AtomicOrder) T

This builtin function atomically modifies memory and then returns the previous value.

T must be a pointer, a bool, a float, an integer or an enum.

Supported operations:

.Xchg - stores the operand unmodified. Supports enums, integers and floats.

.Add - for integers, twos complement wraparound addition. Also supports Floats.

.Sub - for integers, twos complement wraparound subtraction. Also supports Floats.

.And - bitwise and

.Nand - bitwise nand

.Or - bitwise or

.Xor - bitwise xor

.Max - stores the operand if it is larger. Supports integers and floats.

.Min - stores the operand if it is smaller. Supports integers and floats.

@atomicStore

@atomicStore(comptime T: type, ptr: *T, value: T, comptime ordering: builtin.AtomicOrder) void

This builtin function atomically stores a value.

T must be a pointer, a bool, a float, an integer or an enum.

@bitCast

@bitCast(comptime DestType: type, value: anytype) DestType

Converts a value of one type to another type.

Asserts that @sizeOf(@TypeOf(value)) == @sizeOf(DestType).

Asserts that @typeInfo(DestType) != .Pointer. Use @ptrCast or @intToPtr if you need this.

Can be used for these things for example:

Convert f32 to u32 bits

Convert i32 to u32 preserving twos complement

Works at compile-time if value is known at compile time. It's a compile error to bitcast a struct to a scalar type of the same size since structs have undefined layout. However if the struct is packed then it works.

@bitOffsetOf

@bitOffsetOf(comptime T: type, comptime field_name: []const u8) comptime_int

Returns the bit offset of a field relative to its containing struct.

For non packed structs, this will always be divisible by 8. For packed structs, non-byte-aligned fields will share a byte offset, but they will have different bit offsets.

See also:

@byteOffsetOf

@boolToInt

@boolToInt(value: bool) u1

Converts true to u1(1) and false to u1(0).

If the value is known at compile-time, the return type is comptime_int instead of u1.

@bitSizeOf

@bitSizeOf(comptime T: type) comptime_int

This function returns the number of bits it takes to store T in memory if the type were a field in a packed struct/union. The result is a target-specific compile time constant.

This function measures the size at runtime. For types that are disallowed at runtime, such as comptime_int and type, the result is 0.

See also:

@sizeOf

@typeInfo

@breakpoint

@breakpoint()

This function inserts a platform-specific debug trap instruction which causes debuggers to break there.

This function is only valid within function scope.

@mulAdd

@mulAdd(comptime T: type, a: T, b: T, c: T) T

Fused multiply add, similar to (a * b) + c, except only rounds once, and is thus more accurate.

Supports Floats and Vectors of floats.

@byteSwap

@byteSwap(comptime T: type, operand: T) T

T must be an integer type with bit count evenly divisible by 8.

operand may be an integer or vector.

Swaps the byte order of the integer. This converts a big endian integer to a little endian integer, and converts a little endian integer to a big endian integer.

Note that for the purposes of memory layout with respect to endianness, the integer type should be related to the number of bytes reported by @sizeOf bytes. This is demonstrated with u24. @sizeOf(u24) == 4, which means that a u24 stored in memory takes 4 bytes, and those 4 bytes are what are swapped on a little vs big endian system. On the other hand, if T is specified to be u24, then only 3 bytes are reversed.

@bitReverse

@bitReverse(comptime T: type, integer: T) T

T accepts any integer type.

Reverses the bitpattern of an integer value, including the sign bit if applicable.

For example 0b10110110 (u8 = 182, i8 = -74) becomes 0b01101101 (u8 = 109, i8 = 109).

@byteOffsetOf

@byteOffsetOf(comptime T: type, comptime field_name: []const u8) comptime_int

Returns the byte offset of a field relative to its containing struct.

See also:

@bitOffsetOf

@call

@call(options: std.builtin.CallOptions, function: anytype, args: anytype) anytype

Calls a function, in the same way that invoking an expression with parentheses does:

call.zig

const expect = @import("std").testing.expect;

test "noinline function call" {
    try expect(@call(.{}, add, .{3, 9}) == 12);
}

fn add(a: i32, b: i32) i32 {
    return a + b;
}

$ zig test call.zig
Test [1/1] test "noinline function call"...

All 1 tests passed.

@call allows more flexibility than normal function call syntax does. The CallOptions struct is reproduced here:

pub const CallOptions = struct {
    modifier: Modifier = .auto,
    stack: ?[]align(std.Target.stack_align) u8 = null,

    pub const Modifier = enum {
        /// Equivalent to function call syntax.
        auto,

        /// Equivalent to async keyword used with function call syntax.
        async_kw,

        /// Prevents tail call optimization. This guarantees that the return
        /// address will point to the callsite, as opposed to the callsite's
        /// callsite. If the call is otherwise required to be tail-called
        /// or inlined, a compile error is emitted instead.
        never_tail,

        /// Guarantees that the call will not be inlined. If the call is
        /// otherwise required to be inlined, a compile error is emitted instead.
        never_inline,

        /// Asserts that the function call will not suspend. This allows a
        /// non-async function to call an async function.
        no_async,

        /// Guarantees that the call will be generated with tail call optimization.
        /// If this is not possible, a compile error is emitted instead.
        always_tail,

        /// Guarantees that the call will inlined at the callsite.
        /// If this is not possible, a compile error is emitted instead.
        always_inline,

        /// Evaluates the call at compile-time. If the call cannot be completed at
        /// compile-time, a compile error is emitted instead.
        compile_time,
    };
};

@cDefine

@cDefine(comptime name: []u8, value)

This function can only occur inside @cImport.

This appends #define $name $value to the @cImport temporary buffer.

To define without a value, like this:

#define _GNU_SOURCE

Use the void value, like this:

@cDefine("_GNU_SOURCE", {})

See also:

Import from C Header File

@cInclude

@cImport

@cUndef

void

@cImport

@cImport(expression) type

This function parses C code and imports the functions, types, variables, and compatible macro definitions into a new empty struct type, and then returns that type.

expression is interpreted at compile time. The builtin functions @cInclude, @cDefine, and @cUndef work within this expression, appending to a temporary buffer which is then parsed as C code.

Usually you should only have one @cImport in your entire application, because it saves the compiler from invoking clang multiple times, and prevents inline functions from being duplicated.

Reasons for having multiple @cImport expressions would be:

To avoid a symbol collision, for example if foo.h and bar.h both #define CONNECTION_COUNT

To analyze the C code with different preprocessor defines

See also:

Import from C Header File

@cInclude

@cDefine

@cUndef

@cInclude

@cInclude(comptime path: []u8)

This function can only occur inside @cImport.

This appends #include <$path>\n to the c_import temporary buffer.

See also:

Import from C Header File

@cImport

@cDefine

@cUndef

@clz

@clz(comptime T: type, integer: T)

This function counts the number of most-significant (leading in a big-Endian sense) zeroes in integer.

If integer is known at comptime, the return type is comptime_int. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

If integer is zero, @clz returns the bit width of integer type T.

See also:

@ctz

@popCount

@cmpxchgStrong

@cmpxchgStrong(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T

This function performs a strong atomic compare exchange operation. It's the equivalent of this code, except atomic:

fn cmpxchgStrongButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
    const old_value = ptr.*;
    if (old_value == expected_value) {
        ptr.* = new_value;
        return null;
    } else {
        return old_value;
    }
}

If you are using cmpxchg in a loop, @cmpxchgWeak is the better choice, because it can be implemented more efficiently in machine instructions.

T must be a pointer, a bool, a float, an integer or an enum.

@typeInfo(@TypeOf(ptr)).Pointer.alignment must be >= @sizeOf(T).

See also:

Compile Variables

cmpxchgWeak

@cmpxchgWeak

@cmpxchgWeak(comptime T: type, ptr: *T, expected_value: T, new_value: T, success_order: AtomicOrder, fail_order: AtomicOrder) ?T

This function performs a weak atomic compare exchange operation. It's the equivalent of this code, except atomic:

fn cmpxchgWeakButNotAtomic(comptime T: type, ptr: *T, expected_value: T, new_value: T) ?T {
    const old_value = ptr.*;
    if (old_value == expected_value and usuallyTrueButSometimesFalse()) {
        ptr.* = new_value;
        return null;
    } else {
        return old_value;
    }
}

If you are using cmpxchg in a loop, the sporadic failure will be no problem, and cmpxchgWeak is the better choice, because it can be implemented more efficiently in machine instructions. However if you need a stronger guarantee, use @cmpxchgStrong.

T must be a pointer, a bool, a float, an integer or an enum.

@typeInfo(@TypeOf(ptr)).Pointer.alignment must be >= @sizeOf(T).

See also:

Compile Variables

cmpxchgStrong

@compileError

@compileError(comptime msg: []u8)

This function, when semantically analyzed, causes a compile error with the message msg.

There are several ways that code avoids being semantically checked, such as using if or switch with compile time constants, and comptime functions.

@compileLog

@compileLog(args: ...)

This function prints the arguments passed to it at compile-time.

To prevent accidentally leaving compile log statements in a codebase, a compilation error is added to the build, pointing to the compile log statement. This error prevents code from being generated, but does not otherwise interfere with analysis.

This function can be used to do "printf debugging" on compile-time executing code.

test.zig

const print = @import("std").debug.print;

const num1 = blk: {
    var val1: i32 = 99;
    @compileLog("comptime val1 = ", val1);
    val1 = val1 + 1;
    break :blk val1;
};

test "main" {
    @compileLog("comptime in main");

    print("Runtime in main, num1 = {}.\n", .{num1});
}

$ zig test test.zig
| *"comptime in main"
| *"comptime val1 = ", 99
./docgen_tmp/test.zig:11:5: error: found compile log statement
    @compileLog("comptime in main");
    ^
./docgen_tmp/test.zig:1:35: note: referenced here
const print = @import("std").debug.print;
                                  ^
./docgen_tmp/test.zig:13:5: note: referenced here
    print("Runtime in main, num1 = {}.\n", .{num1});
    ^
./docgen_tmp/test.zig:5:5: error: found compile log statement
    @compileLog("comptime val1 = ", val1);
    ^
./docgen_tmp/test.zig:13:46: note: referenced here
    print("Runtime in main, num1 = {}.\n", .{num1});
                                             ^

will ouput:

If all @compileLog calls are removed or not encountered by analysis, the program compiles successfully and the generated executable prints:

test.zig

const print = @import("std").debug.print;

const num1 = blk: {
    var val1: i32 = 99;
    val1 = val1 + 1;
    break :blk val1;
};

test "main" {
    print("Runtime in main, num1 = {}.\n", .{num1});
}

$ zig test test.zig
Test [1/1] test "main"...
Runtime in main, num1 = 100.

All 1 tests passed.

@ctz

@ctz(comptime T: type, integer: T)

This function counts the number of least-significant (trailing in a big-Endian sense) zeroes in integer.

If integer is known at comptime, the return type is comptime_int. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

If integer is zero, @ctz returns the bit width of integer type T.

See also:

@clz

@popCount

@cUndef

@cUndef(comptime name: []u8)

This function can only occur inside @cImport.

This appends #undef $name to the @cImport temporary buffer.

See also:

Import from C Header File

@cImport

@cDefine

@cInclude

@divExact

@divExact(numerator: T, denominator: T) T

Exact division. Caller guarantees denominator != 0 and @divTrunc(numerator, denominator) * denominator == numerator.

@divExact(6, 3) == 2

@divExact(a, b) * b == a

For a function that returns a possible error code, use @import("std").math.divExact.

See also:

@divTrunc

@divFloor

@divFloor

@divFloor(numerator: T, denominator: T) T

Floored division. Rounds toward negative infinity. For unsigned integers it is the same as numerator / denominator. Caller guarantees denominator != 0 and !(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1).

@divFloor(-5, 3) == -2

(@divFloor(a, b) * b) + @mod(a, b) == a

For a function that returns a possible error code, use @import("std").math.divFloor.

See also:

@divTrunc

@divExact

@divTrunc

@divTrunc(numerator: T, denominator: T) T

Truncated division. Rounds toward zero. For unsigned integers it is the same as numerator / denominator. Caller guarantees denominator != 0 and !(@typeInfo(T) == .Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1).

@divTrunc(-5, 3) == -1

(@divTrunc(a, b) * b) + @rem(a, b) == a

For a function that returns a possible error code, use @import("std").math.divTrunc.

See also:

@divFloor

@divExact

@embedFile

@embedFile(comptime path: []const u8) *const [X:0]u8

This function returns a compile time constant pointer to null-terminated, fixed-size array with length equal to the byte count of the file given by path. The contents of the array are the contents of the file. This is equivalent to a string literal with the file contents.

path is absolute or relative to the current file, just like @import.

See also:

@import

@enumToInt

@enumToInt(enum_or_tagged_union: anytype) anytype

Converts an enumeration value into its integer tag type. When a tagged union is passed, the tag value is used as the enumeration value.

If there is only one possible enum value, the resut is a comptime_int known at comptime.

See also:

@intToEnum

@errorName

@errorName(err: anyerror) []const u8

This function returns the string representation of an error. The string representation of error.OutOfMem is "OutOfMem".

If there are no calls to @errorName in an entire application, or all calls have a compile-time known value for err, then no error name table will be generated.

@errorReturnTrace

@errorReturnTrace() ?*builtin.StackTrace

If the binary is built with error return tracing, and this function is invoked in a function that calls a function with an error or error union return type, returns a stack trace object. Otherwise returns null.

@errorToInt

@errorToInt(err: anytype) std.meta.Int(.unsigned, @sizeOf(anyerror) * 8)

Supports the following types:

The Global Error Set

Error Set Type

Error Union Type

Converts an error to the integer representation of an error.

It is generally recommended to avoid this cast, as the integer representation of an error is not stable across source code changes.

See also:

@intToError

@errSetCast

@errSetCast(comptime T: DestType, value: anytype) DestType

Converts an error value from one error set to another error set. Attempting to convert an error which is not in the destination error set results in safety-protected Undefined Behavior.

@export

@export(identifier, comptime options: std.builtin.ExportOptions) void

Creates a symbol in the output object file.

This function can be called from a comptime block to conditionally export symbols. When identifier is a function with the C calling convention and options.linkage is Strong, this is equivalent to the export keyword used on a function:

test.zig

comptime {
    @export(internalName, .{ .name = "foo", .linkage = .Strong });
}

fn internalName() callconv(.C) void {}

$ zig build-obj test.zig

This is equivalent to:

test.zig

export fn foo() void {}

$ zig build-obj test.zig

Note that even when using export, @"foo" syntax can be used to choose any string for the symbol name:

test.zig

export fn @"A function name that is a complete sentence."() void {}

$ zig build-obj test.zig

When looking at the resulting object, you can see the symbol is used verbatim:

00000000000001f0 T A function name that is a complete sentence.

See also:

Exporting a C Library

@extern

@extern(T: type, comptime options: std.builtin.ExternOptions) *T

Creates a reference to an external symbol in the output object file.

See also:

@export

@fence

@fence(order: AtomicOrder)

The fence function is used to introduce happens-before edges between operations.

AtomicOrder can be found with @import("std").builtin.AtomicOrder.

See also:

Compile Variables

@field

@field(lhs: anytype, comptime field_name: []const u8) (field)

Performs field access by a compile-time string. Works on both fields and declarations.

test.zig

const std = @import("std");

const Point = struct {
    x: u32,
    y: u32,

    pub var z: u32 = 1;
};

test "field access by string" {
    const expect = std.testing.expect;
    var p = Point{ .x = 0, .y = 0 };

    @field(p, "x") = 4;
    @field(p, "y") = @field(p, "x") + 1;

    try expect(@field(p, "x") == 4);
    try expect(@field(p, "y") == 5);
}

test "decl access by string" {
    const expect = std.testing.expect;

    try expect(@field(Point, "z") == 1);

    @field(Point, "z") = 2;
    try expect(@field(Point, "z") == 2);
}

$ zig test test.zig
Test [1/2] test "field access by string"...
Test [2/2] test "decl access by string"...

All 2 tests passed.

@fieldParentPtr

@fieldParentPtr(comptime ParentType: type, comptime field_name: []const u8,
    field_ptr: *T) *ParentType

Given a pointer to a field, returns the base pointer of a struct.

@floatCast

@floatCast(comptime DestType: type, value: anytype) DestType

Convert from one float type to another. This cast is safe, but may cause the numeric value to lose precision.

@floatToInt

@floatToInt(comptime DestType: type, float: anytype) DestType

Converts the integer part of a floating point number to the destination type.

If the integer part of the floating point number cannot fit in the destination type, it invokes safety-checked Undefined Behavior.

See also:

@intToFloat

@frame

@frame() *@Frame(func)

This function returns a pointer to the frame for a given function. This type can be coerced to anyframe->T and to anyframe, where T is the return type of the function in scope.

This function does not mark a suspension point, but it does cause the function in scope to become an async function.

@Frame

@Frame(func: anytype) type

This function returns the frame type of a function. This works for Async Functions as well as any function without a specific calling convention.

This type is suitable to be used as the return type of async which allows one to, for example, heap-allocate an async function frame:

test.zig

const std = @import("std");

test "heap allocated frame" {
    const frame = try std.heap.page_allocator.create(@Frame(func));
    frame.* = async func();
}

fn func() void {
    suspend {}
}

$ zig test test.zig
Test [1/1] test "heap allocated frame"...

All 1 tests passed.

@frameAddress

@frameAddress() usize

This function returns the base pointer of the current stack frame.

The implications of this are target specific and not consistent across all platforms. The frame address may not be available in release mode due to aggressive optimizations.

This function is only valid within function scope.

@frameSize

@frameSize() usize

This is the same as @sizeOf(@Frame(func)), where func may be runtime-known.

This function is typically used in conjunction with @asyncCall.

@hasDecl

@hasDecl(comptime Container: type, comptime name: []const u8) bool

Returns whether or not a struct, enum, or union has a declaration matching name.

test.zig

const std = @import("std");
const expect = std.testing.expect;

const Foo = struct {
    nope: i32,

    pub var blah = "xxx";
    const hi = 1;
};

test "@hasDecl" {
    try expect(@hasDecl(Foo, "blah"));

    // Even though `hi` is private, @hasDecl returns true because this test is
    // in the same file scope as Foo. It would return false if Foo was declared
    // in a different file.
    try expect(@hasDecl(Foo, "hi"));

    // @hasDecl is for declarations; not fields.
    try expect(!@hasDecl(Foo, "nope"));
    try expect(!@hasDecl(Foo, "nope1234"));
}

$ zig test test.zig
Test [1/1] test "@hasDecl"...

All 1 tests passed.

See also:

@hasField

@hasField

@hasField(comptime Container: type, comptime name: []const u8) bool

Returns whether the field name of a struct, union, or enum exists.

The result is a compile time constant.

It does not include functions, variables, or constants.

See also:

@hasDecl

@import

@import(comptime path: []u8) type

This function finds a zig file corresponding to path and adds it to the build, if it is not already added.

Zig source files are implicitly structs, with a name equal to the file's basename with the extension truncated. @import returns the struct type corresponding to the file.

Declarations which have the pub keyword may be referenced from a different source file than the one they are declared in.

path can be a relative path or it can be the name of a package. If it is a relative path, it is relative to the file that contains the @import function call.

The following packages are always available:

@import("std") - Zig Standard Library

@import("builtin") - Target-specific information The command zig build-exe --show-builtin outputs the source to stdout for reference.

See also:

Compile Variables

@embedFile

@intCast

@intCast(comptime DestType: type, int: anytype) DestType

Converts an integer to another integer while keeping the same numerical value. Attempting to convert a number which is out of range of the destination type results in safety-protected Undefined Behavior.

If T is comptime_int, then this is semantically equivalent to Type Coercion.

@intToEnum

@intToEnum(comptime DestType: type, int_value: std.meta.Tag(DestType)) DestType

Converts an integer into an enum value.

Attempting to convert an integer which represents no value in the chosen enum type invokes safety-checked Undefined Behavior.

See also:

@enumToInt

@intToError

@intToError(value: std.meta.Int(.unsigned, @sizeOf(anyerror) * 8)) anyerror

Converts from the integer representation of an error into The Global Error Set type.

It is generally recommended to avoid this cast, as the integer representation of an error is not stable across source code changes.

Attempting to convert an integer that does not correspond to any error results in safety-protected Undefined Behavior.

See also:

@errorToInt

@intToFloat

@intToFloat(comptime DestType: type, int: anytype) DestType

Converts an integer to the closest floating point representation. To convert the other way, use @floatToInt. This cast is always safe.

@intToPtr

@intToPtr(comptime DestType: type, address: usize) DestType

Converts an integer to a pointer. To convert the other way, use @ptrToInt.

If the destination pointer type does not allow address zero and address is zero, this invokes safety-checked Undefined Behavior.

@memcpy

@memcpy(noalias dest: [*]u8, noalias source: [*]const u8, byte_count: usize)

This function copies bytes from one region of memory to another. dest and source are both pointers and must not overlap.

This function is a low level intrinsic with no safety mechanisms. Most code should not use this function, instead using something like this:

for (source[0..byte_count]) |b, i| dest[i] = b;

The optimizer is intelligent enough to turn the above snippet into a memcpy.

There is also a standard library function for this:

const mem = @import("std").mem;
mem.copy(u8, dest[0..byte_count], source[0..byte_count]);

@memset

@memset(dest: [*]u8, c: u8, byte_count: usize)

This function sets a region of memory to c. dest is a pointer.

This function is a low level intrinsic with no safety mechanisms. Most code should not use this function, instead using something like this:

for (dest[0..byte_count]) |*b| b.* = c;

The optimizer is intelligent enough to turn the above snippet into a memset.

There is also a standard library function for this:

const mem = @import("std").mem;
mem.set(u8, dest, c);

@wasmMemorySize

@wasmMemorySize(index: u32) u32

This function returns the size of the Wasm memory identified by index as an unsigned value in units of Wasm pages. Note that each Wasm page is 64KB in size.

This function is a low level intrinsic with no safety mechanisms usually useful for allocator designers targeting Wasm. So unless you are writing a new allocator from scratch, you should use something like @import("std").heap.WasmPageAllocator.

See also:

@wasmMemoryGrow

@wasmMemoryGrow

@wasmMemoryGrow(index: u32, delta: u32) i32

This function increases the size of the Wasm memory identified by index by delta in units of unsigned number of Wasm pages. Note that each Wasm page is 64KB in size. On success, returns previous memory size; on failure, if the allocation fails, returns -1.

This function is a low level intrinsic with no safety mechanisms usually useful for allocator designers targeting Wasm. So unless you are writing a new allocator from scratch, you should use something like @import("std").heap.WasmPageAllocator.

test.zig

const std = @import("std");
const native_arch = @import("builtin").target.cpu.arch;
const expect = std.testing.expect;

test "@wasmMemoryGrow" {
    if (native_arch != .wasm32) return error.SkipZigTest;

    var prev = @wasmMemorySize(0);
    try expect(prev == @wasmMemoryGrow(0, 1));
    try expect(prev + 1 == @wasmMemorySize(0));
}

$ zig test test.zig
Test [1/1] test "@wasmMemoryGrow"...
Test [2/1]
test "@wasmMemoryGrow"... SKIP

0 passed; 1 skipped; 0 failed.

See also:

@wasmMemorySize

@mod

@mod(numerator: T, denominator: T) T

Modulus division. For unsigned integers this is the same as numerator % denominator. Caller guarantees denominator > 0.

@mod(-5, 3) == 1

(@divFloor(a, b) * b) + @mod(a, b) == a

For a function that returns an error code, see @import("std").math.mod.

See also:

@rem

@mulWithOverflow

@mulWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs result.* = a * b. If overflow or underflow occurs, stores the overflowed bits in result and returns true. If no overflow or underflow occurs, returns false.

@panic

@panic(message: []const u8) noreturn

Invokes the panic handler function. By default the panic handler function calls the public panic function exposed in the root source file, or if there is not one specified, the std.builtin.default_panic function from std/builtin.zig.

Generally it is better to use @import("std").debug.panic. However, @panic can be useful for 2 scenarios:

From library code, calling the programmer's panic function if they exposed one in the root source file.

When mixing C and Zig code, calling the canonical panic implementation across multiple .o files.

See also:

Root Source File

@popCount

@popCount(comptime T: type, integer: T)

Counts the number of bits set in an integer.

If integer is known at comptime, the return type is comptime_int. Otherwise, the return type is an unsigned integer with the minimum number of bits that can represent the bit count of the integer type.

See also:

@ctz

@clz

@ptrCast

@ptrCast(comptime DestType: type, value: anytype) DestType

Converts a pointer of one type to a pointer of another type.

Optional Pointers are allowed. Casting an optional pointer which is null to a non-optional pointer invokes safety-checked Undefined Behavior.

@ptrToInt

@ptrToInt(value: anytype) usize

Converts value to a usize which is the address of the pointer. value can be one of these types:

*T

?*T

fn()

?fn()

To convert the other way, use @intToPtr

@rem

@rem(numerator: T, denominator: T) T

Remainder division. For unsigned integers this is the same as numerator % denominator. Caller guarantees denominator > 0.

@rem(-5, 3) == -2

(@divTrunc(a, b) * b) + @rem(a, b) == a

For a function that returns an error code, see @import("std").math.rem.

See also:

@mod

@returnAddress

@returnAddress() usize

This function returns the address of the next machine code instruction that will be executed when the current function returns.

The implications of this are target specific and not consistent across all platforms.

This function is only valid within function scope. If the function gets inlined into a calling function, the returned address will apply to the calling function.

@setAlignStack

@setAlignStack(comptime alignment: u29)

Ensures that a function will have a stack alignment of at least alignment bytes.

@setCold

@setCold(is_cold: bool)

Tells the optimizer that a function is rarely called.

@setEvalBranchQuota

@setEvalBranchQuota(new_quota: u32)

Changes the maximum number of backwards branches that compile-time code execution can use before giving up and making a compile error.

If the new_quota is smaller than the default quota (1000) or a previously explicitly set quota, it is ignored.

Example:

test.zig

test "foo" {
    comptime {
        var i = 0;
        while (i < 1001) : (i += 1) {}
    }
}

$ zig test test.zig
./docgen_tmp/test.zig:4:9: error: evaluation exceeded 1000 backwards branches
        while (i < 1001) : (i += 1) {}
        ^
./docgen_tmp/test.zig:1:12: note: referenced here
test "foo" {
           ^

Now we use @setEvalBranchQuota:

test.zig

test "foo" {
    comptime {
        @setEvalBranchQuota(1001);
        var i = 0;
        while (i < 1001) : (i += 1) {}
    }
}

$ zig test test.zig
Test [1/1] test "foo"...

All 1 tests passed.

See also:

comptime

@setFloatMode

@setFloatMode(mode: @import("std").builtin.FloatMode)

Sets the floating point mode of the current scope. Possible values are:

pub const FloatMode = enum {
    Strict,
    Optimized,
};

Strict (default) - Floating point operations follow strict IEEE compliance.

Optimized - Floating point operations may do all of the following:

Assume the arguments and result are not NaN. Optimizations are required to retain defined behavior over NaNs, but the value of the result is undefined.

Assume the arguments and result are not +/-Inf. Optimizations are required to retain defined behavior over +/-Inf, but the value of the result is undefined.

Treat the sign of a zero argument or result as insignificant.

Use the reciprocal of an argument rather than perform division.

Perform floating-point contraction (e.g. fusing a multiply followed by an addition into a fused multiply-and-add).

Perform algebraically equivalent transformations that may change results in floating point (e.g. reassociate).

This is equivalent to -ffast-math in GCC.

The floating point mode is inherited by child scopes, and can be overridden in any scope. You can set the floating point mode in a struct or module scope by using a comptime block.

See also:

Floating Point Operations

@setRuntimeSafety

@setRuntimeSafety(safety_on: bool) void

Sets whether runtime safety checks are enabled for the scope that contains the function call.

test.zig

test "@setRuntimeSafety" {
    // The builtin applies to the scope that it is called in. So here, integer overflow
    // will not be caught in ReleaseFast and ReleaseSmall modes:
    // var x: u8 = 255;
    // x += 1; // undefined behavior in ReleaseFast/ReleaseSmall modes.
    {
        // However this block has safety enabled, so safety checks happen here,
        // even in ReleaseFast and ReleaseSmall modes.
        @setRuntimeSafety(true);
        var x: u8 = 255;
        x += 1;

        {
            // The value can be overridden at any scope. So here integer overflow
            // would not be caught in any build mode.
            @setRuntimeSafety(false);
            // var x: u8 = 255;
            // x += 1; // undefined behavior in all build modes.
        }
    }
}

$ zig test test.zig -OReleaseFast
Test [1/1] test "@setRuntimeSafety"...
thread 16085 panic: integer overflow
error: the following test command crashed:
docgen_tmp/zig-cache/o/5f772ba34b67337c067e5630990ef24e/test /home/andy/Downloads/zig/build-release/zig

Note: it is planned to replace @setRuntimeSafety with @optimizeFor

@shlExact

@shlExact(value: T, shift_amt: Log2T) T

Performs the left shift operation (<<). Caller guarantees that the shift will not shift any 1 bits out.

The type of shift_amt is an unsigned integer with log2(T.bit_count) bits. This is because shift_amt >= T.bit_count is undefined behavior.

See also:

@shrExact

@shlWithOverflow

@shlWithOverflow

@shlWithOverflow(comptime T: type, a: T, shift_amt: Log2T, result: *T) bool

Performs result.* = a << b. If overflow or underflow occurs, stores the overflowed bits in result and returns true. If no overflow or underflow occurs, returns false.

The type of shift_amt is an unsigned integer with log2(T.bit_count) bits. This is because shift_amt >= T.bit_count is undefined behavior.

See also:

@shlExact

@shrExact

@shrExact

@shrExact(value: T, shift_amt: Log2T) T

Performs the right shift operation (>>). Caller guarantees that the shift will not shift any 1 bits out.

The type of shift_amt is an unsigned integer with log2(T.bit_count) bits. This is because shift_amt >= T.bit_count is undefined behavior.

See also:

@shlExact

@shlWithOverflow

@shuffle

@shuffle(comptime E: type, a: std.meta.Vector(a_len, E), b: std.meta.Vector(b_len, E), comptime mask: std.meta.Vector(mask_len, i32)) std.meta.Vector(mask_len, E)

Constructs a new vector by selecting elements from a and b based on mask.

Each element in mask selects an element from either a or b. Positive numbers select from a starting at 0. Negative values select from b, starting at -1 and going down. It is recommended to use the ~ operator from indexes from b so that both indexes can start from 0 (i.e. ~@as(i32, 0) is -1).

For each element of mask, if it or the selected value from a or b is undefined, then the resulting element is undefined.

a_len and b_len may differ in length. Out-of-bounds element indexes in mask result in compile errors.

If a or b is undefined, it is equivalent to a vector of all undefined with the same length as the other vector. If both vectors are undefined, @shuffle returns a vector with all elements undefined.

E must be an integer, float, pointer, or bool. The mask may be any vector length, and its length determines the result length.

See also:

SIMD

@sizeOf

@sizeOf(comptime T: type) comptime_int

This function returns the number of bytes it takes to store T in memory. The result is a target-specific compile time constant.

This size may contain padding bytes. If there were two consecutive T in memory, this would be the offset in bytes between element at index 0 and the element at index 1. For integer, consider whether you want to use @sizeOf(T) or @typeInfo(T).Int.bits.

This function measures the size at runtime. For types that are disallowed at runtime, such as comptime_int and type, the result is 0.

See also:

@bitSizeOf

@typeInfo

@splat

@splat(comptime len: u32, scalar: anytype) std.meta.Vector(len, @TypeOf(scalar))

Produces a vector of length len where each element is the value scalar:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "vector @splat" {
    const scalar: u32 = 5;
    const result = @splat(4, scalar);
    comptime try expect(@TypeOf(result) == std.meta.Vector(4, u32));
    try expect(std.mem.eql(u32, &@as([4]u32, result), &[_]u32{ 5, 5, 5, 5 }));
}

$ zig test test.zig
Test [1/1] test "vector @splat"...

All 1 tests passed.

scalar must be an integer, bool, float, or pointer.

See also:

Vectors

@shuffle

@reduce

@reduce(comptime op: std.builtin.ReduceOp, value: anytype) std.meta.Child(value)

Transforms a vector into a scalar value by performing a sequential horizontal reduction of its elements using the specified operator op.

Not every operator is available for every vector element type:

.And, .Or, .Xor are available for bool vectors,

.Min, .Max, .Add, .Mul are available for floating point vectors,

Every operator is available for integer vectors.

Note that .Add and .Mul reductions on integral types are wrapping; when applied on floating point types the operation associativity is preserved, unless the float mode is set to Optimized.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "vector @reduce" {
    const value: std.meta.Vector(4, i32) = [_]i32{ 1, -1, 1, -1 };
    const result = value > @splat(4, @as(i32, 0));
    // result is { true, false, true, false };
    comptime try expect(@TypeOf(result) == std.meta.Vector(4, bool));
    const is_all_true = @reduce(.And, result);
    comptime try expect(@TypeOf(is_all_true) == bool);
    try expect(is_all_true == false);
}

$ zig test test.zig
Test [1/1] test "vector @reduce"...

All 1 tests passed.

See also:

Vectors

@setFloatMode

@src

@src() std.builtin.SourceLocation

Returns a SourceLocation struct representing the function's name and location in the source code. This must be called in a function.

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "@src" {
    try doTheTest();
}

fn doTheTest() !void {
    const src = @src();

    try expect(src.line == 9);
    try expect(src.column == 17);
    try expect(std.mem.endsWith(u8, src.fn_name, "doTheTest"));
    try expect(std.mem.endsWith(u8, src.file, "test.zig"));
}

$ zig test test.zig
Test [1/1] test "@src"...

All 1 tests passed.

@sqrt

@sqrt(value: anytype) @TypeOf(value)

Performs the square root of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@sin

@sin(value: anytype) @TypeOf(value)

Sine trigometric function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@cos

@cos(value: anytype) @TypeOf(value)

Cosine trigometric function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@exp

@exp(value: anytype) @TypeOf(value)

Base-e exponential function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@exp2

@exp2(value: anytype) @TypeOf(value)

Base-2 exponential function on a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@log

@log(value: anytype) @TypeOf(value)

Returns the natural logarithm of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@log2

@log2(value: anytype) @TypeOf(value)

Returns the logarithm to the base 2 of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@log10

@log10(value: anytype) @TypeOf(value)

Returns the logarithm to the base 10 of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@fabs

@fabs(value: anytype) @TypeOf(value)

Returns the absolute value of a floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@floor

@floor(value: anytype) @TypeOf(value)

Returns the largest integral value not greater than the given floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@ceil

@ceil(value: anytype) @TypeOf(value)

Returns the largest integral value not less than the given floating point number. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@trunc

@trunc(value: anytype) @TypeOf(value)

Rounds the given floating point number to an integer, towards zero. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@round

@round(value: anytype) @TypeOf(value)

Rounds the given floating point number to an integer, away from zero. Uses a dedicated hardware instruction when available.

Supports Floats and Vectors of floats, with the caveat that some float operations are not yet implemented for all float types.

@subWithOverflow

@subWithOverflow(comptime T: type, a: T, b: T, result: *T) bool

Performs result.* = a - b. If overflow or underflow occurs, stores the overflowed bits in result and returns true. If no overflow or underflow occurs, returns false.

@tagName

@tagName(value: anytype) []const u8

Converts an enum value or union value to a slice of bytes representing the name.

If the enum is non-exhaustive and the tag value does not map to a name, it invokes safety-checked Undefined Behavior.

@This

@This() type

Returns the innermost struct, enum, or union that this function call is inside. This can be useful for an anonymous struct that needs to refer to itself:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "@This()" {
    var items = [_]i32{ 1, 2, 3, 4 };
    const list = List(i32){ .items = items[0..] };
    try expect(list.length() == 4);
}

fn List(comptime T: type) type {
    return struct {
        const Self = @This();

        items: []T,

        fn length(self: Self) usize {
            return self.items.len;
        }
    };
}

$ zig test test.zig
Test [1/1] test "@This()"...

All 1 tests passed.

When @This() is used at file scope, it returns a reference to the struct that corresponds to the current file.

@truncate

@truncate(comptime T: type, integer: anytype) T

This function truncates bits from an integer type, resulting in a smaller or same-sized integer type.

The following produces safety-checked Undefined Behavior:

test.zig

test "integer cast panic" {
    var a: u16 = 0xabcd;
    var b: u8 = @intCast(u8, a);
}

$ zig test test.zig
Test [1/1] test "integer cast panic"...
thread 16275 panic: integer cast truncated bits
/home/andy/Downloads/zig/docgen_tmp/test.zig:3:17: 0x206715 in test "integer cast panic" (test)
    var b: u8 = @intCast(u8, a);
                ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22cffa in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x22555a in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2093de in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x208406 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x208212 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

However this is well defined and working code:

truncate.zig

const std = @import("std");
const expect = std.testing.expect;

test "integer truncation" {
    var a: u16 = 0xabcd;
    var b: u8 = @truncate(u8, a);
    try expect(b == 0xcd);
}

$ zig test truncate.zig
Test [1/1] test "integer truncation"...

All 1 tests passed.

This function always truncates the significant bits of the integer, regardless of endianness on the target platform.

@Type

@Type(comptime info: std.builtin.TypeInfo) type

This function is the inverse of @typeInfo. It reifies type information into a type.

It is available for the following types:

type

noreturn

void

bool

Integers - The maximum bit count for an integer type is 65535.

Floats

Pointers

comptime_int

comptime_float

@TypeOf(undefined)

@TypeOf(null)

Arrays

Optionals

Error Set Type

Error Union Type

Vectors

opaque

@Frame

anyframe

struct

enum

Enum Literals

union

For these types, @Type is not available:

Functions

BoundFn

@typeInfo

@typeInfo(comptime T: type) std.builtin.TypeInfo

Provides type reflection.

For structs, unions, enums, and error sets, the fields are guaranteed to be in the same order as declared. For declarations, the order is unspecified.

@typeName

@typeName(T: type) [N]u8

This function returns the string representation of a type, as an array. It is equivalent to a string literal of the type name.

@TypeOf

@TypeOf(...) type

@TypeOf is a special builtin function that takes any (nonzero) number of expressions as parameters and returns the type of the result, using Peer Type Resolution.

The expressions are evaluated, however they are guaranteed to have no runtime side-effects:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "no runtime side effects" {
    var data: i32 = 0;
    const T = @TypeOf(foo(i32, &data));
    comptime try expect(T == i32);
    try expect(data == 0);
}

fn foo(comptime T: type, ptr: *T) T {
    ptr.* += 1;
    return ptr.*;
}

$ zig test test.zig
Test [1/1] test "no runtime side effects"...

All 1 tests passed.

@unionInit

@unionInit(comptime Union: type, comptime active_field_name: []const u8, init_expr) Union

This is the same thing as union initialization syntax, except that the field name is a comptime-known value rather than an identifier token.

@unionInit forwards its result location to init_expr.

Build Mode

Zig has four build modes:

Debug (default)

ReleaseFast

ReleaseSafe

ReleaseSmall

To add standard build options to a build.zig file:

const Builder = @import("std").build.Builder;

pub fn build(b: *Builder) void {
    const exe = b.addExecutable("example", "example.zig");
    exe.setBuildMode(b.standardReleaseOptions());
    b.default_step.dependOn(&exe.step);
}

This causes these options to be available:

  -Drelease-safe=[bool] optimizations on and safety on
  -Drelease-fast=[bool] optimizations on and safety off
  -Drelease-small=[bool] size optimizations on and safety off

Debug

$ zig build-exe example.zig

Fast compilation speed

Safety checks enabled

Slow runtime performance

Large binary size

No reproducible build requirement

ReleaseFast

$ zig build-exe example.zig -O ReleaseFast

Fast runtime performance

Safety checks disabled

Slow compilation speed

Large binary size

Reproducible build

ReleaseSafe

$ zig build-exe example.zig -O ReleaseSafe

Medium runtime performance

Safety checks enabled

Slow compilation speed

Large binary size

Reproducible build

ReleaseSmall

$ zig build-exe example.zig -O ReleaseSmall

Medium runtime performance

Safety checks disabled

Slow compilation speed

Small binary size

Reproducible build

See also:

Compile Variables

Zig Build System

Undefined Behavior

Single Threaded Builds

Zig has a compile option --single-threaded which has the following effects:

All Thread Local Variables are treated as regular Container Level Variables.

The overhead of Async Functions becomes equivalent to function call overhead.

The @import("builtin").single_threaded becomes true and therefore various userland APIs which read this variable become more efficient. For example std.Mutex becomes an empty data structure and all of its functions become no-ops.

Undefined Behavior

Zig has many instances of undefined behavior. If undefined behavior is detected at compile-time, Zig emits a compile error and refuses to continue. Most undefined behavior that cannot be detected at compile-time can be detected at runtime. In these cases, Zig has safety checks. Safety checks can be disabled on a per-block basis with @setRuntimeSafety. The ReleaseFast and ReleaseSmall build modes disable all safety checks (except where overridden by @setRuntimeSafety) in order to facilitate optimizations.

When a safety check fails, Zig crashes with a stack trace, like this:

test.zig

test "safety check" {
    unreachable;
}

$ zig test test.zig
Test [1/1] test "safety check"...
thread 16425 panic: reached unreachable code
/home/andy/Downloads/zig/docgen_tmp/test.zig:2:5: 0x2066ea in test "safety check" (test)
    unreachable;
    ^
/home/andy/Downloads/zig/lib/std/special/test_runner.zig:76:28: 0x22cfca in std.special.main (test)
        } else test_fn.func();
                           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x22552a in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2093ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2083d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x2081e2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
docgen_tmp/zig-cache/o/333a8741816702dafd0ff99af380ebb1/test /home/andy/Downloads/zig/build-release/zig

Reaching Unreachable Code

At compile-time:

test.zig

comptime {
    assert(false);
}
fn assert(ok: bool) void {
    if (!ok) unreachable; // assertion failure
}

$ zig test test.zig
./docgen_tmp/test.zig:5:14: error: reached unreachable code
    if (!ok) unreachable; // assertion failure
             ^
./docgen_tmp/test.zig:2:11: note: called from here
    assert(false);
          ^
./docgen_tmp/test.zig:1:10: note: called from here
comptime {
         ^
./docgen_tmp/test.zig:2:11: note: referenced here
    assert(false);
          ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    std.debug.assert(false);
}

$ zig build-exe test.zig
$ ./test
thread 16491 panic: reached unreachable code
/home/andy/Downloads/zig/lib/std/debug.zig:226:14: 0x2042cb in std.debug.assert (test)
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:4:21: 0x229eea in main (test)
    std.debug.assert(false);
                    ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x2222cc in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20615e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205186 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204f92 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Index out of Bounds

At compile-time:

test.zig

comptime {
    const array: [5]u8 = "hello".*;
    const garbage = array[5];
}

$ zig test test.zig
./docgen_tmp/test.zig:3:26: error: index 5 outside array of size 5
    const garbage = array[5];
                         ^

At runtime:

test.zig

pub fn main() void {
    var x = foo("hello");
}

fn foo(x: []const u8) u8 {
    return x[5];
}

$ zig build-exe test.zig
$ ./test
thread 16553 panic: index out of bounds
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:13: 0x231179 in foo (test)
    return x[5];
            ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:2:16: 0x229f06 in main (test)
    var x = foo("hello");
               ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x2222dc in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20616e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205196 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fa2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Cast Negative Number to Unsigned Integer

At compile-time:

test.zig

comptime {
    const value: i32 = -1;
    const unsigned = @intCast(u32, value);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:22: error: attempt to cast negative value to unsigned integer
    const unsigned = @intCast(u32, value);
                     ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var value: i32 = -1;
    var unsigned = @intCast(u32, value);
    std.debug.print("value: {}\n", .{unsigned});
}

$ zig build-exe test.zig
$ ./test
thread 16614 panic: attempt to cast negative value to unsigned integer
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:20: 0x229f77 in main (test)
    var unsigned = @intCast(u32, value);
                   ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

To obtain the maximum value of an unsigned integer, use std.math.maxInt.

Cast Truncates Data

At compile-time:

test.zig

comptime {
    const spartan_count: u16 = 300;
    const byte = @intCast(u8, spartan_count);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:18: error: cast from 'u16' to 'u8' truncates bits
    const byte = @intCast(u8, spartan_count);
                 ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var spartan_count: u16 = 300;
    const byte = @intCast(u8, spartan_count);
    std.debug.print("value: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
thread 16677 panic: integer cast truncated bits
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:18: 0x229f7c in main (test)
    const byte = @intCast(u8, spartan_count);
                 ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

To truncate bits, use @truncate.

Integer Overflow

Default Operations

The following operators can cause integer overflow:

+ (addition)

- (subtraction)

- (negation)

* (multiplication)

/ (division)

@divTrunc (division)

@divFloor (division)

@divExact (division)

Example with addition at compile-time:

test.zig

comptime {
    var byte: u8 = 255;
    byte += 1;
}

$ zig test test.zig
./docgen_tmp/test.zig:3:10: error: operation caused overflow
    byte += 1;
         ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var byte: u8 = 255;
    byte += 1;
    std.debug.print("value: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
thread 16739 panic: integer overflow
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:10: 0x229f5f in main (test)
    byte += 1;
         ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Standard Library Math Functions

These functions provided by the standard library return possible errors.

@import("std").math.add

@import("std").math.sub

@import("std").math.mul

@import("std").math.divTrunc

@import("std").math.divFloor

@import("std").math.divExact

@import("std").math.shl

Example of catching an overflow for addition:

test.zig

const math = @import("std").math;
const print = @import("std").debug.print;
pub fn main() !void {
    var byte: u8 = 255;

    byte = if (math.add(u8, byte, 1)) |result| result else |err| {
        print("unable to add one: {s}\n", .{@errorName(err)});
        return err;
    };

    print("result: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
unable to add one: Overflow
error: Overflow
/home/andy/Downloads/zig/lib/std/math.zig:449:5: 0x2315ab in std.math.add (test)
    return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:8:9: 0x22a244 in main (test)
        return err;
        ^

Builtin Overflow Functions

These builtins return a bool of whether or not overflow occurred, as well as returning the overflowed bits:

@addWithOverflow

@subWithOverflow

@mulWithOverflow

@shlWithOverflow

Example of @addWithOverflow:

test.zig

const print = @import("std").debug.print;
pub fn main() void {
    var byte: u8 = 255;

    var result: u8 = undefined;
    if (@addWithOverflow(u8, byte, 10, &result)) {
        print("overflowed result: {}\n", .{result});
    } else {
        print("result: {}\n", .{result});
    }
}

$ zig build-exe test.zig
$ ./test
overflowed result: 9

Wrapping Operations

These operations have guaranteed wraparound semantics.

+% (wraparound addition)

-% (wraparound subtraction)

-% (wraparound negation)

*% (wraparound multiplication)

test.zig

const std = @import("std");
const expect = std.testing.expect;
const minInt = std.math.minInt;
const maxInt = std.math.maxInt;

test "wraparound addition and subtraction" {
    const x: i32 = maxInt(i32);
    const min_val = x +% 1;
    try expect(min_val == minInt(i32));
    const max_val = min_val -% 1;
    try expect(max_val == maxInt(i32));
}

$ zig test test.zig
Test [1/1] test "wraparound addition and subtraction"...

All 1 tests passed.

Exact Left Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shlExact(@as(u8, 0b01010101), 2);
}

$ zig test test.zig
./docgen_tmp/test.zig:2:15: error: operation caused overflow
    const x = @shlExact(@as(u8, 0b01010101), 2);
              ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var x: u8 = 0b01010101;
    var y = @shlExact(x, 2);
    std.debug.print("value: {}\n", .{y});
}

$ zig build-exe test.zig
$ ./test
thread 16945 panic: left shift overflowed bits
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:13: 0x229f9b in main (test)
    var y = @shlExact(x, 2);
            ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22233c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ce in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051f6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205002 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Exact Right Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shrExact(@as(u8, 0b10101010), 2);
}

$ zig test test.zig
./docgen_tmp/test.zig:2:15: error: exact shift shifted out 1 bits
    const x = @shrExact(@as(u8, 0b10101010), 2);
              ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var x: u8 = 0b10101010;
    var y = @shrExact(x, 2);
    std.debug.print("value: {}\n", .{y});
}

$ zig build-exe test.zig
$ ./test
thread 17005 panic: right shift overflowed bits
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:13: 0x229f9b in main (test)
    var y = @shrExact(x, 2);
            ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22233c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ce in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051f6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205002 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Division by Zero

At compile-time:

test.zig

comptime {
    const a: i32 = 1;
    const b: i32 = 0;
    const c = a / b;
}

$ zig test test.zig
./docgen_tmp/test.zig:4:17: error: division by zero
    const c = a / b;
                ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var a: u32 = 1;
    var b: u32 = 0;
    var c = a / b;
    std.debug.print("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
thread 17071 panic: division by zero
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:15: 0x229f69 in main (test)
    var c = a / b;
              ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Remainder Division by Zero

At compile-time:

test.zig

comptime {
    const a: i32 = 10;
    const b: i32 = 0;
    const c = a % b;
}

$ zig test test.zig
./docgen_tmp/test.zig:4:17: error: division by zero
    const c = a % b;
                ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var a: u32 = 10;
    var b: u32 = 0;
    var c = a % b;
    std.debug.print("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
thread 17131 panic: remainder division by zero or negative value
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:15: 0x229f8b in main (test)
    var c = a % b;
              ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Exact Division Remainder

At compile-time:

test.zig

comptime {
    const a: u32 = 10;
    const b: u32 = 3;
    const c = @divExact(a, b);
}

$ zig test test.zig
./docgen_tmp/test.zig:4:15: error: exact division had a remainder
    const c = @divExact(a, b);
              ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var a: u32 = 10;
    var b: u32 = 3;
    var c = @divExact(a, b);
    std.debug.print("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
thread 17193 panic: exact division produced remainder
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:13: 0x229fad in main (test)
    var c = @divExact(a, b);
            ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22231c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ae in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051d6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204fe2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Attempt to Unwrap Null

At compile-time:

test.zig

comptime {
    const optional_number: ?i32 = null;
    const number = optional_number.?;
}

$ zig test test.zig
./docgen_tmp/test.zig:3:35: error: unable to unwrap null
    const number = optional_number.?;
                                  ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var optional_number: ?i32 = null;
    var number = optional_number.?;
    std.debug.print("value: {}\n", .{number});
}

$ zig build-exe test.zig
$ ./test
thread 17271 panic: attempt to use null value
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:33: 0x229f6c in main (test)
    var number = optional_number.?;
                                ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22232c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061be in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051e6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204ff2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

One way to avoid this crash is to test for null instead of assuming non-null, with the if expression:

test.zig

const print = @import("std").debug.print;
pub fn main() void {
    const optional_number: ?i32 = null;

    if (optional_number) |number| {
        print("got number: {}\n", .{number});
    } else {
        print("it's null\n", .{});
    }
}

$ zig build-exe test.zig
$ ./test
it's null

See also:

Optionals

Attempt to Unwrap Error

At compile-time:

test.zig

comptime {
    const number = getNumberOrFail() catch unreachable;
}

fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig test test.zig
./docgen_tmp/test.zig:2:38: error: caught unexpected error 'UnableToReturnNumber'
    const number = getNumberOrFail() catch unreachable;
                                     ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    const number = getNumberOrFail() catch unreachable;
    std.debug.print("value: {}\n", .{number});
}

fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig build-exe test.zig
$ ./test
thread 17368 panic: attempt to unwrap error: UnableToReturnNumber
/home/andy/Downloads/zig/docgen_tmp/test.zig:9:5: 0x231254 in getNumberOrFail (test)
    return error.UnableToReturnNumber;
    ^
???:?:?: 0x20c682 in ??? (???)
/home/andy/Downloads/zig/docgen_tmp/test.zig:4:38: 0x229fbb in main (test)
    const number = getNumberOrFail() catch unreachable;
                                     ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22233c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061ce in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2051f6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205002 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

One way to avoid this crash is to test for an error instead of assuming a successful result, with the if expression:

test.zig

const print = @import("std").debug.print;

pub fn main() void {
    const result = getNumberOrFail();

    if (result) |number| {
        print("got number: {}\n", .{number});
    } else |err| {
        print("got error: {s}\n", .{@errorName(err)});
    }
}

fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig build-exe test.zig
$ ./test
got error: UnableToReturnNumber

See also:

Errors

Invalid Error Code

At compile-time:

test.zig

comptime {
    const err = error.AnError;
    const number = @errorToInt(err) + 10;
    const invalid_err = @intToError(number);
}

$ zig test test.zig
./docgen_tmp/test.zig:4:25: error: integer value 11 represents no error
    const invalid_err = @intToError(number);
                        ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var err = error.AnError;
    var number = @errorToInt(err) + 500;
    var invalid_err = @intToError(number);
    std.debug.print("value: {}\n", .{number});
}

$ zig build-exe test.zig
$ ./test
thread 17469 panic: invalid error code
/home/andy/Downloads/zig/docgen_tmp/test.zig:6:23: 0x229fc3 in main (test)
    var invalid_err = @intToError(number);
                      ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22234c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x2061de in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205206 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205012 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Invalid Enum Cast

At compile-time:

test.zig

const Foo = enum {
    a,
    b,
    c,
};
comptime {
    const a: u2 = 3;
    const b = @intToEnum(Foo, a);
}

$ zig test test.zig
./docgen_tmp/test.zig:8:15: error: enum 'Foo' has no tag matching integer value 3
    const b = @intToEnum(Foo, a);
              ^
./docgen_tmp/test.zig:1:13: note: 'Foo' declared here
const Foo = enum {
            ^

At runtime:

test.zig

const std = @import("std");

const Foo = enum {
    a,
    b,
    c,
};

pub fn main() void {
    var a: u2 = 3;
    var b = @intToEnum(Foo, a);
    std.debug.print("value: {s}\n", .{@tagName(b)});
}

$ zig build-exe test.zig
$ ./test
thread 17530 panic: invalid enum value
/home/andy/Downloads/zig/docgen_tmp/test.zig:11:13: 0x229ff9 in main (test)
    var b = @intToEnum(Foo, a);
            ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x2223ac in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20623e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205266 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205072 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Invalid Error Set Cast

At compile-time:

test.zig

const Set1 = error{
    A,
    B,
};
const Set2 = error{
    A,
    C,
};
comptime {
    _ = @errSetCast(Set2, Set1.B);
}

$ zig test test.zig
./docgen_tmp/test.zig:10:9: error: error.B not a member of error set 'Set2'
    _ = @errSetCast(Set2, Set1.B);
        ^

At runtime:

test.zig

const std = @import("std");

const Set1 = error{
    A,
    B,
};
const Set2 = error{
    A,
    C,
};
pub fn main() void {
    foo(Set1.B);
}
fn foo(set1: Set1) void {
    const x = @errSetCast(Set2, set1);
    std.debug.print("value: {}\n", .{x});
}

$ zig build-exe test.zig
$ ./test
thread 17592 panic: invalid error code
/home/andy/Downloads/zig/docgen_tmp/test.zig:15:15: 0x231256 in foo (test)
    const x = @errSetCast(Set2, set1);
              ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:12:8: 0x229fbd in main (test)
    foo(Set1.B);
       ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22239c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20622e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205256 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205062 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Incorrect Pointer Alignment

At compile-time:

test.zig

comptime {
    const ptr = @intToPtr(*align(1) i32, 0x1);
    const aligned = @alignCast(4, ptr);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes
    const aligned = @alignCast(4, ptr);
                                  ^
./docgen_tmp/test.zig:3:21: note: referenced here
    const aligned = @alignCast(4, ptr);
                    ^

At runtime:

test.zig

const mem = @import("std").mem;
pub fn main() !void {
    var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
    const bytes = mem.sliceAsBytes(array[0..]);
    if (foo(bytes) != 0x11111111) return error.Wrong;
}
fn foo(bytes: []u8) u32 {
    const slice4 = bytes[1..5];
    const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
    return int_slice[0];
}

$ zig build-exe test.zig
$ ./test
thread 17652 panic: incorrect alignment
/home/andy/Downloads/zig/docgen_tmp/test.zig:9:59: 0x2314dc in foo (test)
    const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
                                                          ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:5:12: 0x22a135 in main (test)
    if (foo(bytes) != 0x11111111) return error.Wrong;
           ^
/home/andy/Downloads/zig/lib/std/start.zig:471:37: 0x2223ba in std.start.callMain (test)
            const result = root.main() catch |err| {
                                    ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20621e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205246 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x205052 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Wrong Union Field Access

At compile-time:

test.zig

comptime {
    var f = Foo{ .int = 42 };
    f.float = 12.34;
}

const Foo = union {
    float: f32,
    int: u32,
};

$ zig test test.zig
./docgen_tmp/test.zig:3:6: error: accessing union field 'float' while field 'int' is set
    f.float = 12.34;
     ^

At runtime:

test.zig

const std = @import("std");

const Foo = union {
    float: f32,
    int: u32,
};

pub fn main() void {
    var f = Foo{ .int = 42 };
    bar(&f);
}

fn bar(f: *Foo) void {
    f.float = 12.34;
    std.debug.print("value: {}\n", .{f.float});
}

$ zig build-exe test.zig
$ ./test
thread 17714 panic: access of inactive union field
/home/andy/Downloads/zig/docgen_tmp/test.zig:14:6: 0x23e9da in bar (test)
    f.float = 12.34;
     ^
/home/andy/Downloads/zig/docgen_tmp/test.zig:10:8: 0x23773c in main (test)
    bar(&f);
       ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x22fb0c in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x21399e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x2129c6 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x2127d2 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

This safety is not available for extern or packed unions.

To change the active field of a union, assign the entire union, like this:

test.zig

const std = @import("std");

const Foo = union {
    float: f32,
    int: u32,
};

pub fn main() void {
    var f = Foo{ .int = 42 };
    bar(&f);
}

fn bar(f: *Foo) void {
    f.* = Foo{ .float = 12.34 };
    std.debug.print("value: {}\n", .{f.float});
}

$ zig build-exe test.zig
$ ./test
value: 1.23400001e+01

To change the active field of a union when a meaningful value for the field is not known, use undefined, like this:

test.zig

const std = @import("std");

const Foo = union {
    float: f32,
    int: u32,
};

pub fn main() void {
    var f = Foo{ .int = 42 };
    f = Foo{ .float = undefined };
    bar(&f);
    std.debug.print("value: {}\n", .{f.float});
}

fn bar(f: *Foo) void {
    f.float = 12.34;
}

$ zig build-exe test.zig
$ ./test
value: 1.23400001e+01

See also:

union

extern union

Out of Bounds Float to Integer Cast

TODO

Pointer Cast Invalid Null

This happens when casting a pointer with the address 0 to a pointer which may not have the address 0. For example, C Pointers, Optional Pointers, and allowzero pointers allow address zero, but normal Pointers do not.

At compile-time:

test.zig

comptime {
    const opt_ptr: ?*i32 = null;
    const ptr = @ptrCast(*i32, opt_ptr);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:17: error: null pointer casted to type '*i32'
    const ptr = @ptrCast(*i32, opt_ptr);
                ^

At runtime:

test.zig

pub fn main() void {
    var opt_ptr: ?*i32 = null;
    var ptr = @ptrCast(*i32, opt_ptr);
}

$ zig build-exe test.zig
$ ./test
thread 17846 panic: cast causes pointer to be null
/home/andy/Downloads/zig/docgen_tmp/test.zig:3:15: 0x229f10 in main (test)
    var ptr = @ptrCast(*i32, opt_ptr);
              ^
/home/andy/Downloads/zig/lib/std/start.zig:461:22: 0x2222cc in std.start.callMain (test)
            root.main();
                     ^
/home/andy/Downloads/zig/lib/std/start.zig:413:12: 0x20615e in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/andy/Downloads/zig/lib/std/start.zig:332:17: 0x205186 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/home/andy/Downloads/zig/lib/std/start.zig:245:5: 0x204f92 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Memory

The Zig language performs no memory management on behalf of the programmer. This is why Zig has no runtime, and why Zig code works seamlessly in so many environments, including real-time software, operating system kernels, embedded devices, and low latency servers. As a consequence, Zig programmers must always be able to answer the question:

Where are the bytes?

Like Zig, the C programming language has manual memory management. However, unlike Zig, C has a default allocator - malloc, realloc, and free. When linking against libc, Zig exposes this allocator with std.heap.c_allocator. However, by convention, there is no default allocator in Zig. Instead, functions which need to allocate accept an *Allocator parameter. Likewise, data structures such as std.ArrayList accept an *Allocator parameter in their initialization functions:

allocator.zig

const std = @import("std");
const Allocator = std.mem.Allocator;
const expect = std.testing.expect;

test "using an allocator" {
    var buffer: [100]u8 = undefined;
    const allocator = &std.heap.FixedBufferAllocator.init(&buffer).allocator;
    const result = try concat(allocator, "foo", "bar");
    try expect(std.mem.eql(u8, "foobar", result));
}

fn concat(allocator: *Allocator, a: []const u8, b: []const u8) ![]u8 {
    const result = try allocator.alloc(u8, a.len + b.len);
    std.mem.copy(u8, result, a);
    std.mem.copy(u8, result[a.len..], b);
    return result;
}

$ zig test allocator.zig
Test [1/1] test "using an allocator"...

All 1 tests passed.

In the above example, 100 bytes of stack memory are used to initialize a FixedBufferAllocator, which is then passed to a function. As a convenience there is a global FixedBufferAllocator available for quick tests at std.testing.allocator, which will also do perform basic leak detection.

Zig has a general purpose allocator available to be imported with std.heap.GeneralPurposeAllocator. However, it is still recommended to follow the Choosing an Allocator guide.

Choosing an Allocator

What allocator to use depends on a number of factors. Here is a flow chart to help you decide:

Are you making a library? In this case, best to accept an *Allocator as a parameter and allow your library's users to decide what allocator to use.

Are you linking libc? In this case, std.heap.c_allocator is likely the right choice, at least for your main allocator.

Is the maximum number of bytes that you will need bounded by a number known at comptime? In this case, use std.heap.FixedBufferAllocator or std.heap.ThreadSafeFixedBufferAllocator depending on whether you need thread-safety or not.

Is your program a command line application which runs from start to end without any fundamental cyclical pattern (such as a video game main loop, or a web server request handler), such that it would make sense to free everything at once at the end? In this case, it is recommended to follow this pattern:

cli_allocation.zig

const std = @import("std");

pub fn main() !void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();

    const allocator = &arena.allocator;

    const ptr = try allocator.create(i32);
    std.debug.print("ptr={*}\n", .{ptr});
}

$ zig build-exe cli_allocation.zig
$ ./cli_allocation
ptr=i32@7f00d058c018

When using this kind of allocator, there is no need to free anything manually. Everything gets freed at once with the call to arena.deinit().

Are the allocations part of a cyclical pattern such as a video game main loop, or a web server request handler? If the allocations can all be freed at once, at the end of the cycle, for example once the video game frame has been fully rendered, or the web server request has been served, then std.heap.ArenaAllocator is a great candidate. As demonstrated in the previous bullet point, this allows you to free entire arenas at once. Note also that if an upper bound of memory can be established, then std.heap.FixedBufferAllocator can be used as a further optimization.

Are you writing a test, and you want to make sure error.OutOfMemory is handled correctly? In this case, use std.testing.FailingAllocator.

Are you writing a test? In this case, use std.testing.allocator.

Finally, if none of the above apply, you need a general purpose allocator. Zig's general purpose allocator is available as a function that takes a comptime struct of configuration options and returns a type. Generally, you will set up one std.heap.GeneralPurposeAllocator in your main function, and then pass it or sub-allocators around to various parts of your application.

You can also consider Implementing an Allocator.

Where are the bytes?

String literals such as "foo" are in the global constant data section. This is why it is an error to pass a string literal to a mutable slice, like this:

test.zig

fn foo(s: []u8) void {}

test "string literal to mutable slice" {
    foo("hello");
}

$ zig test test.zig
./docgen_tmp/test.zig:4:9: error: expected type '[]u8', found '*const [5:0]u8'
    foo("hello");
        ^

However if you make the slice constant, then it works:

strlit.zig

fn foo(s: []const u8) void {}

test "string literal to constant slice" {
    foo("hello");
}

$ zig test strlit.zig
Test [1/1] test "string literal to constant slice"...

All 1 tests passed.

Just like string literals, const declarations, when the value is known at comptime, are stored in the global constant data section. Also Compile Time Variables are stored in the global constant data section.

var declarations inside functions are stored in the function's stack frame. Once a function returns, any Pointers to variables in the function's stack frame become invalid references, and dereferencing them becomes unchecked Undefined Behavior.

var declarations at the top level or in struct declarations are stored in the global data section.

The location of memory allocated with allocator.alloc or allocator.create is determined by the allocator's implementation.

TODO: thread local variables

Implementing an Allocator

Zig programmers can implement their own allocators by fulfilling the Allocator interface. In order to do this one must read carefully the documentation comments in std/mem.zig and then supply a reallocFn and a shrinkFn.

There are many example allocators to look at for inspiration. Look at std/heap.zig and std.heap.GeneralPurposeAllocator.

Heap Allocation Failure

Many programming languages choose to handle the possibility of heap allocation failure by unconditionally crashing. By convention, Zig programmers do not consider this to be a satisfactory solution. Instead, error.OutOfMemory represents heap allocation failure, and Zig libraries return this error code whenever heap allocation failure prevented an operation from completing successfully.

Some have argued that because some operating systems such as Linux have memory overcommit enabled by default, it is pointless to handle heap allocation failure. There are many problems with this reasoning:

Only some operating systems have an overcommit feature.

Linux has it enabled by default, but it is configurable.

Windows does not overcommit.

Embedded systems do not have overcommit.

Hobby operating systems may or may not have overcommit.

For real-time systems, not only is there no overcommit, but typically the maximum amount of memory per application is determined ahead of time.

When writing a library, one of the main goals is code reuse. By making code handle allocation failure correctly, a library becomes eligible to be reused in more contexts.

Although some software has grown to depend on overcommit being enabled, its existence is the source of countless user experience disasters. When a system with overcommit enabled, such as Linux on default settings, comes close to memory exhaustion, the system locks up and becomes unusable. At this point, the OOM Killer selects an application to kill based on heuristics. This non-deterministic decision often results in an important process being killed, and often fails to return the system back to working order.

Recursion

Recursion is a fundamental tool in modeling software. However it has an often-overlooked problem: unbounded memory allocation.

Recursion is an area of active experimentation in Zig and so the documentation here is not final. You can read a summary of recursion status in the 0.3.0 release notes.

The short summary is that currently recursion works normally as you would expect. Although Zig code is not yet protected from stack overflow, it is planned that a future version of Zig will provide such protection, with some degree of cooperation from Zig code required.

Lifetime and Ownership

It is the Zig programmer's responsibility to ensure that a pointer is not accessed when the memory pointed to is no longer available. Note that a slice is a form of pointer, in that it references other memory.

In order to prevent bugs, there are some helpful conventions to follow when dealing with pointers. In general, when a function returns a pointer, the documentation for the function should explain who "owns" the pointer. This concept helps the programmer decide when it is appropriate, if ever, to free the pointer.

For example, the function's documentation may say "caller owns the returned memory", in which case the code that calls the function must have a plan for when to free that memory. Probably in this situation, the function will accept an *Allocator parameter.

Sometimes the lifetime of a pointer may be more complicated. For example, the std.ArrayList(T).items slice has a lifetime that remains valid until the next time the list is resized, such as by appending new elements.

The API documentation for functions and data structures should take great care to explain the ownership and lifetime semantics of pointers. Ownership determines whose responsibility it is to free the memory referenced by the pointer, and lifetime determines the point at which the memory becomes inaccessible (lest Undefined Behavior occur).

Compile Variables

Compile variables are accessible by importing the "builtin" package, which the compiler makes available to every Zig source file. It contains compile-time constants such as the current target, endianness, and release mode.

const builtin = @import("builtin");
const separator = if (builtin.os.tag == builtin.Os.windows) '\\' else '/';

Example of what is imported with @import("builtin"):

const std = @import("std");
/// Zig version. When writing code that supports multiple versions of Zig, prefer
/// feature detection (i.e. with `@hasDecl` or `@hasField`) over version checks.
pub const zig_version = std.SemanticVersion.parse("0.8.1") catch unreachable;
/// Temporary until self-hosted is feature complete.
pub const zig_is_stage2 = true;
/// Temporary until self-hosted supports the `cpu.arch` value.
pub const stage2_arch: std.Target.Cpu.Arch = .x86_64;

pub const output_mode = std.builtin.OutputMode.Obj;
pub const link_mode = std.builtin.LinkMode.Static;
pub const is_test = false;
pub const single_threaded = false;
pub const abi = std.Target.Abi.gnu;
pub const cpu: std.Target.Cpu = .{
    .arch = .x86_64,
    .model = &std.Target.x86.cpu.skylake,
    .features = std.Target.x86.featureSet(&[_]std.Target.x86.Feature{
        .@"64bit",
        .adx,
        .aes,
        .avx,
        .avx2,
        .bmi,
        .bmi2,
        .clflushopt,
        .cmov,
        .cx16,
        .cx8,
        .ermsb,
        .f16c,
        .false_deps_popcnt,
        .fast_15bytenop,
        .fast_gather,
        .fast_scalar_fsqrt,
        .fast_shld_rotate,
        .fast_variable_shuffle,
        .fast_vector_fsqrt,
        .fma,
        .fsgsbase,
        .fxsr,
        .idivq_to_divl,
        .invpcid,
        .lzcnt,
        .macrofusion,
        .mmx,
        .movbe,
        .nopl,
        .pclmul,
        .popcnt,
        .prfchw,
        .rdrnd,
        .rdseed,
        .sahf,
        .sgx,
        .slow_3ops_lea,
        .sse,
        .sse2,
        .sse3,
        .sse4_1,
        .sse4_2,
        .ssse3,
        .vzeroupper,
        .x87,
        .xsave,
        .xsavec,
        .xsaveopt,
        .xsaves,
    }),
};
pub const os = std.Target.Os{
    .tag = .linux,
    .version_range = .{ .linux = .{
        .range = .{
            .min = .{
                .major = 5,
                .minor = 4,
                .patch = 122,
            },
            .max = .{
                .major = 5,
                .minor = 4,
                .patch = 122,
            },
        },
        .glibc = .{
            .major = 2,
            .minor = 31,
            .patch = 0,
        },
    }},
};
pub const target = std.Target{
    .cpu = cpu,
    .os = os,
    .abi = abi,
};
pub const object_format = std.Target.ObjectFormat.elf;
pub const mode = std.builtin.Mode.Debug;
pub const link_libc = false;
pub const link_libcpp = false;
pub const have_error_return_tracing = true;
pub const valgrind_support = true;
pub const position_independent_code = false;
pub const position_independent_executable = false;
pub const strip_debug_info = false;
pub const code_model = std.builtin.CodeModel.default;

See also:

Build Mode

Root Source File

TODO: explain how root source file finds other files

TODO: pub fn main

TODO: pub fn panic

TODO: if linking with libc you can use export fn main

TODO: order independent top level declarations

TODO: lazy analysis

TODO: using comptime { _ = @import() }

Zig Test

zig test is a tool that can be used to quickly build and run Zig code to make sure behavior meets expectations. @import("builtin").is_test is available for code to detect whether the current build is a test build.

detect_test.zig

const std = @import("std");
const builtin = @import("builtin");
const expect = std.testing.expect;

test "builtin.is_test" {
    try expect(builtin.is_test);
}

$ zig test detect_test.zig
Test [1/1] test "builtin.is_test"...

All 1 tests passed.

Zig has lazy top level declaration analysis, which means that if a function is not called, or otherwise used, it is not analyzed. This means that there may be an undiscovered compile error in a function because it is never called.

unused_fn.zig

fn unused() i32 {
    return "wrong return type";
}
test "unused function" { }

$ zig test unused_fn.zig
Test [1/1] test "unused function"...

All 1 tests passed.

Note that, while in Debug and ReleaseSafe modes, unreachable emits a call to @panic, in ReleaseFast and ReleaseSmall modes, it is really undefined behavior. The implementation of std.debug.assert is as simple as:

pub fn assert(ok: bool) void {
    if (!ok) unreachable;
}

This means that when testing in ReleaseFast or ReleaseSmall mode, assert is not sufficient to check the result of a computation:

const std = @import("std");
const assert = std.debug.assert;

test "assert in release fast mode" {
    assert(false);
}

When compiling this test in ReleaseFast mode, it invokes unchecked Undefined Behavior. Since that could do anything, this documentation cannot show you the output.

Better practice for checking the output when testing is to use std.testing.expect:

test.zig

const std = @import("std");
const expect = std.testing.expect;

test "expect in release fast mode" {
    try expect(false);
}

$ zig test test.zig -O ReleaseFast
Test [1/1] test "expect in release fast mode"...
Test [2/1]
test "expect in release fast mode"... FAIL (TestUnexpectedResult)

0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
docgen_tmp/zig-cache/o/5f772ba34b67337c067e5630990ef24e/test /home/andy/Downloads/zig/build-release/zig

See the rest of the std.testing namespace for more available functions.

zig test has a few command line parameters which affect the compilation. See zig --help for a full list. The most interesting one is --test-filter [text]. This makes the test build only include tests whose name contains the supplied filter text. Again, thanks to lazy analysis, this can allow you to narrow a build to only a few functions in isolation.

Zig Build System

The Zig Build System provides a cross-platform, dependency-free way to declare the logic required to build a project. With this system, the logic to build a project is written in a build.zig file, using the Zig Build System API to declare and configure build artifacts and other tasks.

Some examples of tasks the build system can help with:

Creating build artifacts by executing the Zig compiler. This includes building Zig source code as well as C and C++ source code.

Capturing user-configured options and using those options to configure the build.

Surfacing build configuration as comptime values by providing a file that can be imported by Zig code.

Caching build artifacts to avoid unnecessarily repeating steps.

Executing build artifacts or system-installed tools.

Running tests and verifying the output of executing a build artifact matches the expected value.

Running zig fmt on a codebase or a subset of it.

Custom tasks.

To use the build system, run zig build --help to see a command-line usage help menu. This will include project-specific options that were declared in the build.zig script.

Building an Executable

This build.zig file is automatically generated by zig init-exe.

const Builder = @import("std").build.Builder;

pub fn build(b: *Builder) void {
    // Standard target options allows the person running `zig build` to choose
    // what target to build for. Here we do not override the defaults, which
    // means any target is allowed, and the default is native. Other options
    // for restricting supported target set are available.
    const target = b.standardTargetOptions(.{});

    // Standard release options allow the person running `zig build` to select
    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
    const mode = b.standardReleaseOptions();

    const exe = b.addExecutable("example", "src/main.zig");
    exe.setTarget(target);
    exe.setBuildMode(mode);
    exe.install();

    const run_cmd = exe.run();
    run_cmd.step.dependOn(b.getInstallStep());
    if (b.args) |args| {
        run_cmd.addArgs(args);
    }

    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_cmd.step);
}

Building a Library

This build.zig file is automatically generated by zig init-lib.

const Builder = @import("std").build.Builder;

pub fn build(b: *Builder) void {
    const mode = b.standardReleaseOptions();
    const lib = b.addStaticLibrary("example", "src/main.zig");
    lib.setBuildMode(mode);
    lib.install();

    var main_tests = b.addTest("src/main.zig");
    main_tests.setBuildMode(mode);

    const test_step = b.step("test", "Run library tests");
    test_step.dependOn(&main_tests.step);
}

Compiling C Source Code

lib.addCSourceFile("src/lib.c", &[_][]const u8{
    "-Wall",
    "-Wextra",
    "-Werror",
});

C

Although Zig is independent of C, and, unlike most other languages, does not depend on libc, Zig acknowledges the importance of interacting with existing C code.

There are a few ways that Zig facilitates C interop.

C Type Primitives

These have guaranteed C ABI compatibility and can be used like any other type.

c_short

c_ushort

c_int

c_uint

c_long

c_ulong

c_longlong

c_ulonglong

c_longdouble

c_void

See also:

Primitive Types

Import from C Header File

The @cImport builtin function can be used to directly import symbols from .h files:

test.zig

const c = @cImport({
    // See https://github.com/ziglang/zig/issues/515
    @cDefine("_NO_CRT_STDIO_INLINE", "1");
    @cInclude("stdio.h");
});
pub fn main() void {
    _ = c.printf("hello\n");
}

$ zig build-exe test.zig -lc
$ ./test
hello

The @cImport function takes an expression as a parameter. This expression is evaluated at compile-time and is used to control preprocessor directives and include multiple .h files:

const builtin = @import("builtin");

const c = @cImport({
    @cDefine("NDEBUG", builtin.mode == .ReleaseFast);
    if (something) {
        @cDefine("_GNU_SOURCE", {});
    }
    @cInclude("stdlib.h");
    if (something) {
        @cUndef("_GNU_SOURCE");
    }
    @cInclude("soundio.h");
});

See also:

@cImport

@cInclude

@cDefine

@cUndef

@import

C Pointers

This type is to be avoided whenever possible. The only valid reason for using a C pointer is in auto-generated code from translating C code.

When importing C header files, it is ambiguous whether pointers should be translated as single-item pointers (*T) or many-item pointers ([*]T). C pointers are a compromise so that Zig code can utilize translated header files directly.

[*c]T - C pointer.

Supports all the syntax of the other two pointer types.

Coerces to other pointer types, as well as Optional Pointers. When a C pointer is coerced to a non-optional pointer, safety-checked Undefined Behavior occurs if the address is 0.

Allows address 0. On non-freestanding targets, dereferencing address 0 is safety-checked Undefined Behavior. Optional C pointers introduce another bit to keep track of null, just like ?usize. Note that creating an optional C pointer is unnecessary as one can use normal Optional Pointers.

Supports Type Coercion to and from integers.

Supports comparison with integers.

Does not support Zig-only pointer attributes such as alignment. Use normal Pointers please!

When a C pointer is pointing to a single struct (not an array), dereference the C pointer to access to the struct's fields or member data. That syntax looks like this:

ptr_to_struct.*.struct_member

This is comparable to doing -> in C.

When a C pointer is pointing to an array of structs, the syntax reverts to this:

ptr_to_struct_array[index].struct_member

Exporting a C Library

One of the primary use cases for Zig is exporting a library with the C ABI for other programming languages to call into. The export keyword in front of functions, variables, and types causes them to be part of the library API:

mathtest.zig

export fn add(a: i32, b: i32) i32 {
    return a + b;
}

To make a static library:

$ zig build-lib mathtest.zig

To make a shared library:

$ zig build-lib mathtest.zig -dynamic

Here is an example with the Zig Build System:

test.c

// This header is generated by zig from mathtest.zig
#include "mathtest.h"
#include <stdio.h>

int main(int argc, char **argv) {
    int32_t result = add(42, 1337);
    printf("%d\n", result);
    return 0;
}

build.zig

const Builder = @import("std").build.Builder;

pub fn build(b: *Builder) void {
    const lib = b.addSharedLibrary("mathtest", "mathtest.zig", b.version(1, 0, 0));

    const exe = b.addExecutable("test", null);
    exe.addCSourceFile("test.c", &[_][]const u8{"-std=c99"});
    exe.linkLibrary(lib);
    exe.linkSystemLibrary("c");

    b.default_step.dependOn(&exe.step);

    const run_cmd = exe.run();

    const test_step = b.step("test", "Test the program");
    test_step.dependOn(&run_cmd.step);
}

terminal

$ zig build test
1379

See also:

export

Mixing Object Files

You can mix Zig object files with any other object files that respect the C ABI. Example:

base64.zig

const base64 = @import("std").base64;

export fn decode_base_64(
    dest_ptr: [*]u8,
    dest_len: usize,
    source_ptr: [*]const u8,
    source_len: usize,
) usize {
    const src = source_ptr[0..source_len];
    const dest = dest_ptr[0..dest_len];
    const base64_decoder = base64.standard.Decoder;
    const decoded_size = base64_decoder.calcSizeForSlice(src) catch unreachable;
    base64_decoder.decode(dest[0..decoded_size], src) catch unreachable;
    return decoded_size;
}

test.c

// This header is generated by zig from base64.zig
#include "base64.h"

#include <string.h>
#include <stdio.h>

int main(int argc, char **argv) {
    const char *encoded = "YWxsIHlvdXIgYmFzZSBhcmUgYmVsb25nIHRvIHVz";
    char buf[200];

    size_t len = decode_base_64(buf, 200, encoded, strlen(encoded));
    buf[len] = 0;
    puts(buf);

    return 0;
}

build.zig

const Builder = @import("std").build.Builder;

pub fn build(b: *Builder) void {
    const obj = b.addObject("base64", "base64.zig");

    const exe = b.addExecutable("test", null);
    exe.addCSourceFile("test.c", &[_][]const u8{"-std=c99"});
    exe.addObject(obj);
    exe.linkSystemLibrary("c");
    exe.install();
}

terminal

$ zig build
$ ./zig-out/bin/test
all your base are belong to us

See also:

Targets

Zig Build System

WebAssembly

Zig supports building for WebAssembly out of the box.

Freestanding

For host environments like the web browser and nodejs, build as a dynamic library using the freestanding OS target. Here's an example of running Zig code compiled to WebAssembly with nodejs.

math.zig

extern fn print(i32) void;

export fn add(a: i32, b: i32) void {
    print(a + b);
}

$ zig build-lib math.zig -target wasm32-freestanding -dynamic

test.js

const fs = require('fs');
const source = fs.readFileSync("./math.wasm");
const typedArray = new Uint8Array(source);

WebAssembly.instantiate(typedArray, {
  env: {
    print: (result) => { console.log(`The result is ${result}`); }
  }}).then(result => {
  const add = result.instance.exports.add;
  add(1, 2);
});

$ node test.js
The result is 3

WASI

Zig's support for WebAssembly System Interface (WASI) is under active development. Example of using the standard library and reading command line arguments:

args.zig

const std = @import("std");

pub fn main() !void {
    var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
    const gpa = &general_purpose_allocator.allocator;
    const args = try std.process.argsAlloc(gpa);
    defer std.process.argsFree(gpa, args);

    for (args) |arg, i| {
        std.debug.print("{}: {s}\n", .{ i, arg });
    }
}

$ zig build-exe args.zig -target wasm32-wasi

$ wasmtime args.wasm 123 hello
0: args.wasm
1: 123
2: hello

A more interesting example would be extracting the list of preopens from the runtime. This is now supported in the standard library via std.fs.wasi.PreopenList:

preopens.zig

const std = @import("std");
const PreopenList = std.fs.wasi.PreopenList;

pub fn main() !void {
    var general_purpose_allocator = std.heap.GeneralPurposeAllocator(.{}){};
    const gpa = &general_purpose_allocator.allocator;

    var preopens = PreopenList.init(gpa);
    defer preopens.deinit();

    try preopens.populate();

    for (preopens.asSlice()) |preopen, i| {
        std.debug.print("{}: {}\n", .{ i, preopen });
    }
}

$ zig build-exe preopens.zig -target wasm32-wasi

$ wasmtime --dir=. preopens.wasm
0: Preopen{ .fd = 3, .type = PreopenType{ .Dir = '.' } }

Targets

Zig supports generating code for all targets that LLVM supports. Here is what it looks like to execute zig targets on a Linux x86_64 computer:

$ zig targets
Architectures:
  arm
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
    v7
    v7em
    v7m
    v7s
    v7k
    v7ve
    v6
    v6m
    v6k
    v6t2
    v5
    v5te
    v4t
  armeb
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
    v7
    v7em
    v7m
    v7s
    v7k
    v7ve
    v6
    v6m
    v6k
    v6t2
    v5
    v5te
    v4t
  aarch64
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
  aarch64_be
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
  avr
  bpfel
  bpfeb
  hexagon
  mips
  mipsel
  mips64
  mips64el
  msp430
  powerpc
  powerpc64
  powerpc64le
  r600
  amdgcn
  riscv32
  riscv64
  sparc
  sparcv9
  sparcel
  s390x
  thumb
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
    v7
    v7em
    v7m
    v7s
    v7k
    v7ve
    v6
    v6m
    v6k
    v6t2
    v5
    v5te
    v4t
  thumbeb
    v8_4a
    v8_3a
    v8_2a
    v8_1a
    v8
    v8r
    v8m_baseline
    v8m_mainline
    v7
    v7em
    v7m
    v7s
    v7k
    v7ve
    v6
    v6m
    v6k
    v6t2
    v5
    v5te
    v4t
  i386
  x86_64 (native)
  xcore
  nvptx
  nvptx64
  lanai
  wasm32
  wasm64

Operating Systems:
  freestanding
  ananas
  cloudabi
  dragonfly
  freebsd
  fuchsia
  ios
  kfreebsd
  linux (native)
  lv2
  macos
  netbsd
  openbsd
  solaris
  windows
  haiku
  minix
  rtems
  nacl
  cnk
  aix
  cuda
  nvcl
  amdhsa
  ps4
  elfiamcu
  tvos
  wasi
  watchos
  mesa3d
  contiki
  amdpal
  zen
  uefi

C ABIs:
  none
  gnu (native)
  gnuabin32
  gnuabi64
  gnueabi
  gnueabihf
  gnux32
  code16
  eabi
  eabihf
  android
  musl
  musleabi
  musleabihf
  msvc
  itanium
  cygnus
  coreclr
  simulator

Available libcs:
  aarch64_be-linux-gnu
  aarch64_be-linux-musl
  aarch64-linux-gnu
  aarch64-linux-musleabi
  armeb-linux-gnueabi
  armeb-linux-gnueabihf
  armeb-linux-musleabi
  armeb-linux-musleabihf
  arm-linux-gnueabi
  arm-linux-gnueabihf
  arm-linux-musleabi
  arm-linux-musleabihf
  i386-linux-gnu
  i386-linux-musl
  mips64el-linux-gnuabi64
  mips64el-linux-gnuabin32
  mips64el-linux-musl
  mips64-linux-gnuabi64
  mips64-linux-gnuabin32
  mips64-linux-musl
  mipsel-linux-gnu
  mipsel-linux-musl
  mips-linux-gnu
  mips-linux-musl
  nios2-linux-gnu
  powerpc64le-linux-gnu
  powerpc64le-linux-musl
  powerpc64-linux-gnu
  powerpc64-linux-musl
  powerpc-linux-gnu
  powerpc-linux-musl
  riscv32-linux-musl
  riscv64-linux-gnu
  riscv64-linux-musl
  s390x-linux-gnu
  s390x-linux-musl
  sparc-linux-gnu
  sparcv9-linux-gnu
  wasm32-freestanding-musl
  wasm32-wasi-musl
  x86_64-linux-gnu
  x86_64-linux-gnux32
  x86_64-linux-musl

The Zig Standard Library (@import("std")) has architecture, environment, and operating system abstractions, and thus takes additional work to support more platforms. Not all standard library code requires operating system abstractions, however, so things such as generic data structures work on all above platforms.

The current list of targets supported by the Zig Standard Library is:

Linux x86_64

Windows x86_64

macOS x86_64

Style Guide

These coding conventions are not enforced by the compiler, but they are shipped in this documentation along with the compiler in order to provide a point of reference, should anyone wish to point to an authority on agreed upon Zig coding style.

Whitespace

4 space indentation

Open braces on same line, unless you need to wrap.

If a list of things is longer than 2, put each item on its own line and exercise the ability to put an extra comma at the end.

Line length: aim for 100; use common sense.

Names

Roughly speaking: camelCaseFunctionName, TitleCaseTypeName, snake_case_variable_name. More precisely:

If x is a type then x should be TitleCase, unless it is a struct with 0 fields and is never meant to be instantiated, in which case it is considered to be a "namespace" and uses snake_case.

If x is callable, and x's return type is type, then x should be TitleCase.

If x is otherwise callable, then x should be camelCase.

Otherwise, x should be snake_case.

Acronyms, initialisms, proper nouns, or any other word that has capitalization rules in written English are subject to naming conventions just like any other word. Even acronyms that are only 2 letters long are subject to these conventions.

File names fall into two categories: types and namespaces. If the file (implicitly a struct) has top level fields, it should be named like any other struct with fields using TitleCase. Otherwise, it should use snake_case. Directory names should be snake_case.

These are general rules of thumb; if it makes sense to do something different, do what makes sense. For example, if there is an established convention such as ENOENT, follow the established convention.

Examples

const namespace_name = @import("dir_name/file_name.zig");
const TypeName = @import("dir_name/TypeName.zig");
var global_var: i32 = undefined;
const const_name = 42;
const primitive_type_alias = f32;
const string_alias = []u8;

const StructName = struct {
    field: i32,
};
const StructAlias = StructName;

fn functionName(param_name: TypeName) void {
    var functionPointer = functionName;
    functionPointer();
    functionPointer = otherFunction;
    functionPointer();
}
const functionAlias = functionName;

fn ListTemplateFunction(comptime ChildType: type, comptime fixed_size: usize) type {
    return List(ChildType, fixed_size);
}

fn ShortList(comptime T: type, comptime n: usize) type {
    return struct {
        field_name: [n]T,
        fn methodName() void {}
    };
}

// The word XML loses its casing when used in Zig identifiers.
const xml_document =
    \\<?xml version="1.0" encoding="UTF-8"?>
    \\<document>
    \\</document>
;
const XmlParser = struct {
    field: i32,
};

// The initials BE (Big Endian) are just another word in Zig identifier names.
fn readU32Be() u32 {}

See the Zig Standard Library for more examples.

Doc Comment Guidance

Omit any information that is redundant based on the name of the thing being documented.

Duplicating information onto multiple similar functions is encouraged because it helps IDEs and other tools provide better help text.

Use the word assume to indicate invariants that cause Undefined Behavior when violated.

Use the word assert to indicate invariants that cause safety-checked Undefined Behavior when violated.

Source Encoding

Zig source code is encoded in UTF-8. An invalid UTF-8 byte sequence results in a compile error.

Throughout all zig source code (including in comments), some code points are never allowed:

Ascii control characters, except for U+000a (LF), U+000d (CR), and U+0009 (HT): U+0000 - U+0008, U+000b - U+000c, U+000e - U+0001f, U+007f.

Non-Ascii Unicode line endings: U+0085 (NEL), U+2028 (LS), U+2029 (PS).

LF (byte value 0x0a, code point U+000a, '\n') is the line terminator in Zig source code. This byte value terminates every line of zig source code except the last line of the file. It is recommended that non-empty source files end with an empty line, which means the last byte would be 0x0a (LF).

Each LF may be immediately preceded by a single CR (byte value 0x0d, code point U+000d, '\r') to form a Windows style line ending, but this is discouraged. A CR in any other context is not allowed.

HT hard tabs (byte value 0x09, code point U+0009, '\t') are interchangeable with SP spaces (byte value 0x20, code point U+0020, ' ') as a token separator, but use of hard tabs is discouraged. See Grammar.

Note that running zig fmt on a source file will implement all recommendations mentioned here. Note also that the stage1 compiler does not yet support CR or HT control characters.

Note that a tool reading Zig source code can make assumptions if the source code is assumed to be correct Zig code. For example, when identifying the ends of lines, a tool can use a naive search such as /\n/, or an advanced search such as /\r\n?|[\n\u0085\u2028\u2029]/, and in either case line endings will be correctly identified. For another example, when identifying the whitespace before the first token on a line, a tool can either use a naive search such as /[ \t]/, or an advanced search such as /\s/, and in either case whitespace will be correctly identified.

Keyword Reference

Keyword | Description

align

align can be used to specify the alignment of a pointer. It can also be used after a variable or function declaration to specify the alignment of pointers to that variable or function.

See also Alignment

allowzero

The pointer attribute allowzero allows a pointer to have address zero.

See also allowzero

and

The boolean operator and.

See also Operators

anyframe

anyframe can be used as a type for variables which hold pointers to function frames.

See also Async Functions

anytype

Function parameters and struct fields can be declared with anytype in place of the type. The type will be inferred where the function is called or the struct is instantiated.

See also Function Parameter Type Inference

asm

asm begins an inline assembly expression. This allows for directly controlling the machine code generated on compilation.

See also Assembly

async

async can be used before a function call to get a pointer to the function's frame when it suspends.

See also Async Functions

await

await can be used to suspend the current function until the frame provided after the await completes. await copies the value returned from the target function's frame to the caller.

See also Async Functions

break

break can be used with a block label to return a value from the block. It can also be used to exit a loop before iteration completes naturally.

See also blocks, while, for

catch

catch can be used to evaluate an expression if the expression before it evaluates to an error. The expression after the catch can optionally capture the error value.

See also catch, Operators

comptime

comptime before a declaration can be used to label variables or function parameters as known at compile time. It can also be used to guarantee an expression is run at compile time.

See also comptime

const

const declares a variable that can not be modified. Used as a pointer attribute, it denotes the value referenced by the pointer cannot be modified.

See also Variables

continue

continue can be used in a loop to jump back to the beginning of the loop.

See also while, for

defer

defer will execute an expression when control flow leaves the current block.

See also defer

else

else can be used to provide an alternate branch for if, switch, while, and for expressions.

If used after an if expression, the else branch will be executed if the test value returns false, null, or an error.

If used within a switch expression, the else branch will be executed if the test value matches no other cases.

If used after a loop expression, the else branch will be executed if the loop finishes without breaking.

See also if, switch, while, for

enum

enum defines an enum type.

See also enum

errdefer

errdefer will execute an expression when control flow leaves the current block if the function returns an error.

See also errdefer

error

error defines an error type.

See also Errors

export

export makes a function or variable externally visible in the generated object file. Exported functions default to the C calling convention.

See also Functions

extern

extern can be used to declare a function or variable that will be resolved at link time, when linking statically or at runtime, when linking dynamically.

See also Functions

false

The boolean value false.

See also Primitive Values

fn

fn declares a function.

See also Functions

for

A for expression can be used to iterate over the elements of a slice, array, or tuple.

See also for

if

An if expression can test boolean expressions, optional values, or error unions. For optional values or error unions, the if expression can capture the unwrapped value.

See also if

inline

inline can be used to label a loop expression such that it will be unrolled at compile time. It can also be used to force a function to be inlined at all call sites.

See also inline while, inline for, Functions

noalias

The noalias keyword.

TODO add documentation for noalias

nosuspend

The nosuspend keyword can be used in front of a block, statement or expression, to mark a scope where no suspension points are reached. In particular, inside a nosuspend scope:

Using the suspend keyword results in a compile error.

Using await on a function frame which hasn't completed yet results in safety-checked Undefined Behavior.

Calling an async function may result in safety-checked Undefined Behavior, because it's equivalent to await async some_async_fn(), which contains an await.

Code inside a nosuspend scope does not cause the enclosing function to become an async function.

See also Async Functions

null

The optional value null.

See also null

or

The boolean operator or.

See also Operators

orelse

orelse can be used to evaluate an expression if the expression before it evaluates to null.

See also Optionals, Operators

packed

The packed keyword before a struct definition changes the struct's in-memory layout to the guaranteed packed layout.

See also packed struct

pub

The pub in front of a top level declaration makes the declaration available to reference from a different file than the one it is declared in.

See also import

resume

resume will continue execution of a function frame after the point the function was suspended.

See also Suspend and Resume

return

return exits a function with a value.

See also Functions

linksection

The linksection keyword.

TODO add documentation for linksection

struct

struct defines a struct.

See also struct

suspend

suspend will cause control flow to return to the call site or resumer of the function. suspend can also be used before a block within a function, to allow the function access to its frame before control flow returns to the call site.

See also Suspend and Resume

switch

A switch expression can be used to test values of a common type. switch cases can capture field values of a Tagged union.

See also switch

test

The test keyword can be used to denote a top-level block of code used to make sure behavior meets expectations.

See also Zig Test

threadlocal

threadlocal can be used to specify a variable as thread-local.

See also Thread Local Variables

true

The boolean value true.

See also Primitive Values

try

try evaluates an error union expression. If it is an error, it returns from the current function with the same error. Otherwise, the expression results in the unwrapped value.

See also try

undefined

undefined can be used to leave a value uninitialized.

See also undefined

union

union defines a union.

See also union

unreachable

unreachable can be used to assert that control flow will never happen upon a particular location. Depending on the build mode, unreachable may emit a panic.

Emits a panic in Debug and ReleaseSafe mode, or when using zig test.

Does not emit a panic in ReleaseFast mode, unless zig test is being used.

See also unreachable

usingnamespace

usingnamespace is a top-level declaration that imports all the public declarations of the operand, which must be a struct, union, or enum, into the current scope.

See also usingnamespace

var

var declares a variable that may be modified.

See also Variables

volatile

volatile can be used to denote loads or stores of a pointer have side effects. It can also modify an inline assembly expression to denote it has side effects.

See also volatile, Assembly

while

A while expression can be used to repeatedly test a boolean, optional, or error union expression, and cease looping when that expression evaluates to false, null, or an error, respectively.

See also while

Grammar

Root <- skip container_doc_comment? ContainerMembers eof

# *** Top level ***
ContainerMembers <- ContainerDeclarations (ContainerField COMMA)* (ContainerField / ContainerDeclarations)

ContainerDeclarations
    <- TestDecl ContainerDeclarations
     / TopLevelComptime ContainerDeclarations
     / doc_comment? KEYWORD_pub? TopLevelDecl ContainerDeclarations
     /

TestDecl <- doc_comment? KEYWORD_test STRINGLITERALSINGLE? Block

TopLevelComptime <- doc_comment? KEYWORD_comptime BlockExpr

TopLevelDecl
    <- (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE? / (KEYWORD_inline / KEYWORD_noinline))? FnProto (SEMICOLON / Block)
     / (KEYWORD_export / KEYWORD_extern STRINGLITERALSINGLE?)? KEYWORD_threadlocal? VarDecl
     / KEYWORD_usingnamespace Expr SEMICOLON

FnProto <- KEYWORD_fn IDENTIFIER? LPAREN ParamDeclList RPAREN ByteAlign? LinkSection? CallConv? EXCLAMATIONMARK? TypeExpr

VarDecl <- (KEYWORD_const / KEYWORD_var) IDENTIFIER (COLON TypeExpr)? ByteAlign? LinkSection? (EQUAL Expr)? SEMICOLON

ContainerField <- doc_comment? KEYWORD_comptime? IDENTIFIER (COLON (KEYWORD_anytype / TypeExpr) ByteAlign?)? (EQUAL Expr)?

# *** Block Level ***
Statement
    <- KEYWORD_comptime? VarDecl
     / KEYWORD_comptime BlockExprStatement
     / KEYWORD_nosuspend BlockExprStatement
     / KEYWORD_suspend BlockExprStatement
     / KEYWORD_defer BlockExprStatement
     / KEYWORD_errdefer Payload? BlockExprStatement
     / IfStatement
     / LabeledStatement
     / SwitchExpr
     / AssignExpr SEMICOLON

IfStatement
    <- IfPrefix BlockExpr ( KEYWORD_else Payload? Statement )?
     / IfPrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )

LabeledStatement <- BlockLabel? (Block / LoopStatement)

LoopStatement <- KEYWORD_inline? (ForStatement / WhileStatement)

ForStatement
    <- ForPrefix BlockExpr ( KEYWORD_else Statement )?
     / ForPrefix AssignExpr ( SEMICOLON / KEYWORD_else Statement )

WhileStatement
    <- WhilePrefix BlockExpr ( KEYWORD_else Payload? Statement )?
     / WhilePrefix AssignExpr ( SEMICOLON / KEYWORD_else Payload? Statement )

BlockExprStatement
    <- BlockExpr
     / AssignExpr SEMICOLON

BlockExpr <- BlockLabel? Block

# *** Expression Level ***
AssignExpr <- Expr (AssignOp Expr)?

Expr <- BoolOrExpr

BoolOrExpr <- BoolAndExpr (KEYWORD_or BoolAndExpr)*

BoolAndExpr <- CompareExpr (KEYWORD_and CompareExpr)*

CompareExpr <- BitwiseExpr (CompareOp BitwiseExpr)?

BitwiseExpr <- BitShiftExpr (BitwiseOp BitShiftExpr)*

BitShiftExpr <- AdditionExpr (BitShiftOp AdditionExpr)*

AdditionExpr <- MultiplyExpr (AdditionOp MultiplyExpr)*

MultiplyExpr <- PrefixExpr (MultiplyOp PrefixExpr)*

PrefixExpr <- PrefixOp* PrimaryExpr

PrimaryExpr
    <- AsmExpr
     / IfExpr
     / KEYWORD_break BreakLabel? Expr?
     / KEYWORD_comptime Expr
     / KEYWORD_nosuspend Expr
     / KEYWORD_continue BreakLabel?
     / KEYWORD_resume Expr
     / KEYWORD_return Expr?
     / BlockLabel? LoopExpr
     / Block
     / CurlySuffixExpr

IfExpr <- IfPrefix Expr (KEYWORD_else Payload? Expr)?

Block <- LBRACE Statement* RBRACE

LoopExpr <- KEYWORD_inline? (ForExpr / WhileExpr)

ForExpr <- ForPrefix Expr (KEYWORD_else Expr)?

WhileExpr <- WhilePrefix Expr (KEYWORD_else Payload? Expr)?

CurlySuffixExpr <- TypeExpr InitList?

InitList
    <- LBRACE FieldInit (COMMA FieldInit)* COMMA? RBRACE
     / LBRACE Expr (COMMA Expr)* COMMA? RBRACE
     / LBRACE RBRACE

TypeExpr <- PrefixTypeOp* ErrorUnionExpr

ErrorUnionExpr <- SuffixExpr (EXCLAMATIONMARK TypeExpr)?

SuffixExpr
    <- KEYWORD_async PrimaryTypeExpr SuffixOp* FnCallArguments
     / PrimaryTypeExpr (SuffixOp / FnCallArguments)*

PrimaryTypeExpr
    <- BUILTINIDENTIFIER FnCallArguments
     / CHAR_LITERAL
     / ContainerDecl
     / DOT IDENTIFIER
     / DOT InitList
     / ErrorSetDecl
     / FLOAT
     / FnProto
     / GroupedExpr
     / LabeledTypeExpr
     / IDENTIFIER
     / IfTypeExpr
     / INTEGER
     / KEYWORD_comptime TypeExpr
     / KEYWORD_error DOT IDENTIFIER
     / KEYWORD_false
     / KEYWORD_null
     / KEYWORD_anyframe
     / KEYWORD_true
     / KEYWORD_undefined
     / KEYWORD_unreachable
     / STRINGLITERAL
     / SwitchExpr

ContainerDecl <- (KEYWORD_extern / KEYWORD_packed)? ContainerDeclAuto

ErrorSetDecl <- KEYWORD_error LBRACE IdentifierList RBRACE

GroupedExpr <- LPAREN Expr RPAREN

IfTypeExpr <- IfPrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?

LabeledTypeExpr
    <- BlockLabel Block
     / BlockLabel? LoopTypeExpr

LoopTypeExpr <- KEYWORD_inline? (ForTypeExpr / WhileTypeExpr)

ForTypeExpr <- ForPrefix TypeExpr (KEYWORD_else TypeExpr)?

WhileTypeExpr <- WhilePrefix TypeExpr (KEYWORD_else Payload? TypeExpr)?

SwitchExpr <- KEYWORD_switch LPAREN Expr RPAREN LBRACE SwitchProngList RBRACE

# *** Assembly ***
AsmExpr <- KEYWORD_asm KEYWORD_volatile? LPAREN Expr AsmOutput? RPAREN

AsmOutput <- COLON AsmOutputList AsmInput?

AsmOutputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN (MINUSRARROW TypeExpr / IDENTIFIER) RPAREN

AsmInput <- COLON AsmInputList AsmClobbers?

AsmInputItem <- LBRACKET IDENTIFIER RBRACKET STRINGLITERAL LPAREN Expr RPAREN

AsmClobbers <- COLON StringList

# *** Helper grammar ***
BreakLabel <- COLON IDENTIFIER

BlockLabel <- IDENTIFIER COLON

FieldInit <- DOT IDENTIFIER EQUAL Expr

WhileContinueExpr <- COLON LPAREN AssignExpr RPAREN

LinkSection <- KEYWORD_linksection LPAREN Expr RPAREN

# Fn specific
CallConv <- KEYWORD_callconv LPAREN Expr RPAREN

ParamDecl
    <- doc_comment? (KEYWORD_noalias / KEYWORD_comptime)? (IDENTIFIER COLON)? ParamType
     / DOT3

ParamType
    <- KEYWORD_anytype
     / TypeExpr

# Control flow prefixes
IfPrefix <- KEYWORD_if LPAREN Expr RPAREN PtrPayload?

WhilePrefix <- KEYWORD_while LPAREN Expr RPAREN PtrPayload? WhileContinueExpr?

ForPrefix <- KEYWORD_for LPAREN Expr RPAREN PtrIndexPayload

# Payloads
Payload <- PIPE IDENTIFIER PIPE

PtrPayload <- PIPE ASTERISK? IDENTIFIER PIPE

PtrIndexPayload <- PIPE ASTERISK? IDENTIFIER (COMMA IDENTIFIER)? PIPE

# Switch specific
SwitchProng <- SwitchCase EQUALRARROW PtrPayload? AssignExpr

SwitchCase
    <- SwitchItem (COMMA SwitchItem)* COMMA?
     / KEYWORD_else

SwitchItem <- Expr (DOT3 Expr)?

# Operators
AssignOp
    <- ASTERISKEQUAL
     / SLASHEQUAL
     / PERCENTEQUAL
     / PLUSEQUAL
     / MINUSEQUAL
     / LARROW2EQUAL
     / RARROW2EQUAL
     / AMPERSANDEQUAL
     / CARETEQUAL
     / PIPEEQUAL
     / ASTERISKPERCENTEQUAL
     / PLUSPERCENTEQUAL
     / MINUSPERCENTEQUAL
     / EQUAL

CompareOp
    <- EQUALEQUAL
     / EXCLAMATIONMARKEQUAL
     / LARROW
     / RARROW
     / LARROWEQUAL
     / RARROWEQUAL

BitwiseOp
    <- AMPERSAND
     / CARET
     / PIPE
     / KEYWORD_orelse
     / KEYWORD_catch Payload?

BitShiftOp
    <- LARROW2
     / RARROW2

AdditionOp
    <- PLUS
     / MINUS
     / PLUS2
     / PLUSPERCENT
     / MINUSPERCENT

MultiplyOp
    <- PIPE2
     / ASTERISK
     / SLASH
     / PERCENT
     / ASTERISK2
     / ASTERISKPERCENT

PrefixOp
    <- EXCLAMATIONMARK
     / MINUS
     / TILDE
     / MINUSPERCENT
     / AMPERSAND
     / KEYWORD_try
     / KEYWORD_await

PrefixTypeOp
    <- QUESTIONMARK
     / KEYWORD_anyframe MINUSRARROW
     / SliceTypeStart (ByteAlign / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
     / PtrTypeStart (KEYWORD_align LPAREN Expr (COLON INTEGER COLON INTEGER)? RPAREN / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
     / ArrayTypeStart

SuffixOp
    <- LBRACKET Expr (DOT2 (Expr? (COLON Expr)?)?)? RBRACKET
     / DOT IDENTIFIER
     / DOTASTERISK
     / DOTQUESTIONMARK

FnCallArguments <- LPAREN ExprList RPAREN

# Ptr specific
SliceTypeStart <- LBRACKET (COLON Expr)? RBRACKET

PtrTypeStart
    <- ASTERISK
     / ASTERISK2
     / LBRACKET ASTERISK (LETTERC / COLON Expr)? RBRACKET

ArrayTypeStart <- LBRACKET Expr (COLON Expr)? RBRACKET

# ContainerDecl specific
ContainerDeclAuto <- ContainerDeclType LBRACE container_doc_comment? ContainerMembers RBRACE

ContainerDeclType
    <- KEYWORD_struct
     / KEYWORD_opaque
     / KEYWORD_enum (LPAREN Expr RPAREN)?
     / KEYWORD_union (LPAREN (KEYWORD_enum (LPAREN Expr RPAREN)? / Expr) RPAREN)?

# Alignment
ByteAlign <- KEYWORD_align LPAREN Expr RPAREN

# Lists
IdentifierList <- (doc_comment? IDENTIFIER COMMA)* (doc_comment? IDENTIFIER)?

SwitchProngList <- (SwitchProng COMMA)* SwitchProng?

AsmOutputList <- (AsmOutputItem COMMA)* AsmOutputItem?

AsmInputList <- (AsmInputItem COMMA)* AsmInputItem?

StringList <- (STRINGLITERAL COMMA)* STRINGLITERAL?

ParamDeclList <- (ParamDecl COMMA)* ParamDecl?

ExprList <- (Expr COMMA)* Expr?

# *** Tokens ***
eof <- !.
bin <- [01]
bin_ <- '_'? bin
oct <- [0-7]
oct_ <- '_'? oct
hex <- [0-9a-fA-F]
hex_ <- '_'? hex
dec <- [0-9]
dec_ <- '_'? dec

bin_int <- bin bin_*
oct_int <- oct oct_*
dec_int <- dec dec_*
hex_int <- hex hex_*

ox80_oxBF <- [\200-\277]
oxF4 <- '\364'
ox80_ox8F <- [\200-\217]
oxF1_oxF3 <- [\361-\363]
oxF0 <- '\360'
ox90_0xBF <- [\220-\277]
oxEE_oxEF <- [\356-\357]
oxED <- '\355'
ox80_ox9F <- [\200-\237]
oxE1_oxEC <- [\341-\354]
oxE0 <- '\340'
oxA0_oxBF <- [\240-\277]
oxC2_oxDF <- [\302-\337]

# From https://lemire.me/blog/2018/05/09/how-quickly-can-you-check-that-a-string-is-valid-unicode-utf-8/
# First Byte      Second Byte     Third Byte      Fourth Byte
# [0x00,0x7F]
# [0xC2,0xDF]     [0x80,0xBF]
#    0xE0         [0xA0,0xBF]     [0x80,0xBF]
# [0xE1,0xEC]     [0x80,0xBF]     [0x80,0xBF]
#    0xED         [0x80,0x9F]     [0x80,0xBF]
# [0xEE,0xEF]     [0x80,0xBF]     [0x80,0xBF]
#    0xF0         [0x90,0xBF]     [0x80,0xBF]     [0x80,0xBF]
# [0xF1,0xF3]     [0x80,0xBF]     [0x80,0xBF]     [0x80,0xBF]
#    0xF4         [0x80,0x8F]     [0x80,0xBF]     [0x80,0xBF]

mb_utf8_literal <-
       oxF4      ox80_ox8F ox80_oxBF ox80_oxBF
     / oxF1_oxF3 ox80_oxBF ox80_oxBF ox80_oxBF
     / oxF0      ox90_0xBF ox80_oxBF ox80_oxBF
     / oxEE_oxEF ox80_oxBF ox80_oxBF
     / oxED      ox80_ox9F ox80_oxBF
     / oxE1_oxEC ox80_oxBF ox80_oxBF
     / oxE0      oxA0_oxBF ox80_oxBF
     / oxC2_oxDF ox80_oxBF

ascii_char_not_nl_slash_squote <- [\000-\011\013-\046-\050-\133\135-\177]

char_escape
    <- "\\x" hex hex
     / "\\u{" hex+ "}"
     / "\\" [nr\\t'"]
char_char
    <- mb_utf8_literal
     / char_escape
     / ascii_char_not_nl_slash_squote

string_char
    <- char_escape
     / [^\\"\n]

container_doc_comment <- ('//!' [^\n]* [ \n]*)+
doc_comment <- ('///' [^\n]* [ \n]*)+
line_comment <- '//' ![!/][^\n]* / '////' [^\n]*
line_string <- ("\\\\" [^\n]* [ \n]*)+
skip <- ([ \n] / line_comment)*

CHAR_LITERAL <- "'" char_char "'" skip
FLOAT
    <- "0x" hex_int "." hex_int ([pP] [-+]? dec_int)? skip
     /      dec_int "." dec_int ([eE] [-+]? dec_int)? skip
     / "0x" hex_int "."? [pP] [-+]? dec_int skip
     /      dec_int "."? [eE] [-+]? dec_int skip
INTEGER
    <- "0b" bin_int skip
     / "0o" oct_int skip
     / "0x" hex_int skip
     /      dec_int   skip
STRINGLITERALSINGLE <- "\"" string_char* "\"" skip
STRINGLITERAL
    <- STRINGLITERALSINGLE
     / (line_string                 skip)+
IDENTIFIER
    <- !keyword [A-Za-z_] [A-Za-z0-9_]* skip
     / "@\"" string_char* "\""                            skip
BUILTINIDENTIFIER <- "@"[A-Za-z_][A-Za-z0-9_]* skip

AMPERSAND            <- '&'      ![=]      skip
AMPERSANDEQUAL       <- '&='               skip
ASTERISK             <- '*'      ![*%=]    skip
ASTERISK2            <- '**'               skip
ASTERISKEQUAL        <- '*='               skip
ASTERISKPERCENT      <- '*%'     ![=]      skip
ASTERISKPERCENTEQUAL <- '*%='              skip
CARET                <- '^'      ![=]      skip
CARETEQUAL           <- '^='               skip
COLON                <- ':'                skip
COMMA                <- ','                skip
DOT                  <- '.'      ![*.?]    skip
DOT2                 <- '..'     ![.]      skip
DOT3                 <- '...'              skip
DOTASTERISK          <- '.*'               skip
DOTQUESTIONMARK      <- '.?'               skip
EQUAL                <- '='      ![>=]     skip
EQUALEQUAL           <- '=='               skip
EQUALRARROW          <- '=>'               skip
EXCLAMATIONMARK      <- '!'      ![=]      skip
EXCLAMATIONMARKEQUAL <- '!='               skip
LARROW               <- '<'      ![<=]     skip
LARROW2              <- '<<'     ![=]      skip
LARROW2EQUAL         <- '<<='              skip
LARROWEQUAL          <- '<='               skip
LBRACE               <- '{'                skip
LBRACKET             <- '['                skip
LPAREN               <- '('                skip
MINUS                <- '-'      ![%=>]    skip
MINUSEQUAL           <- '-='               skip
MINUSPERCENT         <- '-%'     ![=]      skip
MINUSPERCENTEQUAL    <- '-%='              skip
MINUSRARROW          <- '->'               skip
PERCENT              <- '%'      ![=]      skip
PERCENTEQUAL         <- '%='               skip
PIPE                 <- '|'      ![|=]     skip
PIPE2                <- '||'               skip
PIPEEQUAL            <- '|='               skip
PLUS                 <- '+'      ![%+=]    skip
PLUS2                <- '++'               skip
PLUSEQUAL            <- '+='               skip
PLUSPERCENT          <- '+%'     ![=]      skip
PLUSPERCENTEQUAL     <- '+%='              skip
LETTERC              <- 'c'                skip
QUESTIONMARK         <- '?'                skip
RARROW               <- '>'      ![>=]     skip
RARROW2              <- '>>'     ![=]      skip
RARROW2EQUAL         <- '>>='              skip
RARROWEQUAL          <- '>='               skip
RBRACE               <- '}'                skip
RBRACKET             <- ']'                skip
RPAREN               <- ')'                skip
SEMICOLON            <- ';'                skip
SLASH                <- '/'      ![=]      skip
SLASHEQUAL           <- '/='               skip
TILDE                <- '~'                skip

end_of_word <- ![a-zA-Z0-9_] skip
KEYWORD_align       <- 'align'       end_of_word
KEYWORD_allowzero   <- 'allowzero'   end_of_word
KEYWORD_and         <- 'and'         end_of_word
KEYWORD_anyframe    <- 'anyframe'    end_of_word
KEYWORD_anytype     <- 'anytype'     end_of_word
KEYWORD_asm         <- 'asm'         end_of_word
KEYWORD_async       <- 'async'       end_of_word
KEYWORD_await       <- 'await'       end_of_word
KEYWORD_break       <- 'break'       end_of_word
KEYWORD_callconv    <- 'callconv'    end_of_word
KEYWORD_catch       <- 'catch'       end_of_word
KEYWORD_comptime    <- 'comptime'    end_of_word
KEYWORD_const       <- 'const'       end_of_word
KEYWORD_continue    <- 'continue'    end_of_word
KEYWORD_defer       <- 'defer'       end_of_word
KEYWORD_else        <- 'else'        end_of_word
KEYWORD_enum        <- 'enum'        end_of_word
KEYWORD_errdefer    <- 'errdefer'    end_of_word
KEYWORD_error       <- 'error'       end_of_word
KEYWORD_export      <- 'export'      end_of_word
KEYWORD_extern      <- 'extern'      end_of_word
KEYWORD_false       <- 'false'       end_of_word
KEYWORD_fn          <- 'fn'          end_of_word
KEYWORD_for         <- 'for'         end_of_word
KEYWORD_if          <- 'if'          end_of_word
KEYWORD_inline      <- 'inline'      end_of_word
KEYWORD_noalias     <- 'noalias'     end_of_word
KEYWORD_nosuspend   <- 'nosuspend'   end_of_word
KEYWORD_noinline    <- 'noinline'    end_of_word
KEYWORD_null        <- 'null'        end_of_word
KEYWORD_opaque      <- 'opaque'      end_of_word
KEYWORD_or          <- 'or'          end_of_word
KEYWORD_orelse      <- 'orelse'      end_of_word
KEYWORD_packed      <- 'packed'      end_of_word
KEYWORD_pub         <- 'pub'         end_of_word
KEYWORD_resume      <- 'resume'      end_of_word
KEYWORD_return      <- 'return'      end_of_word
KEYWORD_linksection <- 'linksection' end_of_word
KEYWORD_struct      <- 'struct'      end_of_word
KEYWORD_suspend     <- 'suspend'     end_of_word
KEYWORD_switch      <- 'switch'      end_of_word
KEYWORD_test        <- 'test'        end_of_word
KEYWORD_threadlocal <- 'threadlocal' end_of_word
KEYWORD_true        <- 'true'        end_of_word
KEYWORD_try         <- 'try'         end_of_word
KEYWORD_undefined   <- 'undefined'   end_of_word
KEYWORD_union       <- 'union'       end_of_word
KEYWORD_unreachable <- 'unreachable' end_of_word
KEYWORD_usingnamespace <- 'usingnamespace' end_of_word
KEYWORD_var         <- 'var'         end_of_word
KEYWORD_volatile    <- 'volatile'    end_of_word
KEYWORD_while       <- 'while'       end_of_word

keyword <- KEYWORD_align / KEYWORD_allowzero / KEYWORD_and / KEYWORD_anyframe
         / KEYWORD_anytype / KEYWORD_asm / KEYWORD_async / KEYWORD_await
         / KEYWORD_break / KEYWORD_callconv / KEYWORD_catch / KEYWORD_comptime
         / KEYWORD_const / KEYWORD_continue / KEYWORD_defer / KEYWORD_else
         / KEYWORD_enum / KEYWORD_errdefer / KEYWORD_error / KEYWORD_export
         / KEYWORD_extern / KEYWORD_false / KEYWORD_fn / KEYWORD_for / KEYWORD_if
         / KEYWORD_inline / KEYWORD_noalias / KEYWORD_nosuspend / KEYWORD_noinline
         / KEYWORD_null / KEYWORD_opaque / KEYWORD_or / KEYWORD_orelse / KEYWORD_packed
         / KEYWORD_pub / KEYWORD_resume / KEYWORD_return / KEYWORD_linksection
         / KEYWORD_struct / KEYWORD_suspend / KEYWORD_switch
         / KEYWORD_test / KEYWORD_threadlocal / KEYWORD_true / KEYWORD_try
         / KEYWORD_undefined / KEYWORD_union / KEYWORD_unreachable
         / KEYWORD_usingnamespace / KEYWORD_var / KEYWORD_volatile / KEYWORD_while

Zen

Communicate intent precisely.

Edge cases matter.

Favor reading code over writing code.

Only one obvious way to do things.

Runtime crashes are better than bugs.

Compile errors are better than runtime crashes.

Incremental improvements.

Avoid local maximums.

Reduce the amount one must remember.

Focus on code rather than style.

Resource allocation may fail; resource deallocation must succeed.

Memory is a resource.

Together we serve the users.