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Introduction

Zig is an open-source programming language designed for robustness, optimality, and clarity.

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

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

Clear - precisely communicate your intent to the compiler and other programmers. The language imposes a low overhead to reading code.

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.

Index

Introduction

Index

Hello World

Comments

Doc comments

Values

Primitive Types

Primitive Values

String Literals and Character Literals

Escape Sequences

Multiline String Literals

Assignment

undefined

Variables

Global Variables

Thread Local Variables

Local Variables

Integers

Integer Literals

Runtime Integer Values

Floats

Float Literals

Floating Point Operations

Operators

Table of Operators

Precedence

Arrays

Vectors

SIMD

Pointers

volatile

Alignment

allowzero

Slices

struct

extern struct

packed struct

struct Naming

enum

extern enum

packed enum

union

Tagged union

extern union

packed union

blocks

Shadowing

switch

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 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

Implicit Casts

Implicit Cast: Stricter Qualification

Implicit Cast: Integer and Float Widening

Implicit Cast: Arrays and Pointers

Implicit Cast: Optionals

Implicit Cast: Error Unions

Implicit Cast: Compile-Time Known Numbers

Implicit Cast: unions and enums

Implicit Cast: Zero Bit Types

Implicit Cast: undefined

Explicit Casts

Peer Type Resolution

Zero Bit Types

void

Pointers to Zero Bit Types

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

Coroutines

Minimal Coroutine Example

Suspend and Resume

Resuming from Suspend Blocks

Await

Open Issues

Builtin Functions

@addWithOverflow

@alignCast

@alignOf

@ArgType

@atomicLoad

@atomicRmw

@bitCast

@bitOffsetOf

@boolToInt

@breakpoint

@bswap

@bitreverse

@byteOffsetOf

@bytesToSlice

@cDefine

@cImport

@cInclude

@clz

@cmpxchgStrong

@cmpxchgWeak

@compileError

@compileLog

@ctz

@cUndef

@divExact

@divFloor

@divTrunc

@embedFile

@enumToInt

@errorName

@errorReturnTrace

@errorToInt

@errSetCast

@export

@fence

@field

@fieldParentPtr

@floatCast

@floatToInt

@frameAddress

@handle

@import

@inlineCall

@intCast

@intToEnum

@intToError

@intToFloat

@intToPtr

@IntType

@memberCount

@memberName

@memberType

@memcpy

@memset

@mod

@mulWithOverflow

@newStackCall

@noInlineCall

@OpaqueType

@panic

@popCount

@ptrCast

@ptrToInt

@rem

@returnAddress

@setAlignStack

@setCold

@setEvalBranchQuota

@setFloatMode

@setGlobalLinkage

@setRuntimeSafety

@shlExact

@shlWithOverflow

@shrExact

@sizeOf

@sliceToBytes

@sqrt

@subWithOverflow

@tagName

@TagType

@This

@truncate

@typeId

@typeInfo

@typeName

@typeOf

@Vector

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

Slice Widen 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

C

C Type Primitives

C String Literals

Import from C Header File

C Pointers

Exporting a C Library

Mixing Object Files

Targets

Style Guide

Whitespace

Names

Examples

Source Encoding

Keyword Reference

Keyword: pub

Grammar

Zen

Hello World

hello.zig

const std = @import("std");

pub fn main() !void {
    // If this program is run without stdout attached, exit with an error.
    const stdout_file = try std.io.getStdOut();
    // If this program encounters pipe failure when printing to stdout, exit
    // with an error.
    try stdout_file.write("Hello, world!\n");
}

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

Usually you don't want to write to stdout. You want to write to stderr. And you don't care if it fails. It's more like a warning message that you want to emit. For that you can use a simpler API:

hello.zig

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

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

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

Note that we also left off the ! from the return type. In Zig, if your main function cannot fail, you must use the void return type.

See also:

Values

@import

Errors

Root Source File

Comments

comments.zig

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

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.

    //assert(false);

    const x = true;  // another comment
    assert(x);
}

$ zig test comments.zig
Test 1/1 comments...OK
All 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 have a doc comment in an unexpected place, such as in the middle of an expression, or just before a non-doc comment.

Values

values.zig

// Top-level declarations are order-independent:
const warn = std.debug.warn;
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;
    warn("1 + 1 = {}\n", one_plus_one);

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

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

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

    warn("\noptional 1\ntype: {}\nvalue: {}\n",
        @typeName(@typeOf(optional_value)), optional_value);

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

    warn("\noptional 2\ntype: {}\nvalue: {}\n",
        @typeName(@typeOf(optional_value)), optional_value);

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

    warn("\nerror union 1\ntype: {}\nvalue: {}\n",
        @typeName(@typeOf(number_or_error)), number_or_error);

    number_or_error = 1234;

    warn("\nerror union 2\ntype: {}\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

@IntType

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 Character Literals

String literals are UTF-8 encoded byte arrays.

Character literals have type comptime_int, the same as Integer Literals. All Escape Sequences are valid in both string literals and character literals. Once https://github.com/ziglang/zig/issues/2097 is implemented, character literals will be allowed to have a single UTF-8 encoded codepoint.

test.zig

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

test "string literals" {
    // In Zig a string literal is an array of bytes.
    const normal_bytes = "hello";
    assert(@typeOf(normal_bytes) == [5]u8);
    assert(normal_bytes.len == 5);
    assert(normal_bytes[1] == 'e');
    assert('e' == '\x65');
    assert('\U01f4a9' == 128169);
    assert(mem.eql(u8, "hello", "h\x65llo"));

    // A C string literal is a null terminated pointer.
    const null_terminated_bytes = c"hello";
    assert(@typeOf(null_terminated_bytes) == [*]const u8);
    assert(null_terminated_bytes[5] == 0);
}

$ zig test test.zig
Test 1/1 string literals...OK
All 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 character code (2 digits)
\uNNNN | hexadecimal 16-bit Unicode character code UTF-8 encoded (4 digits)
\UNNNNNN | hexadecimal 24-bit Unicode character code UTF-8 encoded (6 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;
    \\}
;

For a multiline C string literal, prepend c to each \\:

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

In this example the variable c_string_literal has type [*]const u8 and has a terminating null byte.

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 global 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
/home/andy/dev/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 assert = @import("std").debug.assert;

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

    y += 1;

    assert(y == 5679);
}

$ zig test test.zig
Test 1/1 var...OK
All tests passed.

Variables must be initialized:

test.zig

test "initialization" {
    var x: i32;

    x = 1;
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:2:5: error: variables must be initialized
    var x: i32;
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:4:5: error: use of undeclared identifier 'x'
    x = 1;
    ^

undefined

Use undefined to leave variables uninitialized:

test.zig

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

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

$ zig test test.zig
Test 1/1 init with undefined...OK
All tests passed.

undefined can be implicitly cast 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.

Global Variables

Global variables are considered to be a top level declaration, which means that they are order-independent and lazily analyzed. The initialization value of global variables is implicitly comptime. If a global variable is const then its value is comptime-known, otherwise it is runtime-known.

global_variables.zig

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

test "global variables" {
    assert(x == 46);
    assert(y == 56);
}

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

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

$ zig test global_variables.zig
Test 1/1 global variables...OK
All tests passed.

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

namespaced_global.zig

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

test "namespaced global variable" {
    assert(foo() == 1235);
    assert(foo() == 1236);
}

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

$ zig test namespaced_global.zig
Test 1/1 namespaced global variable...OK
All tests passed.

The extern keyword 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.os.spawnThread({}, testTls);
    const thread2 = try std.os.spawnThread({}, 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 thread local storage...OK
All tests passed.

For Single Threaded Builds, all thread local variables are treated as Global 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 assert = std.debug.assert;

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

    x += 1;
    y += 1;

    assert(x == 2);
    assert(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 comptime vars...OK
All 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;

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 implicitly cast 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;

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 builtin = @import("builtin");
const big = f64(1 << 40);

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

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

$ zig build-obj foo.zig --release-fast

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 warn = @import("std").debug.warn;

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

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

$ zig build-exe float_mode.zig --object 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.

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.

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.

-%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.

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.

For non-compile-time-known signed integers, must use @divTrunc, @divFloor, or @divExact instead of /.

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.

For non-compile-time-known signed integers, must use @rem or @mod instead of %.

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.

~u8(0b0101111) == 0b1010000

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

Arrays

arrays.zig

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 an array literal
const same_message = "hello";

comptime {
    assert(mem.eql(u8, message, same_message));
    assert(@typeOf(message) == @typeOf(same_message));
}

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

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

test "modify an array" {
    for (some_integers) |*item, i| {
        item.* = @intCast(i32, i);
    }
    assert(some_integers[10] == 10);
    assert(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 initalization" {
    assert(fancy_array[4].x == 4);
    assert(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" {
    assert(more_points[4].x == 3);
    assert(more_points[4].y == 6);
    assert(more_points.len == 10);
}

$ zig test arrays.zig
Test 1/4 iterate over an array...OK
Test 2/4 modify an array...OK
Test 3/4 compile-time array initalization...OK
Test 4/4 array initialization with function calls...OK
All tests passed.

See also:

for

Slices

Vectors

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

*T - pointer to exactly one item.

Supports deref syntax: ptr.*

[*]T - 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 @OpaqueType.

These types are closely related to Arrays and Slices:

*[N]T - pointer to N items, same as single-item pointer to 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 assert = @import("std").debug.assert;

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

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

    // When you get the address of a const variable, you get a const pointer to a single item.
    assert(@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;
    assert(@typeOf(y_ptr) == *i32);
    y_ptr.* += 1;
    assert(y_ptr.* == 5679);
}

test "pointer array access" {
    // Taking an address of an individual element gives a
    // pointer to a single item. 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];
    assert(@typeOf(ptr) == *u8);

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

$ zig test test.zig
Test 1/2 address of syntax...OK
Test 2/2 pointer array access...OK
All tests passed.

In Zig, we prefer slices over pointers to null-terminated arrays. 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 assert = @import("std").debug.assert;

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

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

$ zig test test.zig
Test 1/1 pointer slicing...OK
All tests passed.

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

test.zig

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

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

$ zig test test.zig
Test 1/1 comptime pointers...OK
All tests passed.

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

test.zig

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

test "@ptrToInt and @intToPtr" {
    const ptr = @intToPtr(*i32, 0xdeadbeef);
    const addr = @ptrToInt(ptr);
    assert(@typeOf(addr) == usize);
    assert(addr == 0xdeadbeef);
}

$ zig test test.zig
Test 1/1 @ptrToInt and @intToPtr...OK
All tests passed.

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

test.zig

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

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

$ zig test test.zig
Test 1/1 comptime @intToPtr...OK
All 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 assert = @import("std").debug.assert;

test "volatile" {
    const mmio_ptr = @intToPtr(*volatile u8, 0x12345678);
    assert(@typeOf(mmio_ptr) == *volatile u8);
}

$ zig test test.zig
Test 1/1 volatile...OK
All 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 assert = @import("std").debug.assert;

test "pointer casting" {
    const bytes align(@alignOf(u32)) = []u8{ 0x12, 0x12, 0x12, 0x12 };
    const u32_ptr = @ptrCast(*const u32, &bytes);
    assert(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 = @bytesToSlice(u32, bytes[0..])[0];
    assert(u32_value == 0x12121212);

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

test "pointer child type" {
    // pointer types have a `child` field which tells you the type they point to.
    assert((*u32).Child == u32);
}

$ zig test test.zig
Test 1/2 pointer casting...OK
Test 2/2 pointer child type...OK
All 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 assert = @import("std").debug.assert;
const builtin = @import("builtin");

test "variable alignment" {
    var x: i32 = 1234;
    const align_of_i32 = @alignOf(@typeOf(x));
    assert(@typeOf(&x) == *i32);
    assert(*i32 == *align(align_of_i32) i32);
    if (builtin.arch == builtin.Arch.x86_64) {
        assert((*i32).alignment == 4);
    }
}

$ zig test test.zig
Test 1/1 variable alignment...OK
All tests passed.

In the same way that a *i32 can be implicitly cast 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 assert = @import("std").debug.assert;

var foo: u8 align(4) = 100;

test "global variable alignment" {
    assert(@typeOf(&foo).alignment == 4);
    assert(@typeOf(&foo) == *align(4) u8);
    const slice = (*[1]u8)(&foo)[0..];
    assert(@typeOf(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" {
    assert(derp() == 1234);
    assert(@typeOf(noop1) == fn() align(1) void);
    assert(@typeOf(noop4) == fn() align(4) void);
    noop1();
    noop4();
}

$ zig test test.zig
Test 1/2 global variable alignment...OK
Test 2/2 function alignment...OK
All 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 assert = @import("std").debug.assert;

test "pointer alignment safety" {
    var array align(4) = []u32{ 0x11111111, 0x11111111 };
    const bytes = @sliceToBytes(array[0..]);
    assert(foo(bytes) == 0x11111111);
}
fn foo(bytes: []u8) u32 {
    const slice4 = bytes[1..5];
    const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
    return int_slice[0];
}

$ zig test test.zig
Test 1/1 pointer alignment safety...incorrect alignment
/home/andy/dev/zig/docgen_tmp/test.zig:10:56: 0x204346 in ??? (test)
    const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
                                                       ^
/home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x20412b in ??? (test)
    assert(foo(bytes) == 0x11111111);
              ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x225d2b in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x2254b6 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225221 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

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. 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 assert = std.debug.assert;

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

$ zig test allowzero.zig
Test 1/1 allowzero...OK
All tests passed.

Slices

test.zig

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

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.
    const slice = array[0..array.len];
    assert(&slice[0] == &array[0]);
    assert(slice.len == array.len);

    // Using the address-of operator on a slice gives a pointer to a single
    // item, while using the `ptr` field gives an unknown length pointer.
    assert(@typeOf(slice.ptr) == [*]i32);
    assert(@typeOf(&slice[0]) == *i32);
    assert(@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 basic slices...index out of bounds
/home/andy/dev/zig/docgen_tmp/test.zig:21:10: 0x2041e5 in ??? (test)
    slice[10] += 1;
         ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x225d0b in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x225496 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225201 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

This is one reason we prefer slices to pointers.

slices.zig

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

test "using slices for strings" {
    // Zig has no concept of strings. String literals are arrays of u8, and
    // in general the string type is []u8 (slice of u8).
    // Here we implicitly cast [5]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, "{} {}", 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.
    assert(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;
    assert(slice[2] == 3);
    // The slice is mutable because we sliced a mutable pointer.
    assert(@typeOf(slice) == []u8);

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

test "slice widening" {
    // Zig supports slice widening and slice narrowing. Cast a slice of u8
    // to a slice of anything else, and Zig will perform the length conversion.
    const array align(@alignOf(u32)) = []u8{ 0x12, 0x12, 0x12, 0x12, 0x13, 0x13, 0x13, 0x13 };
    const slice = @bytesToSlice(u32, array[0..]);
    assert(slice.len == 2);
    assert(slice[0] == 0x12121212);
    assert(slice[1] == 0x13131313);
}

$ zig test slices.zig
Test 1/3 using slices for strings...OK
Test 2/3 slice pointer...OK
Test 3/3 slice widening...OK
All tests passed.

See also:

Pointers

for

Arrays

struct

structs.zig

// Declare a struct.
// Zig gives no guarantees about the order of fields and whether or
// not there will be padding.
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 assert = @import("std").debug.assert;
test "dot product" {
    const v1 = Vec3.init(1.0, 0.0, 0.0);
    const v2 = Vec3.init(0.0, 1.0, 0.0);
    assert(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:
    assert(Vec3.dot(v1, v2) == 0.0);
}

// Structs can have global declarations.
// Structs can have 0 fields.
const Empty = struct {
    pub const PI = 3.14;
};
test "struct namespaced variable" {
    assert(Empty.PI == 3.14);
    assert(@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);
    assert(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:
    assert(LinkedList(i32) == LinkedList(i32));

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

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

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

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

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

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

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

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

$ zig test test.zig
Test 1/1 @bitCast between packed structs...OK
All tests passed.

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

test.zig

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

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;
    assert(ptr.* == 2);
}

$ zig test test.zig
Test 1/1 pointer to non-byte-aligned field...OK
All 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 assert = std.debug.assert;

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" {
    assert(bar(&bit_field.b) == 2);
}

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

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:17:26: error: expected type '*const u3', found '*align(:3:1) u3'
    assert(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 assert = std.debug.assert;

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" {
    assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.b));
    assert(@ptrToInt(&bit_field.a) == @ptrToInt(&bit_field.c));
}

$ zig test test.zig
Test 1/1 pointer to non-bit-aligned field...OK
All tests passed.

This can be observed with @bitOffsetOf and byteOffsetOf:

test.zig

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

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

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

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

$ zig test test.zig
Test 1/1 pointer to non-bit-aligned field...OK
All 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
/home/andy/dev/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 planned to be able to set alignment of struct fields.

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.warn("variable: {}\n", @typeName(Foo));
    std.debug.warn("anonymous: {}\n", @typeName(struct {}));
    std.debug.warn("function: {}\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:6:49
function: List(i32)

See also:

comptime

@fieldParentPtr

enum

enums.zig

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

// Declare an enum.
const Type = enum {
    Ok,
    NotOk,
};

// 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" {
    assert(@enumToInt(Value.Zero) == 0);
    assert(@enumToInt(Value.One) == 1);
    assert(@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" {
    assert(@enumToInt(Value2.Hundred) == 100);
    assert(@enumToInt(Value2.Thousand) == 1000);
    assert(@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;
    assert(!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",
    };
    assert(mem.eql(u8, what_is_it, "this is a number"));
}

// @TagType can be used to access the integer tag type of an enum.
const Small = enum {
    One,
    Two,
    Three,
    Four,
};
test "@TagType" {
    assert(@TagType(Small) == u2);
}

// @memberCount tells how many fields an enum has:
test "@memberCount" {
    assert(@memberCount(Small) == 4);
}

// @memberName tells the name of a field in an enum:
test "@memberName" {
    assert(mem.eql(u8, @memberName(Small, 1), "Two"));
}

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

$ zig test enums.zig
Test 1/8 enum ordinal value...OK
Test 2/8 set enum ordinal value...OK
Test 3/8 enum method...OK
Test 4/8 enum variant switch...OK
Test 5/8 @TagType...OK
Test 6/8 @memberCount...OK
Test 7/8 @memberName...OK
Test 8/8 @tagName...OK
All tests passed.

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
/home/andy/dev/zig/docgen_tmp/test.zig:2:22: error: parameter of type 'Foo' not allowed in function with calling convention 'ccc'
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

packed enum

By default, the size of enums is not guaranteed.

packed enum causes the size of the enum to be the same as the size of the integer tag type of the enum:

test.zig

const std = @import("std");

test "packed enum" {
    const Number = packed enum(u8) {
        One,
        Two,
        Three,
    };
    std.debug.assert(@sizeOf(Number) == @sizeOf(u8));
}

$ zig test test.zig
Test 1/1 packed enum...OK
All tests passed.

This makes the enum eligible to be in a packed struct.

See also:

@memberName

@memberCount

@tagName

@sizeOf

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,
    Bool: bool,
};
test "simple union" {
    var payload = Payload{ .Int = 1234 };
    payload.Float = 12.34;
}

$ zig test test.zig
Test 1/1 simple union...access of inactive union field
/home/andy/dev/zig/docgen_tmp/test.zig:8:12: 0x20410f in ??? (test)
    payload.Float = 12.34;
           ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x225bdb in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x225366 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x2250d1 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

You can activate another field by assigning the entire union:

test.zig

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

const Payload = union {
    Int: i64,
    Float: f64,
    Bool: bool,
};
test "simple union" {
    var payload = Payload{ .Int = 1234 };
    assert(payload.Int == 1234);
    payload = Payload{ .Float = 12.34 };
    assert(payload.Float == 12.34);
}

$ zig test test.zig
Test 1/1 simple union...OK
All tests passed.

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

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. One can use @TagType to obtain the enum type from the union type.

test.zig

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

const ComplexTypeTag = enum {
    Ok,
    NotOk,
};
const ComplexType = union(ComplexTypeTag) {
    Ok: u8,
    NotOk: void,
};

test "switch on tagged union" {
    const c = ComplexType{ .Ok = 42 };
    assert(ComplexTypeTag(c) == ComplexTypeTag.Ok);

    switch (c) {
        ComplexTypeTag.Ok => |value| assert(value == 42),
        ComplexTypeTag.NotOk => unreachable,
    }
}

test "@TagType" {
    assert(@TagType(ComplexType) == ComplexTypeTag);
}

$ zig test test.zig
Test 1/2 switch on tagged union...OK
Test 2/2 @TagType...OK
All 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 assert = std.debug.assert;

const ComplexTypeTag = enum {
    Ok,
    NotOk,
};
const ComplexType = union(ComplexTypeTag) {
    Ok: u8,
    NotOk: void,
};

test "modify tagged union in switch" {
    var c = ComplexType{ .Ok = 42 };
    assert(ComplexTypeTag(c) == ComplexTypeTag.Ok);

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

    assert(c.Ok == 43);
}

$ zig test test.zig
Test 1/1 modify tagged union in switch...OK
All 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 assert = std.debug.assert;

const Variant = union(enum) {
    Int: i32,
    Bool: 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.Bool => |x_bool| x_bool,
            Variant.None => false,
        };
    }
};

test "union method" {
    var v1 = Variant{ .Int = 1 };
    var v2 = Variant{ .Bool = false };

    assert(v1.truthy());
    assert(!v2.truthy());
}

$ zig test test.zig
Test 1/1 union method...OK
All tests passed.

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

test.zig

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

const Small2 = union(enum) {
    A: i32,
    B: bool,
    C: u8,
};
test "@tagName" {
    assert(std.mem.eql(u8, @tagName(Small2.C), "C"));
}

$ zig test test.zig
Test 1/1 @tagName...OK
All 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.

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
/home/andy/dev/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 assert = std.debug.assert;

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

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

$ zig test test.zig
Test 1/1 labeled break from labeled block expression...OK
All 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
/home/andy/dev/zig/docgen_tmp/test.zig:6:9: error: redefinition of 'pi'
        var pi: i32 = 1234;
        ^
/home/andy/dev/zig/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 separate scopes...OK
All tests passed.

switch

switch.zig

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

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,
    };

    assert(b == 1);
}

// Switch expressions can be used outside a function:
const os_msg = switch (builtin.os) {
    builtin.Os.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 (builtin.os) {
        builtin.Os.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 switch simple...OK
Test 2/2 switch inside function...OK
All 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 assert = @import("std").debug.assert;

test "switch on tagged union" {
    const Point = struct {
        x: u8,
        y: u8,
    };
    const Item = union(enum) {
        A: u32,
        C: Point,
        D,
    };

    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.
        Item.A => |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,
    };

    assert(b == 6);
    assert(a.C.x == 2);
}

$ zig test test.zig
Test 1/1 switch on tagged union...OK
All tests passed.

See also:

comptime

enum

@compileError

Compile Variables

while

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

while.zig

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

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

$ zig test while.zig
Test 1/1 while basic...OK
All tests passed.

Use break to exit a while loop early.

while.zig

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

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

$ zig test while.zig
Test 1/1 while break...OK
All tests passed.

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

while.zig

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

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

$ zig test while.zig
Test 1/1 while continue...OK
All 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 assert = @import("std").debug.assert;

test "while loop continue expression" {
    var i: usize = 0;
    while (i < 10) : (i += 1) {}
    assert(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;
        assert(my_ij < 2000);
    }
}

$ zig test while.zig
Test 1/2 while loop continue expression...OK
Test 2/2 while loop continue expression, more complicated...OK
All 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 assert = @import("std").debug.assert;

test "while else" {
    assert(rangeHasNumber(0, 10, 5));
    assert(!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 while else...OK
All 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 nested break...OK
Test 2/2 nested continue...OK
All 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 assert = @import("std").debug.assert;

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

    var sum2: u32 = 0;
    numbers_left = 3;
    while (eventuallyNullSequence()) |value| {
        sum2 += value;
    } else {
        assert(sum1 == 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 while null capture...OK
All 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 assert = @import("std").debug.assert;

test "while error union capture" {
    var sum1: u32 = 0;
    numbers_left = 3;
    while (eventuallyErrorSequence()) |value| {
        sum1 += value;
    } else |err| {
        assert(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 while error union capture...OK
All 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 assert = @import("std").debug.assert;

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);
    }
    assert(sum == 9);
}

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

$ zig test test.zig
Test 1/1 inline while loop...OK
All 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 assert = @import("std").debug.assert;

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;
    }
    assert(sum == 16);

    // To iterate over a portion of a slice, reslice.
    for (items[0..1]) |value| {
        sum += value;
    }
    assert(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| {
        assert(@typeOf(i) == usize);
        sum2 += @intCast(i32, i);
    }
    assert(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;
    }

    assert(items[0] == 4);
    assert(items[1] == 5);
    assert(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.
    var sum: i32 = 0;
    const result = for (items) |value| {
        if (value == null) {
            break 9;
        } else {
            sum += value.?;
        }
    } else blk: {
        assert(sum == 7);
        break :blk sum;
    };
}

$ zig test for.zig
Test 1/3 for basics...OK
Test 2/3 for reference...OK
Test 3/3 for else...OK
All 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 assert = std.debug.assert;

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;
        }
    }
    assert(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;
        }
    }

    assert(count == 8);
}

$ zig test test.zig
Test 1/2 nested break...OK
Test 2/2 nested continue...OK
All 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 assert = @import("std").debug.assert;

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);
    }
    assert(sum == 9);
}

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

$ zig test test.zig
Test 1/1 inline for loop...OK
All 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 assert = @import("std").debug.assert;

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

    // If expressions are used instead of a ternary expression.
    const result = if (a != b) 47 else 3089;
    assert(result == 47);
}

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

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

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

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

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

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

    if (c) |value| {
        assert(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| {
        assert(value == 0);
    } else |err| {
        unreachable;
    }

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

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

    // To check only the error value, use an empty block expression.
    if (b) |_| {} else |err| {
        assert(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| {
        assert(value == 9);
    } else |err| {
        unreachable;
    }
}

$ zig test if.zig
Test 1/3 if boolean...OK
Test 2/3 if optional...OK
Test 3/3 if error union...OK
All tests passed.

See also:

Optionals

Errors

defer

defer.zig

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

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

    {
        defer a = 2;
        a = 1;
    }
    assert(a == 2);

    a = 5;
    return a;
}

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

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

    defer {
        warn("1 ");
    }
    defer {
        warn("2 ");
    }
    if (false) {
        // defers are not run if they are never executed.
        defer {
            warn("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 {
    warn("\nstart of function\n");

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

    errdefer {
        warn("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 defer basics...OK
Test 2/3 defer unwinding...
2 1 OK
Test 3/3 errdefer unwinding...
start of function
end of function

start of function
encountered an error!
end of function
OK
All 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 basic math...OK
All tests passed.

In fact, this is how assert is implemented:

test.zig

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 this will fail...reached unreachable code
/home/andy/dev/zig/docgen_tmp/test.zig:2:14: 0x20424c in ??? (test)
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/dev/zig/docgen_tmp/test.zig:7:11: 0x2040cb in ??? (test)
    assert(false);
          ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x225beb in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x225376 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x2250e1 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

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
/home/andy/dev/zig/docgen_tmp/test.zig:10:16: error: unreachable code
        assert(@typeOf(unreachable) == noreturn);
               ^

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 noreturn...OK
All tests passed.

Another use case for noreturn is the exit function:

test.zig

pub extern "kernel32" stdcallcc fn ExitProcess(exit_code: c_uint) noreturn;

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

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

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

$ zig test test.zig
Created /home/andy/dev/zig/docgen_tmp/test.exe but skipping execution because it is non-native.

Functions

functions.zig

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

// 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 stdcallcc specifier changes the calling convention of the function.
extern "kernel32" stdcallcc fn ExitProcess(exit_code: u32) 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 nakedcc specifier makes a function not have any function prologue or epilogue.
// This can be useful when integrating with assembly.
nakedcc fn _start() noreturn {
    abort();
}

// The inline specifier forces a function to be inlined at all call sites.
// If the function cannot be inlined, it is a compile-time error.
inline fn shiftLeftOne(a: u32) 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" {
    assert(do_op(add, 5, 6) == 11);
    assert(do_op(sub2, 5, 6) == -1);
}

$ zig test functions.zig
Test 1/1 function...OK
All 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 assert = @import("std").debug.assert;

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

$ zig test test.zig
Test 1/1 pass struct to function...OK
All tests passed.

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

Function Reflection

test.zig

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

test "fn reflection" {
    assert(@typeOf(assert).ReturnType == void);
    assert(@typeOf(assert).is_var_args == false);
}

$ zig test test.zig
Test 1/1 fn reflection...OK
All 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 implicitly cast an error from a subset to its superset:

test.zig

const std = @import("std");

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

const AllocationError = error {
    OutOfMemory,
};

test "implicit cast subset to superset" {
    const err = foo(AllocationError.OutOfMemory);
    std.debug.assert(err == FileOpenError.OutOfMemory);
}

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

$ zig test test.zig
Test 1/1 implicit cast subset to superset...OK
All tests passed.

But you cannot implicitly cast an error from a superset to a subset:

test.zig

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

const AllocationError = error {
    OutOfMemory,
};

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

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

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:16:12: error: expected type 'AllocationError', found 'FileOpenError'
    return err;
           ^
/home/andy/dev/zig/docgen_tmp/test.zig:2:5: note: 'error.AccessDenied' not a member of destination error set
    AccessDenied,
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:4:5: note: 'error.FileNotFound' not a member of destination error set
    FileNotFound,
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:15:28: note: return type declared here
fn foo(err: FileOpenError) AllocationError {
                           ^

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 implicitly cast 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);
    std.debug.assert(result == 1234);
}

$ zig test test.zig
Test 1/1 parse u64...OK
All 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 implicitly cast 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 assert = @import("std").debug.assert;

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

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

    // Implicitly cast from an error set:
    foo = error.SomeError;

    // Use compile-time reflection to access the payload type of an error union:
    comptime assert(@typeOf(foo).Payload == i32);

    // Use compile-time reflection to access the error set type of an error union:
    comptime assert(@typeOf(foo).ErrorSet == anyerror);
}

$ zig test test.zig
Test 1/1 error union...OK
All 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 merge error sets...OK
All 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 inferred error set...OK
All 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/dev/zig/docgen_tmp/test.zig:39:5: 0x225da9 in ??? (test)
    return error.FileNotFound;
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:23:5: 0x225c8c in ??? (test)
    try bang1();
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:43:5: 0x225c59 in ??? (test)
    return error.PermissionDenied;
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:31:5: 0x225d7c in ??? (test)
    try bang2();
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:17:31: 0x225c27 in ??? (test)
        error.FileNotFound => try hello(),
                              ^
/home/andy/dev/zig/docgen_tmp/test.zig:7:9: 0x225b19 in ??? (test)
        try bar();
        ^
/home/andy/dev/zig/docgen_tmp/test.zig:2:5: 0x2259c1 in ??? (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
PermissionDenied
/home/andy/dev/zig/docgen_tmp/test.zig:38:5: 0x225887 in ??? (test)
    @panic("PermissionDenied");
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:30:10: 0x2258c9 in ??? (test)
    bang2();
         ^
/home/andy/dev/zig/docgen_tmp/test.zig:17:14: 0x22586b in ??? (test)
        hello();
             ^
/home/andy/dev/zig/docgen_tmp/test.zig:7:12: 0x225836 in ??? (test)
        bar();
           ^
/home/andy/dev/zig/docgen_tmp/test.zig:2:8: 0x22581e in ??? (test)
    foo(12);
       ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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

test "optional type" {
    // Declare an optional and implicitly cast from null:
    var foo: ?i32 = null;

    // Implicitly cast from child type of an optional
    foo = 1234;

    // Use compile-time reflection to access the child type of the optional:
    comptime assert(@typeOf(foo).Child == i32);
}

$ zig test test.zig
Test 1/1 optional type...OK
All 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 assert = @import("std").debug.assert;

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;

    assert(ptr.?.* == 1);

    // Optional pointers are the same size as normal pointers, because pointer
    // value 0 is used as the null value.
    assert(@sizeOf(?*i32) == @sizeOf(*i32));
}

$ zig test test.zig
Test 1/1 optional pointers...OK
All tests passed.

Casting

A type cast converts a value of one type to another. Zig has Implicit Casts 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.

Implicit Casts

An implicit cast occurs when one type is expected, but different type is provided:

test.zig

test "implicit cast - variable declaration" {
    var a: u8 = 1;
    var b: u16 = a;
}

test "implicit cast - function call" {
    var a: u8 = 1;
    foo(a);
}

fn foo(b: u16) void {}

test "implicit cast - invoke a type as a function" {
    var a: u8 = 1;
    var b = u16(a);
}

$ zig test test.zig
Test 1/3 implicit cast - variable declaration...OK
Test 2/3 implicit cast - function call...OK
Test 3/3 implicit cast - invoke a type as a function...OK
All tests passed.

Implicit casts 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.

Implicit Cast: 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 "implicit cast - 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 implicit cast - const qualification...OK
All tests passed.

In addition, pointers implicitly cast to const optional pointers:

test.zig

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

test "cast *[1][*]const u8 to [*]const ?[*]const u8" {
    const window_name = [1][*]const u8{c"window name"};
    const x: [*]const ?[*]const u8 = &window_name;
    assert(mem.eql(u8, std.cstr.toSliceConst(x[0].?), "window name"));
}

$ zig test test.zig
Test 1/1 cast *[1][*]const u8 to [*]const ?[*]const u8...OK
All tests passed.

Implicit Cast: Integer and Float Widening

Integers implicitly cast to integer types which can represent every value of the old type, and likewise Floats implicitly cast to float types which can represent every value of the old type.

test.zig

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

test "implicit unsigned integer to signed integer" {
    var a: u8 = 250;
    var b: i16 = a;
    assert(b == 250);
}

test "float widening" {
    var a: f16 = 12.34;
    var b: f32 = a;
    var c: f64 = b;
    var d: f128 = c;
    assert(d == a);
}

$ zig test test.zig
Test 1/3 integer widening...OK
Test 2/3 implicit unsigned integer to signed integer...OK
Test 3/3 float widening...OK
All tests passed.

Implicit Cast: Arrays and Pointers

test.zig

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

// This cast exists primarily so that string literals can be
// passed to functions that accept const slices. However
// it is probably going to be removed from the language when
// https://github.com/ziglang/zig/issues/265 is implemented.
test "[N]T to []const T" {
    var x1: []const u8 = "hello";
    var x2: []const u8 = [5]u8{ 'h', 'e', 'l', 'l', 111 };
    assert(std.mem.eql(u8, x1, x2));

    var y: []const f32 = [2]f32{ 1.2, 3.4 };
    assert(y[0] == 1.2);
}

// Likewise, it works when the destination type is an error union.
test "[N]T to E![]const T" {
    var x1: anyerror![]const u8 = "hello";
    var x2: anyerror![]const u8 = [5]u8{ 'h', 'e', 'l', 'l', 111 };
    assert(std.mem.eql(u8, try x1, try x2));

    var y: anyerror![]const f32 = [2]f32{ 1.2, 3.4 };
    assert((try y)[0] == 1.2);
}

// Likewise, it works when the destination type is an optional.
test "[N]T to ?[]const T" {
    var x1: ?[]const u8 = "hello";
    var x2: ?[]const u8 = [5]u8{ 'h', 'e', 'l', 'l', 111 };
    assert(std.mem.eql(u8, x1.?, x2.?));

    var y: ?[]const f32 = [2]f32{ 1.2, 3.4 };
    assert(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;
    assert(std.mem.eql(u8, x, "hello"));

    const buf2 = [2]f32{ 1.2, 3.4 };
    const x2: []const f32 = &buf2;
    assert(std.mem.eql(f32, x2, [2]f32{ 1.2, 3.4 }));
}

// Single-item pointers to arrays can be implicitly casted to
// unknown length pointers.
test "*[N]T to [*]T" {
    var buf: [5]u8 = "hello";
    const x: [*]u8 = &buf;
    assert(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;
    assert(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;
    assert(z[0] == 1234);
}

$ zig test test.zig
Test 1/7 [N]T to []const T...OK
Test 2/7 [N]T to E![]const T...OK
Test 3/7 [N]T to ?[]const T...OK
Test 4/7 *[N]T to []T...OK
Test 5/7 *[N]T to [*]T...OK
Test 6/7 *[N]T to ?[*]T...OK
Test 7/7 *T to *[1]T...OK
All tests passed.

See also:

C Pointers

Implicit Cast: Optionals

The payload type of Optionals, as well as null, implicitly cast to the optional type.

test.zig

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

test "implicit casting to optionals" {
    const x: ?i32 = 1234;
    const y: ?i32 = null;

    assert(x.? == 1234);
    assert(y == null);
}

$ zig test test.zig
Test 1/1 implicit casting to optionals...OK
All tests passed.

It works nested inside the Error Union Type, too:

test.zig

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

test "implicit casting to optionals wrapped in error union" {
    const x: anyerror!?i32 = 1234;
    const y: anyerror!?i32 = null;

    assert((try x).? == 1234);
    assert((try y) == null);
}

$ zig test test.zig
Test 1/1 implicit casting to optionals wrapped in error union...OK
All tests passed.

Implicit Cast: Error Unions

The the payload type of an Error Union Type as well as the Error Set Type implicitly cast to the error union type:

test.zig

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

test "implicit casting to error unions" {
    const x: anyerror!i32 = 1234;
    const y: anyerror!i32 = error.Failure;

    assert((try x) == 1234);
    std.testing.expectError(error.Failure, y);
}

$ zig test test.zig
Test 1/1 implicit casting to error unions...OK
All tests passed.

Implicit Cast: Compile-Time Known Numbers

When a number is comptime-known to be representable in the destination type, it may be implicitly casted:

test.zig

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

test "implicit casting large integer type to smaller one when value is comptime known to fit" {
    const x: u64 = 255;
    const y: u8 = x;
    assert(y == 255);
}

$ zig test test.zig
Test 1/1 implicit casting large integer type to smaller one when value is comptime known to fit...OK
All tests passed.

Implicit Cast: unions and enums

Tagged unions can be implicitly cast to enums, and enums can be implicitly casted 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 assert = std.debug.assert;

const E = enum {
    One,
    Two,
    Three,
};

const U = union(E) {
    One: i32,
    Two: f32,
    Three,
};

test "implicit casting between unions and enums" {
    var u = U{ .Two = 12.34 };
    var e: E = u;
    assert(e == E.Two);

    const three = E.Three;
    var another_u: U = three;
    assert(another_u == E.Three);
}

$ zig test test.zig
Test 1/1 implicit casting between unions and enums...OK
All tests passed.

See also:

union

enum

Implicit Cast: Zero Bit Types

Zero Bit Types may be implicitly casted 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 "implicit casting of zero bit types" {
    var x: void = {};
    var y: *void = x;
    //var z: void = y; // TODO
}

$ zig test test.zig
Test 1/1 implicit casting of zero bit types...OK
All tests passed.

Implicit Cast: 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

@bytesToSlice - convert a slice of bytes to a slice of another type

@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

@sliceToBytes - convert a slice of anything to a slice of bytes

@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 implicitly cast into. Here are some examples:

test.zig

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

test "peer resolve int widening" {
    var a: i8 = 12;
    var b: i16 = 34;
    var c = a + b;
    assert(c == 46);
    assert(@typeOf(c) == i16);
}

test "peer resolve arrays of different size to const slice" {
    assert(mem.eql(u8, boolToStr(true), "true"));
    assert(mem.eql(u8, boolToStr(false), "false"));
    comptime assert(mem.eql(u8, boolToStr(true), "true"));
    comptime assert(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" {
    testPeerResolveArrayConstSlice(true);
    comptime testPeerResolveArrayConstSlice(true);
}
fn testPeerResolveArrayConstSlice(b: bool) void {
    const value1 = if (b) "aoeu" else ([]const u8)("zz");
    const value2 = if (b) ([]const u8)("zz") else "aoeu";
    assert(mem.eql(u8, value1, "aoeu"));
    assert(mem.eql(u8, value2, "zz"));
}

test "peer type resolution: ?T and T" {
    assert(peerTypeTAndOptionalT(true, false).? == 0);
    assert(peerTypeTAndOptionalT(false, false).? == 3);
    comptime {
        assert(peerTypeTAndOptionalT(true, false).? == 0);
        assert(peerTypeTAndOptionalT(false, false).? == 3);
    }
}
fn peerTypeTAndOptionalT(c: bool, b: bool) ?usize {
    if (c) {
        return if (b) null else usize(0);
    }

    return usize(3);
}

test "peer type resolution: [0]u8 and []const u8" {
    assert(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
    assert(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
    comptime {
        assert(peerTypeEmptyArrayAndSlice(true, "hi").len == 0);
        assert(peerTypeEmptyArrayAndSlice(false, "hi").len == 1);
    }
}
fn peerTypeEmptyArrayAndSlice(a: bool, slice: []const u8) []const u8 {
    if (a) {
        return []const u8{};
    }

    return slice[0..1];
}
test "peer type resolution: [0]u8, []const u8, and anyerror![]u8" {
    {
        var data = "hi";
        const slice = data[0..];
        assert((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
        assert((try peerTypeEmptyArrayAndSliceAndError(false, slice)).len == 1);
    }
    comptime {
        var data = "hi";
        const slice = data[0..];
        assert((try peerTypeEmptyArrayAndSliceAndError(true, slice)).len == 0);
        assert((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, 0x123456789);
    const b = @intToPtr(?*usize, 0x123456789);
    assert(a == b);
    assert(b == a);
}

$ zig test test.zig
Test 1/7 peer resolve int widening...OK
Test 2/7 peer resolve arrays of different size to const slice...OK
Test 3/7 peer resolve array and const slice...OK
Test 4/7 peer type resolution: ?T and T...OK
Test 5/7 peer type resolution: [0]u8 and []const u8...OK
Test 6/7 peer type resolution: [0]u8, []const u8, and anyerror![]u8...OK
Test 7/7 peer type resolution: *const T and ?*T...OK
All 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.

An 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 assert = std.debug.assert;

test "turn HashMap into a set with void" {
    var map = std.HashMap(i32, void, hash_i32, eql_i32).init(std.debug.global_allocator);
    defer map.deinit();

    _ = try map.put(1, {});
    _ = try map.put(2, {});

    assert(map.contains(2));
    assert(!map.contains(3));

    _ = map.remove(2);
    assert(!map.contains(2));
}

fn hash_i32(x: i32) u32 {
    return @bitCast(u32, x);
}

fn eql_i32(a: i32, b: i32) bool {
    return a == b;
}

$ zig test test.zig
Test 1/1 turn HashMap into a set with void...OK
All tests passed.

Note that this is different than 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, which is defined like this: pub const c_void = @OpaqueType();. 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
/home/andy/dev/zig/docgen_tmp/test.zig:2:8: error: expression value is ignored
    foo();
       ^

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 void is ignored...OK
Test 2/2 explicitly ignoring expression value...OK
All 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 assert = std.debug.assert;

test "pointer to empty struct" {
    const Empty = struct {};
    var a = Empty{};
    var b = Empty{};
    var ptr_a = &a;
    var ptr_b = &b;
    comptime assert(ptr_a == ptr_b);
}

$ zig test test.zig
Test 1/1 pointer to empty struct...OK
All 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
/home/andy/dev/zig/docgen_tmp/test.zig:5:20: error: pointer to size 0 type has no address
    _ = @ptrToInt(&a);
                   ^
/home/andy/dev/zig/docgen_tmp/test.zig:9:19: error: type '*Empty' has 0 bits and cannot store information
    _ = @intToPtr(*Empty, 0x1);
                  ^

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
/home/andy/dev/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
/home/andy/dev/zig/docgen_tmp/test.zig:2:18: error: operator not allowed for type 'bool'
    return if (a > b) a else b;
                 ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:12: note: called from here
    _ = max(bool, true, false);
           ^

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" {
    @import("std").debug.assert(max(bool, false, true) == true);
}

$ zig test test.zig
Test 1/1 try to compare bools...OK
All 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 assert = @import("std").debug.assert;

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" {
    assert(performFn('t', 1) == 6);
    assert(performFn('o', 0) == 1);
    assert(performFn('w', 99) == 99);
}

$ zig test comptime_vars.zig
Test 1/1 perform fn...OK
All 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:
// assert(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:
// assert(performFn('o', 0) == 1);
fn performFn(start_value: i32) i32 {
    var result: i32 = start_value;
    result = one(result);
    return result;
}

// From the line:
// assert(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
/home/andy/dev/zig/docgen_tmp/test.zig:5:9: error: unable to evaluate constant expression
        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 assert = @import("std").debug.assert;

fn fibonacci(index: u32) u32 {
    if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    // test fibonacci at run-time
    assert(fibonacci(7) == 13);

    // test fibonacci at compile-time
    comptime {
        assert(fibonacci(7) == 13);
    }
}

$ zig test test.zig
Test 1/1 fibonacci...OK
All tests passed.

Imagine if we had forgotten the base case of the recursive function and tried to run the tests:

test.zig

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

fn fibonacci(index: u32) u32 {
    //if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        assert(fibonacci(7) == 13);
    }
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:5:28: error: operation caused overflow
    return fibonacci(index - 1) + fibonacci(index - 2);
                           ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:10:25: note: called from here
        assert(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 assert = @import("std").debug.assert;

fn fibonacci(index: i32) i32 {
    //if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        assert(fibonacci(7) == 13);
    }
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: error: evaluation exceeded 1000 backwards branches
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:21: note: called from here
    return fibonacci(index - 1) + fibonacci(index - 2);
                    ^

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 assert line?

test.zig

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

fn fibonacci(index: i32) i32 {
    if (index < 2) return index;
    return fibonacci(index - 1) + fibonacci(index - 2);
}

test "fibonacci" {
    comptime {
        assert(fibonacci(7) == 99999);
    }
}

$ zig test test.zig
/home/andy/dev/zig/build/lib/zig/std/debug.zig:169:14: error: unable to evaluate constant expression
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/dev/zig/docgen_tmp/test.zig:10:15: note: called from here
        assert(fibonacci(7) == 99999);
              ^

What happened is Zig started interpreting the assert function with the parameter ok set to false. When the interpreter hit unreachable it emitted a compile error, because reaching unreachable code is undefined behavior, and undefined behavior causes a compile error if it is detected at compile-time.

In the global scope (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" {
    @import("std").debug.assert(sum_of_first_25_primes == 1060);
}

$ zig test test.zig
Test 1/1 variable values...OK
All 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 warn = @import("std").debug.warn;

const a_number: i32 = 1234;
const a_string = "foobar";

pub fn main() void {
    warn("here is a string: '{}' 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: *OutStream, comptime format: []const u8, args: ...) anyerror!void {
    const State = enum {
        Start,
        OpenBrace,
        CloseBrace,
    };

    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.OpenBrace;
                },
                '}' => {
                    if (start_index < i) try self.write(format[start_index..i]);
                    state = State.CloseBrace;
                },
                else => {},
            },
            State.OpenBrace => 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.CloseBrace => 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: *OutStream, 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: *OutStream, value: var) !void {
    const T = @typeOf(value);
    if (@isInteger(T)) {
        return self.printInt(T, value);
    } else if (@isFloat(T)) {
        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 warn = @import("std").debug.warn;

const a_number: i32 = 1234;
const a_string = "foobar";

test "printf too many arguments" {
    warn("here is a string: '{}' here is a number: {}\n",
        a_string, a_number, a_number);
}

$ zig test test.zig
/home/andy/dev/zig/build/lib/zig/std/fmt.zig:94:13: error: Unused arguments
            @compileError("Unused arguments");
            ^
/home/andy/dev/zig/build/lib/zig/std/io.zig:222:34: note: called from here
            return std.fmt.format(self, Error, self.writeFn, format, args);
                                 ^
/home/andy/dev/zig/build/lib/zig/std/debug.zig:46:17: note: called from here
    stderr.print(fmt, args) catch return;
                ^
/home/andy/dev/zig/docgen_tmp/test.zig:7:9: note: called from here
    warn("here is a string: '{}' here is a number: {}\n",
        ^

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 is implicitly castable to a []const u8:

printf.zig

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

const a_number: i32 = 1234;
const a_string = "foobar";
const fmt = "here is a string: '{}' here is a number: {}\n";

pub fn main() void {
    warn(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
$ ./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/8.0.0/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 top 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 assert = std.debug.assert;

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" {
    assert(my_func(12, 34) == 46);
}

$ zig test global-asm.zig
Test 1/1 global assembly...OK
All tests passed.

Atomics

TODO: @fence()

TODO: @atomic rmw

TODO: builtin atomic memory ordering enum

Coroutines

A coroutine is a generalization of a function.

When you call a function, it creates a stack frame, and then the function runs until it reaches a return statement, and then the stack frame is destroyed. At the callsite, the next line of code does not run until the function returns.

A coroutine is like a function, but it can be suspended and resumed any number of times, and then it must be explicitly destroyed. When a coroutine suspends, it returns to the resumer.

Minimal Coroutine Example

Declare a coroutine with the async keyword. The expression in angle brackets must evaluate to a struct which has these fields:

allocFn: fn (self: *Allocator, byte_count: usize, alignment: u29) Error![]u8 - where Error can be any error set.

freeFn: fn (self: *Allocator, old_mem: []u8) void

You may notice that this corresponds to the std.mem.Allocator interface. This makes it convenient to integrate with existing allocators. Note, however, that the language feature does not depend on the standard library, and any struct which has these fields is allowed.

Omitting the angle bracket expression when defining an async function makes the function generic. Zig will infer the allocator type when the async function is called.

Call a coroutine with the async keyword. Here, the expression in angle brackets is a pointer to the allocator struct that the coroutine expects.

The result of an async function call is a promise->T type, where T is the return type of the async function. Once a promise has been created, it must be consumed, either with cancel or await:

Async functions start executing when created, so in the following example, the entire async function completes before it is canceled:

test.zig

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

var x: i32 = 1;

test "create a coroutine and cancel it" {
    const p = try async<std.debug.global_allocator> simpleAsyncFn();
    comptime assert(@typeOf(p) == promise->void);
    cancel p;
    assert(x == 2);
}
async<*std.mem.Allocator> fn simpleAsyncFn() void {
    x += 1;
}

$ zig test test.zig
Test 1/1 create a coroutine and cancel it...OK
All tests passed.

Suspend and Resume

At any point, an async function may suspend itself. This causes control flow to return to the caller or resumer. The following code demonstrates where control flow goes:

test.zig

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

test "coroutine suspend, resume, cancel" {
    seq('a');
    const p = try async<std.debug.global_allocator> testAsyncSeq();
    seq('c');
    resume p;
    seq('f');
    cancel p;
    seq('g');

    assert(std.mem.eql(u8, points, "abcdefg"));
}
async fn testAsyncSeq() void {
    defer seq('e');

    seq('b');
    suspend;
    seq('d');
}
var points = []u8{0} ** "abcdefg".len;
var index: usize = 0;

fn seq(c: u8) void {
    points[index] = c;
    index += 1;
}

$ zig test test.zig
Test 1/1 coroutine suspend, resume, cancel...OK
All tests passed.

When an async function suspends itself, it must be sure that it will be resumed or canceled somehow, for example by registering its promise handle in an event loop. Use a suspend capture block to gain access to the promise:

test.zig

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

test "coroutine suspend with block" {
    const p = try async<std.debug.global_allocator> testSuspendBlock();
    std.debug.assert(!result);
    resume a_promise;
    std.debug.assert(result);
    cancel p;
}

var a_promise: promise = undefined;
var result = false;
async fn testSuspendBlock() void {
    suspend {
        comptime assert(@typeOf(@handle()) == promise->void);
        a_promise = @handle();
    }
    result = true;
}

$ zig test test.zig
Test 1/1 coroutine suspend with block...OK
All tests passed.

Every suspend point in an async function represents a point at which the coroutine could be destroyed. If that happens, defer expressions that are in scope are run, as well as errdefer expressions.

Await counts as a suspend point.

Resuming from Suspend Blocks

Upon entering a suspend block, the coroutine is already considered suspended, and can be resumed. For example, if you started another kernel thread, and had that thread call resume on the promise handle provided by the suspend block, the new thread would begin executing after the suspend block, while the old thread continued executing the suspend block.

However, the coroutine 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 assert = std.debug.assert;

test "resume from suspend" {
    var buf: [500]u8 = undefined;
    var a = &std.heap.FixedBufferAllocator.init(buf[0..]).allocator;
    var my_result: i32 = 1;
    const p = try async<a> testResumeFromSuspend(&my_result);
    cancel p;
    std.debug.assert(my_result == 2);
}
async fn testResumeFromSuspend(my_result: *i32) void {
    suspend {
        resume @handle();
    }
    my_result.* += 1;
    suspend;
    my_result.* += 1;
}

$ zig test test.zig
Test 1/1 resume from suspend...OK
All tests passed.

This is guaranteed to be a tail call, and therefore will not cause a new stack frame.

Await

The await keyword is used to coordinate with an async function's return statement.

await is valid only in an async function, and it takes as an operand a promise handle. If the async function associated with the promise handle has already returned, then await destroys the target async function, and gives the return value. Otherwise, await suspends the current async function, registering its promise handle with the target coroutine. It becomes the target coroutine's responsibility to have ensured that it will be resumed or destroyed. When the target coroutine reaches its return statement, it gives the return value to the awaiter, destroys itself, and then resumes the awaiter.

A promise handle must be consumed exactly once after it is created, either by cancel or await.

await counts as a suspend point, and therefore at every await, a coroutine can be potentially destroyed, which would run defer and errdefer expressions.

test.zig

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

var a_promise: promise = undefined;
var final_result: i32 = 0;

test "coroutine await" {
    seq('a');
    const p = async<std.debug.global_allocator> amain() catch unreachable;
    seq('f');
    resume a_promise;
    seq('i');
    assert(final_result == 1234);
    assert(std.mem.eql(u8, seq_points, "abcdefghi"));
}
async fn amain() void {
    seq('b');
    const p = async another() catch unreachable;
    seq('e');
    final_result = await p;
    seq('h');
}
async fn another() i32 {
    seq('c');
    suspend {
        seq('d');
        a_promise = @handle();
    }
    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 coroutine await...OK
All 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.

Open Issues

There are a few issues with coroutines that are considered unresolved. Best be aware of them, as the situation is likely to change before 1.0.0:

Async functions have optimizations disabled - even in release modes - due to an LLVM bug.

There are some situations where we can know statically that there will not be memory allocation failure, but Zig still forces us to handle it. TODO file an issue for this and link it here.

Zig does not take advantage of LLVM's allocation elision optimization for coroutines. It crashed LLVM when I tried to do it the first time. This is related to the other 2 bullet points here. See #802.

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: var) var

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 assert = @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

@ArgType

@ArgType(comptime T: type, comptime n: usize) type

This builtin function takes a function type and returns the type of the parameter at index n.

T must be a function type.

Note: This function is deprecated. Use @typeInfo instead.

@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 type, a bool, or an integer whose bit count meets these requirements:

At least 8

At most the same as usize

Power of 2

TODO right now bool is not accepted. Also I think we could make non powers of 2 work fine, maybe we can remove this restriction

@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 type, a bool, or an integer whose bit count meets these requirements:

At least 8

At most the same as usize

Power of 2

TODO right now bool is not accepted. Also I think we could make non powers of 2 work fine, maybe we can remove this restriction

@bitCast

@bitCast(comptime DestType: type, value: var) DestType

Converts a value of one type to another type.

Asserts that @sizeOf(@typeOf(value)) == @sizeOf(DestType).

Asserts that @typeId(DestType) != @import("builtin").TypeId.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.

@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.

@bswap

@bswap(comptime T: type, value: T) T

T must be an integer type with bit count evenly divisible by 8.

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.

@bitreverse

@bitreverse(comptime T: type, value: 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

@bytesToSlice

@bytesToSlice(comptime Element: type, bytes: []u8) []Element

Converts a slice of bytes or array of bytes into a slice of Element. The resulting slice has the same pointer properties as the parameter.

Attempting to convert a number of bytes with a length that does not evenly divide into a slice of elements results in safety-protected Undefined Behavior.

@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(x: T) U

This function counts the number of leading zeroes in x which is an integer type T.

The return type U is an unsigned integer with the minimum number of bits that can represent the value T.bit_count.

If x is zero, @clz returns T.bit_count.

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.

AtomicOrder can be found with @import("builtin").AtomicOrder.

@typeOf(ptr).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.

AtomicOrder can be found with @import("builtin").AtomicOrder.

@typeOf(ptr).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 warn = @import("std").debug.warn;

const num1 = blk: {
    var val1: i32 = 99;
    @compileLog("comptime val1 = ", val1);
    val1 = val1 + 1;
    break :blk val1;
};

test "main" {
    @compileLog("comptime in main");

    warn("Runtime in main, num1 = {}.\n", num1);
}

$ zig test test.zig
| "comptime in main"
| "comptime val1 = ", 99
/home/andy/dev/zig/docgen_tmp/test.zig:11:5: error: found compile log statement
    @compileLog("comptime in main");
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:5:5: error: found compile log statement
    @compileLog("comptime val1 = ", val1);
    ^

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 warn = @import("std").debug.warn;

const num1 = blk: {
    var val1: i32 = 99;
    val1 = val1 + 1;
    break :blk val1;
};

test "main" {
    warn("Runtime in main, num1 = {}.\n", num1);
}

$ zig test test.zig
Test 1/1 main...Runtime in main, num1 = 100.
OK
All tests passed.

@ctz

@ctz(x: T) U

This function counts the number of trailing zeroes in x which is an integer type T.

The return type U is an unsigned integer with the minimum number of bits that can represent the value T.bit_count.

If x is zero, @ctz returns T.bit_count.

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 !(@typeId(T) == builtin.TypeId.Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1).

@divFloor(-5, 3) == -2

@divFloor(a, 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 !(@typeId(T) == builtin.TypeId.Int and T.is_signed and numerator == std.math.minInt(T) and denominator == -1).

@divTrunc(-5, 3) == -1

@divTrunc(a, 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) [X]u8

This function returns a compile time constant 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.

path is absolute or relative to the current file, just like @import.

See also:

@import

@enumToInt

@enumToInt(enum_or_tagged_union: var) var

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: var) @IntType(false, @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: var) 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(comptime name: []const u8, target: var, linkage: builtin.GlobalLinkage) []const u8

Creates a symbol in the output object file.

This function can be called from a comptime block to conditionally export symbols. When target is a function with the C calling convention and linkage is Strong, this is equivalent to the export keyword used on a function:

test.zig

const builtin = @import("builtin");

comptime {
    @export("foo", internalName, builtin.GlobalLinkage.Strong);
}

extern fn internalName() 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

@fence

@fence(order: AtomicOrder)

The fence function is used to introduce happens-before edges between operations.

AtomicOrder can be found with @import("builtin").AtomicOrder.

See also:

Compile Variables

@field

@field(lhs: var, comptime field_name: []const u8) (field)

Preforms field access equivalent to lhs.field_name, except instead of the field "field_name", it accesses the field named by the string value of field_name.

@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: var) 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: var) 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

@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.

@handle

@handle()

This function returns a promise->T type, where T is the return type of the async function in scope.

This function is only valid within an async function scope.

@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 or absolute 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") - Compiler-provided types and variables. The command zig builtin outputs the source to stdout for reference.

See also:

Compile Variables

@embedFile

@inlineCall

@inlineCall(function: X, args: ...) Y

This calls a function, in the same way that invoking an expression with parentheses does:

test.zig

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

test "inline function call" {
    assert(@inlineCall(add, 3, 9) == 12);
}

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

$ zig test test.zig
Test 1/1 inline function call...OK
All tests passed.

Unlike a normal function call, however, @inlineCall guarantees that the call will be inlined. If the call cannot be inlined, a compile error is emitted.

See also:

@noInlineCall

@intCast

@intCast(comptime DestType: type, int: var) 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.

@intToEnum

@intToEnum(comptime DestType: type, int_value: @TagType(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: @IntType(false, @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: var) 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.

@IntType

@IntType(comptime is_signed: bool, comptime bit_count: u16) type

This function returns an integer type with the given signness and bit count. The maximum bit count for an integer type is 65535.

@memberCount

@memberCount(comptime T: type) comptime_int

This function returns the number of members in a struct, enum, or union type.

The result is a compile time constant.

It does not include functions, variables, or constants.

@memberName

@memberName(comptime T: type, comptime index: usize) [N]u8

Returns the field name of a struct, union, or enum.

The result is a compile time constant.

It does not include functions, variables, or constants.

@memberType

@memberType(comptime T: type, comptime index: usize) type

Returns the field type of a struct or union.

@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);

@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) + @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.

@newStackCall

@newStackCall(new_stack: []u8, function: var, args: ...) var

This calls a function, in the same way that invoking an expression with parentheses does. However, instead of using the same stack as the caller, the function uses the stack provided in the new_stack parameter.

test.zig

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

var new_stack_bytes: [1024]u8 = undefined;

test "calling a function with a new stack" {
    const arg = 1234;

    const a = @newStackCall(new_stack_bytes[0..512], targetFunction, arg);
    const b = @newStackCall(new_stack_bytes[512..], targetFunction, arg);
    _ = targetFunction(arg);

    assert(arg == 1234);
    assert(a < b);
}

fn targetFunction(x: i32) usize {
    assert(x == 1234);

    var local_variable: i32 = 42;
    const ptr = &local_variable;
    ptr.* += 1;

    assert(local_variable == 43);
    return @ptrToInt(ptr);
}

$ zig test test.zig
Test 1/1 calling a function with a new stack...OK
All tests passed.

@noInlineCall

@noInlineCall(function: var, args: ...) var

This calls a function, in the same way that invoking an expression with parentheses does:

test.zig

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

test "noinline function call" {
    assert(@noInlineCall(add, 3, 9) == 12);
}

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

$ zig test test.zig
Test 1/1 noinline function call...OK
All tests passed.

Unlike a normal function call, however, @noInlineCall guarantees that the call will not be inlined. If the call must be inlined, a compile error is emitted.

See also:

@inlineCall

@OpaqueType

@OpaqueType() type

Creates a new type with an unknown (but non-zero) size and alignment.

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

test.zig

const Derp = @OpaqueType();
const Wat = @OpaqueType();

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

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

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

@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, invokes the one provided in std/special/panic.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(integer: var) var

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: var) 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: var) 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) + @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: usize)

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
/home/andy/dev/zig/docgen_tmp/test.zig:4:9: error: evaluation exceeded 1000 backwards branches
        while (i < 1001) : (i += 1) {}
        ^

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 foo...OK
All tests passed.

See also:

comptime

@setFloatMode

@setFloatMode(mode: @import("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

@setGlobalLinkage

@setGlobalLinkage(global_variable_name, comptime linkage: GlobalLinkage)

GlobalLinkage can be found with @import("builtin").GlobalLinkage.

See also:

Compile Variables

@setRuntimeSafety

@setRuntimeSafety(safety_on: bool)

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 --release-fast
Test 1/1 @setRuntimeSafety...integer overflow

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

@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

@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.

See also:

@typeInfo

@sliceToBytes

@sliceToBytes(value: var) []u8

Converts a slice or array to a slice of u8. The resulting slice has the same pointer properties as the parameter.

@sqrt

@sqrt(comptime T: type, value: T) T

Performs the square root of a floating point number. Uses a dedicated hardware instruction when available. Currently only supports f32 and f64 at runtime. f128 at runtime is TODO.

This is a low-level intrinsic. Most code can use std.math.sqrt instead.

@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: var) []const u8

Converts an enum value or union value to a slice of bytes representing the name.

@TagType

@TagType(T: type) type

For an enum, returns the integer type that is used to store the enumeration value.

For a union, returns the enum type that is used to store the tag value.

@This

@This() type

Returns the innermost struct 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 assert = std.debug.assert;

test "@This()" {
    var items = []i32{ 1, 2, 3, 4 };
    const list = List(i32){ .items = items[0..] };
    assert(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 @This()...OK
All tests passed.

When @This() is used at global scope, it returns a reference to the current import. There is a proposal to remove the import type and use an empty struct type instead. See #1047 for details.

@truncate

@truncate(comptime T: type, integer: var) T

This function truncates bits from an integer type, resulting in a smaller integer type.

The following produces a crash in Debug mode and Undefined Behavior in ReleaseFast mode:

const a: u16 = 0xabcd;
const b: u8 = u8(a);

However this is well defined and working code:

const a: u16 = 0xabcd;
const b: u8 = @truncate(u8, a);
// b is now 0xcd

This function always truncates the significant bits of the integer, regardless of endianness on the target platform.

If T is comptime_int, then this is semantically equivalent to an implicit cast.

@typeId

@typeId(comptime T: type) @import("builtin").TypeId

Returns which kind of type something is. Possible values:

pub const TypeId = enum {
    Type,
    Void,
    Bool,
    NoReturn,
    Int,
    Float,
    Pointer,
    Array,
    Struct,
    ComptimeFloat,
    ComptimeInt,
    Undefined,
    Null,
    Optional,
    ErrorUnion,
    Error,
    Enum,
    Union,
    Fn,
    Block,
    BoundFn,
    ArgTuple,
    Opaque,
};

@typeInfo

@typeInfo(comptime T: type) @import("builtin").TypeInfo

Returns information on the type. Returns a value of the following union:

pub const TypeInfo = union(TypeId) {
    Type: void,
    Void: void,
    Bool: void,
    NoReturn: void,
    Int: Int,
    Float: Float,
    Pointer: Pointer,
    Array: Array,
    Struct: Struct,
    ComptimeFloat: void,
    ComptimeInt: void,
    Undefined: void,
    Null: void,
    Optional: Optional,
    ErrorUnion: ErrorUnion,
    ErrorSet: ErrorSet,
    Enum: Enum,
    Union: Union,
    Fn: Fn,
    BoundFn: Fn,
    ArgTuple: void,
    Opaque: void,
    Promise: Promise,
    Vector: Vector,
    EnumLiteral: void,

    pub const Int = struct {
        is_signed: bool,
        bits: comptime_int,
    };

    pub const Float = struct {
        bits: comptime_int,
    };

    pub const Pointer = struct {
        size: Size,
        is_const: bool,
        is_volatile: bool,
        alignment: comptime_int,
        child: type,
        is_allowzero: bool,

        pub const Size = enum {
            One,
            Many,
            Slice,
            C,
        };
    };

    pub const Array = struct {
        len: comptime_int,
        child: type,
    };

    pub const ContainerLayout = enum {
        Auto,
        Extern,
        Packed,
    };

    pub const StructField = struct {
        name: []const u8,
        offset: ?comptime_int,
        field_type: type,
    };

    pub const Struct = struct {
        layout: ContainerLayout,
        fields: []StructField,
        defs: []Definition,
    };

    pub const Optional = struct {
        child: type,
    };

    pub const ErrorUnion = struct {
        error_set: type,
        payload: type,
    };

    pub const Error = struct {
        name: []const u8,
        value: comptime_int,
    };

    pub const ErrorSet = ?[]Error;

    pub const EnumField = struct {
        name: []const u8,
        value: comptime_int,
    };

    pub const Enum = struct {
        layout: ContainerLayout,
        tag_type: type,
        fields: []EnumField,
        defs: []Definition,
    };

    pub const UnionField = struct {
        name: []const u8,
        enum_field: ?EnumField,
        field_type: type,
    };

    pub const Union = struct {
        layout: ContainerLayout,
        tag_type: ?type,
        fields: []UnionField,
        defs: []Definition,
    };

    pub const CallingConvention = enum {
        Unspecified,
        C,
        Cold,
        Naked,
        Stdcall,
        Async,
    };

    pub const FnArg = struct {
        is_generic: bool,
        is_noalias: bool,
        arg_type: ?type,
    };

    pub const Fn = struct {
        calling_convention: CallingConvention,
        is_generic: bool,
        is_var_args: bool,
        return_type: ?type,
        async_allocator_type: ?type,
        args: []FnArg,
    };

    pub const Promise = struct {
        child: ?type,
    };

    pub const Vector = struct {
        len: comptime_int,
        child: type,
    };

    pub const Definition = struct {
        name: []const u8,
        is_pub: bool,
        data: Data,

        pub const Data = union(enum) {
            Type: type,
            Var: type,
            Fn: FnDef,

            pub const FnDef = struct {
                fn_type: type,
                inline_type: Inline,
                calling_convention: CallingConvention,
                is_var_args: bool,
                is_extern: bool,
                is_export: bool,
                lib_name: ?[]const u8,
                return_type: type,
                arg_names: [][] const u8,

                pub const Inline = enum {
                    Auto,
                    Always,
                    Never,
                };
            };
        };
    };
};

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(expression) type

This function returns a compile-time constant, which is the type of the expression passed as an argument. The expression is evaluated.

@Vector

@Vector(comptime len: u32, comptime ElemType: type) type

This function returns a vector type for SIMD.

ElemType must be an integer, a float, or a pointer.

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 --release-fast

Fast runtime performance

Safety checks disabled

Slow compilation speed

Large binary size

Reproducible build

ReleaseSafe

$ zig build-exe example.zig --release-safe

Medium runtime performance

Safety checks enabled

Slow compilation speed

Large binary size

Reproducible build

ReleaseSmall

$ zig build-exe example.zig --release-small

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 Global Variables.

The overhead of Coroutines becomes equivalent to function call overhead. TODO: please note this will not be implemented until the upcoming Coroutine Rewrite

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 build mode disables all safety checks 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 safety check...reached unreachable code
/home/andy/dev/zig/docgen_tmp/test.zig:2:5: 0x2040d7 in ??? (test)
    unreachable;
    ^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x225bab in ??? (test)
        if (test_fn.func()) |_| {
                        ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x225336 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x2250a1 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

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
/home/andy/dev/zig/docgen_tmp/test.zig:5:14: error: unable to evaluate constant expression
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/dev/zig/docgen_tmp/test.zig:2:11: note: called from here
    assert(false);
          ^
/home/andy/dev/zig/docgen_tmp/test.zig:1:10: note: called from here
comptime {
         ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    std.debug.assert(false);
}

$ zig build-exe test.zig
$ ./test
reached unreachable code
/home/andy/dev/zig/build/lib/zig/std/debug.zig:169:14: 0x20402c in ??? (test)
    if (!ok) unreachable; // assertion failure
             ^
/home/andy/dev/zig/docgen_tmp/test.zig:4:21: 0x22581b in ??? (test)
    std.debug.assert(false);
                    ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Index out of Bounds

At compile-time:

test.zig

comptime {
    const array = "hello";
    const garbage = array[5];
}

$ zig test test.zig
/home/andy/dev/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
index out of bounds
/home/andy/dev/zig/docgen_tmp/test.zig:6:13: 0x22585a in ??? (test)
    return x[5];
            ^
/home/andy/dev/zig/docgen_tmp/test.zig:2:16: 0x225827 in ??? (test)
    var x = foo("hello");
               ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/zig/docgen_tmp/test.zig:3:36: error: cannot cast negative value -1 to unsigned integer type 'u32'
    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.warn("value: {}\n", unsigned);
}

$ zig build-exe test.zig
$ ./test
attempt to cast negative value to unsigned integer
/home/andy/dev/zig/docgen_tmp/test.zig:5:20: 0x225851 in ??? (test)
    var unsigned = @intCast(u32, value);
                   ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/zig/docgen_tmp/test.zig:3:31: error: integer value 300 cannot be implicitly casted to type 'u8'
    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.warn("value: {}\n", byte);
}

$ zig build-exe test.zig
$ ./test
integer cast truncated bits
/home/andy/dev/zig/docgen_tmp/test.zig:5:18: 0x22585a in ??? (test)
    const byte = @intCast(u8, spartan_count);
                 ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", byte);
}

$ zig build-exe test.zig
$ ./test
integer overflow
/home/andy/dev/zig/docgen_tmp/test.zig:5:10: 0x225841 in ??? (test)
    byte += 1;
         ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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 warn = @import("std").debug.warn;
pub fn main() !void {
    var byte: u8 = 255;

    byte = if (math.add(u8, byte, 1)) |result| result else |err| {
        warn("unable to add one: {}\n", @errorName(err));
        return err;
    };

    warn("result: {}\n", byte);
}

$ zig build-exe test.zig
$ ./test
unable to add one: Overflow
error: Overflow
/home/andy/dev/zig/build/lib/zig/std/math.zig:268:5: 0x225c4e in ??? (test)
    return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    ^
/home/andy/dev/zig/docgen_tmp/test.zig:8:9: 0x225a3e in ??? (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 warn = @import("std").debug.warn;
pub fn main() void {
    var byte: u8 = 255;

    var result: u8 = undefined;
    if (@addWithOverflow(u8, byte, 10, &result)) {
        warn("overflowed result: {}\n", result);
    } else {
        warn("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 assert = std.debug.assert;
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;
    assert(min_val == minInt(i32));
    const max_val = min_val -% 1;
    assert(max_val == maxInt(i32));
}

$ zig test test.zig
Test 1/1 wraparound addition and subtraction...OK
All tests passed.

Exact Left Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shlExact(u8(0b01010101), 2);
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:2:15: error: operation caused overflow
    const x = @shlExact(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.warn("value: {}\n", y);
}

$ zig build-exe test.zig
$ ./test
left shift overflowed bits
/home/andy/dev/zig/docgen_tmp/test.zig:5:13: 0x225858 in ??? (test)
    var y = @shlExact(x, 2);
            ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Exact Right Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shrExact(u8(0b10101010), 2);
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:2:15: error: exact shift shifted out 1 bits
    const x = @shrExact(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.warn("value: {}\n", y);
}

$ zig build-exe test.zig
$ ./test
right shift overflowed bits
/home/andy/dev/zig/docgen_tmp/test.zig:5:13: 0x225858 in ??? (test)
    var y = @shrExact(x, 2);
            ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", c);
}

$ zig build-exe test.zig
$ ./test
division by zero
/home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x22584a in ??? (test)
    var c = a / b;
              ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", c);
}

$ zig build-exe test.zig
$ ./test
remainder division by zero or negative value
/home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x225865 in ??? (test)
    var c = a % b;
              ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", c);
}

$ zig build-exe test.zig
$ ./test
exact division produced remainder
/home/andy/dev/zig/docgen_tmp/test.zig:6:13: 0x225887 in ??? (test)
    var c = @divExact(a, b);
            ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Slice Widen Remainder

At compile-time:

test.zig

comptime {
    var bytes = [5]u8{ 1, 2, 3, 4, 5 };
    var slice = @bytesToSlice(u32, bytes);
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:3:17: error: unable to convert [5]u8 to []align(1) const u32: size mismatch
    var slice = @bytesToSlice(u32, bytes);
                ^
/home/andy/dev/zig/docgen_tmp/test.zig:3:31: note: u32 has size 4; remaining bytes: 1
    var slice = @bytesToSlice(u32, bytes);
                              ^

At runtime:

test.zig

const std = @import("std");

pub fn main() void {
    var bytes = [5]u8{ 1, 2, 3, 4, 5 };
    var slice = @bytesToSlice(u32, bytes[0..]);
    std.debug.warn("value: {}\n", slice[0]);
}

$ zig build-exe test.zig
$ ./test
slice widening size mismatch
/home/andy/dev/zig/docgen_tmp/test.zig:5:17: 0x2258ae in ??? (test)
    var slice = @bytesToSlice(u32, bytes[0..]);
                ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Attempt to Unwrap Null

At compile-time:

test.zig

comptime {
    const optional_number: ?i32 = null;
    const number = optional_number.?;
}

$ zig test test.zig
/home/andy/dev/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.warn("value: {}\n", number);
}

$ zig build-exe test.zig
$ ./test
attempt to unwrap null
/home/andy/dev/zig/docgen_tmp/test.zig:5:33: 0x225853 in ??? (test)
    var number = optional_number.?;
                                ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

One way to avoid this crash is to test for null instead of assuming non-null, with the if expression:

test.zig

const warn = @import("std").debug.warn;
pub fn main() void {
    const optional_number: ?i32 = null;

    if (optional_number) |number| {
        warn("got number: {}\n", number);
    } else {
        warn("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
/home/andy/dev/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.warn("value: {}\n", number);
}

fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig build-exe test.zig
$ ./test
attempt to unwrap error: UnableToReturnNumber
/home/andy/dev/zig/docgen_tmp/test.zig:9:5: 0x2258bb in ??? (test)
    return error.UnableToReturnNumber;
    ^
???:?:?: 0x225948 in ??? (???)

/home/andy/dev/zig/docgen_tmp/test.zig:4:38: 0x22587c in ??? (test)
    const number = getNumberOrFail() catch unreachable;
                                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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 warn = @import("std").debug.warn;

pub fn main() void {
    const result = getNumberOrFail();

    if (result) |number| {
        warn("got number: {}\n", number);
    } else |err| {
        warn("got error: {}\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
/home/andy/dev/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.warn("value: {}\n", number);
}

$ zig build-exe test.zig
$ ./test
invalid error code
/home/andy/dev/zig/docgen_tmp/test.zig:6:23: 0x22586d in ??? (test)
    var invalid_err = @intToError(number);
                      ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/zig/docgen_tmp/test.zig:8:15: error: enum 'Foo' has no tag matching integer value 3
    const b = @intToEnum(Foo, a);
              ^
/home/andy/dev/zig/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.warn("value: {}\n", @tagName(b));
}

$ zig build-exe test.zig
$ ./test
invalid enum value
/home/andy/dev/zig/docgen_tmp/test.zig:11:13: 0x225850 in ??? (test)
    var b = @intToEnum(Foo, a);
            ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", x);
}

$ zig build-exe test.zig
$ ./test
invalid error code
/home/andy/dev/zig/docgen_tmp/test.zig:15:15: 0x225880 in ??? (test)
    const x = @errSetCast(Set2, set1);
              ^
/home/andy/dev/zig/docgen_tmp/test.zig:12:8: 0x22581e in ??? (test)
    foo(Set1.B);
       ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

Incorrect Pointer Alignment

At compile-time:

test.zig

comptime {
    const ptr = @intToPtr(*i32, 0x1);
    const aligned = @alignCast(4, ptr);
}

$ zig test test.zig
/home/andy/dev/zig/docgen_tmp/test.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes
    const aligned = @alignCast(4, ptr);
                                  ^

At runtime:

test.zig

pub fn main() !void {
    var array align(4) = []u32{ 0x11111111, 0x11111111 };
    const bytes = @sliceToBytes(array[0..]);
    if (foo(bytes) != 0x11111111) return error.Wrong;
}
fn foo(bytes: []u8) u32 {
    const slice4 = bytes[1..5];
    const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
    return int_slice[0];
}

$ zig build-exe test.zig
$ ./test
incorrect alignment
/home/andy/dev/zig/docgen_tmp/test.zig:8:56: 0x225be6 in ??? (test)
    const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
                                                       ^
/home/andy/dev/zig/docgen_tmp/test.zig:4:12: 0x225a22 in ??? (test)
    if (foo(bytes) != 0x11111111) return error.Wrong;
           ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:122:22: 0x225316 in ??? (test)
            root.main() catch |err| {
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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
/home/andy/dev/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.warn("value: {}\n", f.float);
}

$ zig build-exe test.zig
$ ./test
access of inactive union field
/home/andy/dev/zig/docgen_tmp/test.zig:14:6: 0x22c88b in ??? (test)
    f.float = 12.34;
     ^
/home/andy/dev/zig/docgen_tmp/test.zig:10:8: 0x22c82d in ??? (test)
    bar(&f);
       ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22c28b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x22c081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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.warn("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.warn("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
/home/andy/dev/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
cast causes pointer to be null
/home/andy/dev/zig/docgen_tmp/test.zig:3:15: 0x225841 in ??? (test)
    var ptr = @ptrCast(*i32, opt_ptr);
              ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:112:22: 0x22528b in ??? (test)
            root.main();
                     ^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:43:5: 0x225081 in ??? (test)
    @noInlineCall(posixCallMainAndExit);
    ^

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 assert = std.debug.assert;

test "using an allocator" {
    var buffer: [100]u8 = undefined;
    const allocator = &std.heap.FixedBufferAllocator.init(&buffer).allocator;
    const result = try concat(allocator, "foo", "bar");
    assert(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 using an allocator...OK
All 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.debug.global_allocator, however it is deprecated and should be avoided in favor of directly using a FixedBufferAllocator as in the example above.

Currently Zig has no general purpose allocator, but there is one under active development. Once it is merged into the Zig standard library it will become available to import with std.heap.default_allocator. However, it will still be 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 direct_allocator = std.heap.DirectAllocator.init();
    defer direct_allocator.deinit();

    var arena = std.heap.ArenaAllocator.init(&direct_allocator.allocator);
    defer arena.deinit();

    const allocator = &arena.allocator;

    const ptr = try allocator.create(i32);
    std.debug.warn("ptr={*}\n", ptr);
}

$ zig build-exe cli_allocation.zig
$ ./cli_allocation
ptr=i32@7f01f88d7020

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.debug.FailingAllocator.

Finally, if none of the above apply, you need a general purpose allocator. Zig does not yet have a general purpose allocator in the standard library, but one is being actively developed. 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
/home/andy/dev/zig/docgen_tmp/test.zig:4:9: error: expected type '[]u8', found '[5]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 string literal to constant slice...OK
All 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 at this work-in-progress general purpose allocator. TODO: once #21 is done, link to the docs here.

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, when using std.ArrayList(T).toSlice(), the returned 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 == builtin.Os.windows) '\\' else '/';

Example of what is imported with @import("builtin"):

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

pub const PanicFn = fn([]const u8, ?*StackTrace) noreturn;

pub const Os = enum {
    freestanding,
    ananas,
    cloudabi,
    dragonfly,
    freebsd,
    fuchsia,
    ios,
    kfreebsd,
    linux,
    lv2,
    macosx,
    netbsd,
    openbsd,
    solaris,
    windows,
    haiku,
    minix,
    rtems,
    nacl,
    cnk,
    aix,
    cuda,
    nvcl,
    amdhsa,
    ps4,
    elfiamcu,
    tvos,
    watchos,
    mesa3d,
    contiki,
    amdpal,
    hermit,
    hurd,
    wasi,
    zen,
    uefi,
};

pub const Arch = union(enum) {
    arm: Arm32,
    armeb: Arm32,
    aarch64: Arm64,
    aarch64_be: Arm64,
    arc,
    avr,
    bpfel,
    bpfeb,
    hexagon,
    mips,
    mipsel,
    mips64,
    mips64el,
    msp430,
    powerpc,
    powerpc64,
    powerpc64le,
    r600,
    amdgcn,
    riscv32,
    riscv64,
    sparc,
    sparcv9,
    sparcel,
    s390x,
    tce,
    tcele,
    thumb: Arm32,
    thumbeb: Arm32,
    i386,
    x86_64,
    xcore,
    nvptx,
    nvptx64,
    le32,
    le64,
    amdil,
    amdil64,
    hsail,
    hsail64,
    spir,
    spir64,
    kalimba: Kalimba,
    shave,
    lanai,
    wasm32,
    wasm64,
    renderscript32,
    renderscript64,
    pub const Arm32 = enum {
        v8_5a,
        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,
    };
    pub const Arm64 = enum {
        v8_5a,
        v8_4a,
        v8_3a,
        v8_2a,
        v8_1a,
        v8,
        v8r,
        v8m_baseline,
        v8m_mainline,
    };
    pub const Kalimba = enum {
        v5,
        v4,
        v3,
    };
    pub const Mips = enum {
        r6,
    };
};

pub const Abi = enum {
    none,
    gnu,
    gnuabin32,
    gnuabi64,
    gnueabi,
    gnueabihf,
    gnux32,
    code16,
    eabi,
    eabihf,
    android,
    musl,
    musleabi,
    musleabihf,
    msvc,
    itanium,
    cygnus,
    coreclr,
    simulator,
};

pub const ObjectFormat = enum {
    unknown,
    coff,
    elf,
    macho,
    wasm,
};

pub const GlobalLinkage = enum {
    Internal,
    Strong,
    Weak,
    LinkOnce,
};

pub const AtomicOrder = enum {
    Unordered,
    Monotonic,
    Acquire,
    Release,
    AcqRel,
    SeqCst,
};

pub const AtomicRmwOp = enum {
    Xchg,
    Add,
    Sub,
    And,
    Nand,
    Or,
    Xor,
    Max,
    Min,
};

pub const Mode = enum {
    Debug,
    ReleaseSafe,
    ReleaseFast,
    ReleaseSmall,
};

pub const TypeId = enum {
    Type,
    Void,
    Bool,
    NoReturn,
    Int,
    Float,
    Pointer,
    Array,
    Struct,
    ComptimeFloat,
    ComptimeInt,
    Undefined,
    Null,
    Optional,
    ErrorUnion,
    ErrorSet,
    Enum,
    Union,
    Fn,
    BoundFn,
    ArgTuple,
    Opaque,
    Promise,
    Vector,
    EnumLiteral,
};

pub const TypeInfo = union(TypeId) {
    Type: void,
    Void: void,
    Bool: void,
    NoReturn: void,
    Int: Int,
    Float: Float,
    Pointer: Pointer,
    Array: Array,
    Struct: Struct,
    ComptimeFloat: void,
    ComptimeInt: void,
    Undefined: void,
    Null: void,
    Optional: Optional,
    ErrorUnion: ErrorUnion,
    ErrorSet: ErrorSet,
    Enum: Enum,
    Union: Union,
    Fn: Fn,
    BoundFn: Fn,
    ArgTuple: void,
    Opaque: void,
    Promise: Promise,
    Vector: Vector,
    EnumLiteral: void,

    pub const Int = struct {
        is_signed: bool,
        bits: comptime_int,
    };

    pub const Float = struct {
        bits: comptime_int,
    };

    pub const Pointer = struct {
        size: Size,
        is_const: bool,
        is_volatile: bool,
        alignment: comptime_int,
        child: type,
        is_allowzero: bool,

        pub const Size = enum {
            One,
            Many,
            Slice,
            C,
        };
    };

    pub const Array = struct {
        len: comptime_int,
        child: type,
    };

    pub const ContainerLayout = enum {
        Auto,
        Extern,
        Packed,
    };

    pub const StructField = struct {
        name: []const u8,
        offset: ?comptime_int,
        field_type: type,
    };

    pub const Struct = struct {
        layout: ContainerLayout,
        fields: []StructField,
        defs: []Definition,
    };

    pub const Optional = struct {
        child: type,
    };

    pub const ErrorUnion = struct {
        error_set: type,
        payload: type,
    };

    pub const Error = struct {
        name: []const u8,
        value: comptime_int,
    };

    pub const ErrorSet = ?[]Error;

    pub const EnumField = struct {
        name: []const u8,
        value: comptime_int,
    };

    pub const Enum = struct {
        layout: ContainerLayout,
        tag_type: type,
        fields: []EnumField,
        defs: []Definition,
    };

    pub const UnionField = struct {
        name: []const u8,
        enum_field: ?EnumField,
        field_type: type,
    };

    pub const Union = struct {
        layout: ContainerLayout,
        tag_type: ?type,
        fields: []UnionField,
        defs: []Definition,
    };

    pub const CallingConvention = enum {
        Unspecified,
        C,
        Cold,
        Naked,
        Stdcall,
        Async,
    };

    pub const FnArg = struct {
        is_generic: bool,
        is_noalias: bool,
        arg_type: ?type,
    };

    pub const Fn = struct {
        calling_convention: CallingConvention,
        is_generic: bool,
        is_var_args: bool,
        return_type: ?type,
        async_allocator_type: ?type,
        args: []FnArg,
    };

    pub const Promise = struct {
        child: ?type,
    };

    pub const Vector = struct {
        len: comptime_int,
        child: type,
    };

    pub const Definition = struct {
        name: []const u8,
        is_pub: bool,
        data: Data,

        pub const Data = union(enum) {
            Type: type,
            Var: type,
            Fn: FnDef,

            pub const FnDef = struct {
                fn_type: type,
                inline_type: Inline,
                calling_convention: CallingConvention,
                is_var_args: bool,
                is_extern: bool,
                is_export: bool,
                lib_name: ?[]const u8,
                return_type: type,
                arg_names: [][] const u8,

                pub const Inline = enum {
                    Auto,
                    Always,
                    Never,
                };
            };
        };
    };
};

pub const FloatMode = enum {
    Strict,
    Optimized,
};

pub const Endian = enum {
    Big,
    Little,
};

pub const endian = Endian.Little;
pub const is_test = false;
pub const single_threaded = false;
pub const os = Os.linux;
pub const arch = Arch.x86_64;
pub const abi = Abi.gnu;
pub const object_format = ObjectFormat.elf;
pub const mode = Mode.Debug;
pub const link_libc = false;
pub const have_error_return_tracing = false;
pub const valgrind_support = true;
pub const position_independent_code = false;

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 assert = std.debug.assert;

test "builtin.is_test" {
    assert(builtin.is_test);
}

$ zig test detect_test.zig
Test 1/1 builtin.is_test...OK
All 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 unused function...OK
All 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:

assert.zig

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

test "assert in release fast mode" {
    assert(false);
}

$ zig test assert.zig --release-fast
Test 1/1 assert in release fast mode...OK
All tests passed.

Note that although the above example shows the test passing, this is invoking unchecked Undefined Behavior. This documentation is showing only one possible outcome of this test.

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 "assert in release fast mode" {
    expect(false);
}

$ zig test test.zig --release-fast
Test 1/1 assert in release fast mode...test failure

Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test

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

TODO: explain purpose, it's supposed to replace make/cmake

TODO: example of building a zig executable

TODO: example of building a C library

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

C String Literals

test.zig

extern fn puts([*]const u8) void;

pub fn main() void {
    puts(c"this has a null terminator");
    puts(
        c\\and so
        c\\does this
        c\\multiline C string literal
    );
}

$ zig build-exe test.zig --library c
$ ./test
this has a null terminator
and so
does this
multiline C string literal

See also:

String Literals and Character Literals

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(c"hello\n");
}

$ zig build-exe test.zig --library c
$ ./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 == 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 unknown-length 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.

Implicitly casts to other pointer types, as well as Optional Pointers. When a C pointer is implicitly casted 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 implicit casting to and from integers.

Supports comparison with integers.

Does not support Zig-only pointer attributes such as alignment. Use normal Pointers please!

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 shared library:

$ zig build-lib mathtest.zig

To make a static library:

$ zig build-lib mathtest.zig --static

Here is an example with the Zig Build System:

test.c

// This header is generated by zig from mathtest.zig
#include "mathtest.h"
#include <assert.h>

int main(int argc, char **argv) {
    assert(add(42, 1337) == 1379);
    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.addCExecutable("test");
    exe.addCompileFlags([][]const u8{"-std=c99"});
    exe.addSourceFile("test.c");
    exe.linkLibrary(lib);

    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
$ echo $?
0

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_unsafe;
    const decoded_size = base64_decoder.calcSize(src);
    base64_decoder.decode(dest[0..decoded_size], src);
    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.addCExecutable("test");
    exe.addCompileFlags([][]const u8 {
        "-std=c99",
    });
    exe.addSourceFile("test.c");
    exe.addObject(obj);
    exe.setOutputPath(".");

    b.default_step.dependOn(&exe.step);
}

terminal

$ zig build
$ ./test
all your base are belong to us

See also:

Targets

Zig Build System

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
  macosx
  netbsd
  openbsd
  solaris
  windows
  haiku
  minix
  rtems
  nacl
  cnk
  aix
  cuda
  nvcl
  amdhsa
  ps4
  elfiamcu
  tvos
  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
  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 an 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.

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");
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.

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 codepoints are never allowed:

Ascii control characters, except for U+000a (LF): U+0000 - U+0009, U+000b - U+0001f, U+007f. (Note that Windows line endings (CRLF) are not allowed, and hard tabs are not allowed.)

Non-Ascii Unicode line endings: U+0085 (NEL), U+2028 (LS), U+2029 (PS).

The codepoint U+000a (LF) (which is encoded as the single-byte value 0x0a) is the line terminator character. This character always terminates a line of zig source code (except possbly the last line of the file).

For some discussion on the rationale behind these design decisions, see issue #663

Keyword Reference

TODO the rest of the keywords. Most of these can just be links to the relevant section.

Keyword: 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.

TODO delete pub syntax for fields, or make it do something.

See also:

@import

Grammar

Root <- skip ContainerMembers eof

# *** Top level ***
ContainerMembers
    <- TestDecl ContainerMembers
     / TopLevelComptime ContainerMembers
     / KEYWORD_pub? TopLevelDecl ContainerMembers
     / KEYWORD_pub? ContainerField COMMA ContainerMembers
     / KEYWORD_pub? ContainerField
     /

TestDecl <- KEYWORD_test STRINGLITERAL Block

TopLevelComptime <- KEYWORD_comptime BlockExpr

TopLevelDecl
    <- (KEYWORD_export / KEYWORD_extern STRINGLITERAL? / KEYWORD_inline)? FnProto (SEMICOLON / Block)
     / (KEYWORD_export / KEYWORD_extern STRINGLITERAL?)? KEYWORD_threadlocal? VarDecl
     / KEYWORD_use Expr SEMICOLON

FnProto <- FnCC? KEYWORD_fn IDENTIFIER? LPAREN ParamDeclList RPAREN ByteAlign? LinkSection? EXCLAMATIONMARK? (KEYWORD_var / TypeExpr)

VarDecl <- (KEYWORD_const / KEYWORD_var) IDENTIFIER (COLON TypeExpr)? ByteAlign? LinkSection? (EQUAL Expr)? SEMICOLON

ContainerField <- IDENTIFIER (COLON TypeExpr)? (EQUAL Expr)?

# *** Block Level ***
Statement
    <- KEYWORD_comptime? VarDecl
     / KEYWORD_comptime BlockExprStatement
     / KEYWORD_suspend (SEMICOLON / BlockExprStatement)
     / KEYWORD_defer BlockExprStatement
     / KEYWORD_errdefer 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 <- KEYWORD_try* 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_cancel Expr
     / KEYWORD_comptime Expr
     / KEYWORD_continue BreakLabel?
     / KEYWORD_resume Expr
     / KEYWORD_return Expr?
     / BlockLabel? LoopExpr
     / Block
     / CurlySuffixExpr

IfExpr <- IfPrefix Expr (KEYWORD_else Payload? Expr)?

LabeledExpr <- BlockLabel? (Block / LoopExpr)

Block <- LBRACE Statement* RBRACE

LoopExpr <- KEYWORD_inline? (ForExpr / WhileExpr)

ForExpr <- ForPrefix Expr (KEYWORD_else Expr)?

WhileExpr <- WhilePrefix Expr (KEYWORD_else Payload? Expr)?

InitList
    <- LBRACE FieldInit (COMMA FieldInit)* COMMA? RBRACE
     / LBRACE Expr (COMMA Expr)* COMMA? RBRACE
     / LBRACE RBRACE

TypeExpr <- PrefixTypeOp* ErrorUnionExpr

ErrorUnionExpr <- SuffixExpr (EXCLAMATIONMARK TypeExpr)?

SuffixExpr
    <- AsyncPrefix PrimaryTypeExpr SuffixOp* FnCallArguments
     / PrimaryTypeExpr (SuffixOp / FnCallArguments)*

PrimaryTypeExpr
    <- BUILTINIDENTIFIER FnCallArguments
     / CHAR_LITERAL
     / ContainerDecl
     / ErrorSetDecl
     / FLOAT
     / FnProto
     / GroupedExpr
     / LabeledTypeExpr
     / IDENTIFIER
     / IfTypeExpr
     / INTEGER
     / KEYWORD_anyerror
     / KEYWORD_comptime TypeExpr
     / KEYWORD_error DOT IDENTIFIER
     / KEYWORD_false
     / KEYWORD_null
     / KEYWORD_promise
     / 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 STRINGLITERAL 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
FnCC
    <- KEYWORD_nakedcc
     / KEYWORD_stdcallcc
     / KEYWORD_extern
     / KEYWORD_async (LARROW TypeExpr RARROW)?

ParamDecl <- (KEYWORD_noalias / KEYWORD_comptime)? (IDENTIFIER COLON)? ParamType

ParamType
    <- KEYWORD_var
     / DOT3
     / 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_promise MINUSRARROW
     / ArrayTypeStart (ByteAlign / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*
     / PtrTypeStart (KEYWORD_align LPAREN Expr (COLON INTEGER COLON INTEGER)? RPAREN / KEYWORD_const / KEYWORD_volatile / KEYWORD_allowzero)*

SuffixOp
    <- LBRACKET Expr (DOT2 Expr?)? RBRACKET
     / DOT IDENTIFIER
     / DOTASTERISK
     / DOTQUESTIONMARK

AsyncPrefix <- KEYWORD_async (LARROW PrefixExpr RARROW)?

FnCallArguments <- LPAREN ExprList RPAREN

# Ptr specific
ArrayTypeStart <- LBRACKET Expr? RBRACKET

PtrTypeStart
    <- ASTERISK
     / ASTERISK2
     / PTRUNKNOWN
     / PTRC

# ContainerDecl specific
ContainerDeclAuto <- ContainerDeclType LBRACE ContainerMembers RBRACE

ContainerDeclType
    <- (KEYWORD_struct / KEYWORD_enum) (LPAREN Expr RPAREN)?
     / KEYWORD_union (LPAREN (KEYWORD_enum (LPAREN Expr RPAREN)? / Expr) RPAREN)?

# Alignment
ByteAlign <- KEYWORD_align LPAREN Expr RPAREN

# Lists
IdentifierList <- (IDENTIFIER COMMA)* 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 <- !.
hex <- [0-9a-fA-F]
char_escape
    <- "\\x" hex hex
     / "\\u" hex hex hex hex
     / "\\U" hex hex hex hex hex hex
     / "\\" [nr\\t'"]
char_char
    <- char_escape
     / [^\\'\n]
string_char
    <- char_escape
     / [^\\"\n]

line_comment <- '//'[^\n]*
line_string <- ("\\\\" [^\n]* [ \n]*)+
line_cstring <- ("c\\\\" [^\n]* [ \n]*)+
skip <- ([ \n] / line_comment)*

CHAR_LITERAL <- "'" char_char "'" skip
FLOAT
    <- "0x" hex+   "." hex+   ([pP] [-+]? hex+)?   skip
     /      [0-9]+ "." [0-9]+ ([eE] [-+]? [0-9]+)? skip
     / "0x" hex+   "."? [pP] [-+]? hex+   skip
     /      [0-9]+ "."? [eE] [-+]? [0-9]+ skip
INTEGER
    <- "0b" [01]+  skip
     / "0o" [0-7]+ skip
     / "0x" hex+   skip
     /      [0-9]+ skip
STRINGLITERAL
    <- "c"? "\"" string_char* "\"" skip
     / line_string                 skip
     / line_cstring                skip
IDENTIFIER
    <- !keyword ("c" !["\\] / [A-Zabd-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
PTRC                 <- '[*c]'             skip
PTRUNKNOWN           <- '[*]'              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_anyerror    <- 'anyerror'    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_cancel      <- 'cancel'      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_nakedcc     <- 'nakedcc'     end_of_word
KEYWORD_noalias     <- 'noalias'     end_of_word
KEYWORD_null        <- 'null'        end_of_word
KEYWORD_or          <- 'or'          end_of_word
KEYWORD_orelse      <- 'orelse'      end_of_word
KEYWORD_packed      <- 'packed'      end_of_word
KEYWORD_promise     <- 'promise'     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_stdcallcc   <- 'stdcallcc'   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_use         <- 'use'         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_and / KEYWORD_allowzero / KEYWORD_anyerror / KEYWORD_asm
         / KEYWORD_async / KEYWORD_await / KEYWORD_break / KEYWORD_cancel
         / 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_nakedcc / KEYWORD_noalias / KEYWORD_null / KEYWORD_or
         / KEYWORD_orelse / KEYWORD_packed / KEYWORD_promise / KEYWORD_pub
         / KEYWORD_resume / KEYWORD_return / KEYWORD_linksection
         / KEYWORD_stdcallcc / KEYWORD_struct / KEYWORD_suspend
         / KEYWORD_switch / KEYWORD_test / KEYWORD_threadlocal / KEYWORD_true / KEYWORD_try
         / KEYWORD_undefined / KEYWORD_union / KEYWORD_unreachable
         / KEYWORD_use / 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.

Minimize energy spent on coding style.

Together we serve end users.