Zig is an open-source programming language designed for robustness, optimality, and clarity.
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.
hello.zig
const std = @import("std");
pub fn main() !void {
// If this program is run without stdout attached, exit with an error.
var 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:
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.
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.zig
const std = @import("std");
const warn = std.debug.warn;
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: error!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: error!i32
value: error.ArgNotFound
error union 2
type: error!i32
value: 1234
| 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 |
error |
(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.
See also:
| 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:
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(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:
| 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 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:
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;
^
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.
const decimal_int = 98222;
const hex_int = 0xff;
const another_hex_int = 0xFF;
const octal_int = 0o755;
const binary_int = 0b11110000;
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.
See also:
Zig has the following floating point types:
f16
- IEEE-754-2008 binary16f32
- IEEE-754-2008 binary32f64
- IEEE-754-2008 binary64f128
- IEEE-754-2008 binary128c_longdouble
- matches long double
for the target C ABI
Float literals have type comptime_float
which is guaranteed to hold at least all possible values
that the largest other floating point type can hold. Float literals implicitly cast to any other type.
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;
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:
| Syntax | Relevant Types | Description | Example |
|---|---|---|---|
|
Addition.
|
|
|
|
Wrapping Addition.
|
|
|
|
Subtraction.
|
|
|
|
Wrapping Subtraction.
|
|
|
|
Negation.
|
|
|
|
Wrapping Negation.
|
|
|
|
Multiplication.
|
|
|
|
Wrapping Multiplication.
|
|
|
|
Divison.
|
|
|
|
Remainder Division.
|
|
|
|
Bit Shift Left.
|
|
|
|
Bit Shift Right.
|
|
|
|
Bitwise AND.
|
|
|
|
Bitwise OR.
|
|
|
|
Bitwise XOR.
|
|
|
|
Bitwise NOT. |
|
|
|
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.
|
|
|
|
Equivalent to:
|
|
|
|
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 .
|
|
|
|
If a is false , returns false
without evaluating b . Otherwise, returns b .
|
|
|
|
If a is true , returns true
without evaluating b . Otherwise, returns b .
|
|
|
|
Boolean NOT. |
|
|
|
Returns true if a and b are equal, otherwise returns false .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Returns true if a is null , otherwise returns false .
|
|
|
|
Returns false if a and b are equal, otherwise returns true .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Returns true if a is greater than b, otherwise returns false .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Returns true if a is greater than or equal to b, otherwise returns false .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Returns true if a is less than b, otherwise returns false .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Returns true if a is less than or equal to b, otherwise returns false .
Invokes Peer Type Resolution for the operands.
|
|
|
|
Array concatenation.
|
|
|
|
Array multiplication.
|
|
|
|
Pointer dereference. |
|
|
|
All types | Address of. |
|
|
Merging Error Sets |
|
x() x[] x.y
a!b
!x -x -%x ~x &x ?x
x{} x.* x.?
! * / % ** *% ||
+ - ++ +% -%
<< >>
&
^
|
== != < > <= >=
and
or
orelse catch
= *= /= %= += -= <<= >>= &= ^= |=
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:
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;
// Deference a pointer:
assert(x_ptr.* == 1234);
// When you get the address of a const variable, you get a const pointer.
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);
}
test "pointer slicing" {
// In Zig, we prefer slices over pointers to null-terminated arrays.
// You can turn an array into a slice using slice syntax:
var array = []u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
const slice = array[2..4];
assert(slice.len == 2);
// Slices have bounds checking and are therefore protected
// against this kind of undefined behavior. This is one reason
// we prefer slices to pointers.
assert(array[3] == 4);
slice[1] += 1;
assert(array[3] == 5);
}
comptime {
// Pointers work at compile-time too, as long as you don't use
// @ptrCast.
var x: i32 = 1;
const ptr = &x;
ptr.* += 1;
x += 1;
assert(ptr.* == 3);
}
test "@ptrToInt and @intToPtr" {
// To convert an integer address into a pointer, use @intToPtr:
const ptr = @intToPtr(*i32, 0xdeadbeef);
// To convert a pointer to an integer, use @ptrToInt:
const addr = @ptrToInt(ptr);
assert(@typeOf(addr) == usize);
assert(addr == 0xdeadbeef);
}
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);
}
test "volatile" {
// In Zig, 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`:
const mmio_ptr = @intToPtr(*volatile u8, 0x12345678);
// Now loads and stores with mmio_ptr are guaranteed to all happen
// and in the same order as in source code.
assert(@typeOf(mmio_ptr) == *volatile u8);
}
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));
}
test "pointer casting" {
// 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.
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 1/8 address of syntax...OK
Test 2/8 pointer array access...OK
Test 3/8 pointer slicing...OK
Test 4/8 @ptrToInt and @intToPtr...OK
Test 5/8 volatile...OK
Test 6/8 optional pointers...OK
Test 7/8 pointer casting...OK
Test 8/8 pointer child type...OK
All tests passed.
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: 0x2052bf in ??? (test)
const int_slice = @bytesToSlice(u32, @alignCast(4, slice4));
^
/home/andy/dev/zig/docgen_tmp/test.zig:6:15: 0x2050a7 in ??? (test)
assert(foo(bytes) == 0x11111111);
^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x222aca in ??? (test)
if (test_fn.func()) |_| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:96:22: 0x22287b in ??? (test)
root.main() catch |err| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:70:20: 0x2227f5 in ??? (test)
return callMain();
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:64:39: 0x222658 in ??? (test)
std.os.posix.exit(callMainWithArgs(argc, argv, envp));
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:37:5: 0x222510 in ??? (test)
@noInlineCall(posixCallMainAndExit);
^
Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test
Zig uses Type Based Alias Analysis (also known as Strict Aliasing) to
perform some optimizations. This means that pointers of different types must
not alias the same memory, with the exception of u8
. Pointers to
u8
can alias any memory.
As an example, this code produces undefined behavior:
@ptrCast(*u32, f32(12.34)).*
Instead, use @bitCast:
@bitCast(u32, f32(12.34))
As an added benefit, the @bitCast
version works at compile-time.
See also:
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: 0x205156 in ??? (test)
slice[10] += 1;
^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x222a8a in ??? (test)
if (test_fn.func()) |_| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:96:22: 0x22283b in ??? (test)
root.main() catch |err| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:70:20: 0x2227b5 in ??? (test)
return callMain();
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:64:39: 0x222618 in ??? (test)
std.os.posix.exit(callMainWithArgs(argc, argv, envp));
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:37:5: 0x2224d0 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:
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: *const Vec3, other: *const 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.
packed
structs have guaranteed in-memory layout.
TODO bit fields
TODO alignment
TODO endianness
TODO @bitOffsetOf and @byteOffsetOf
TODO mention how volatile loads and stores of bit packed fields could be more efficient when done by hand instead of with packed struct
Since all structs are anonymous, Zig infers the type name based on a few rules.
return
expression, it gets named after
the function it is returning from, with the parameter values serialized.(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: (anonymous struct at /home/andy/dev/zig/docgen_tmp/struct_name.zig:6:49)
function: List(i32)
See also:
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.
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
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.
See also:
union.zig
const assert = @import("std").debug.assert;
const mem = @import("std").mem;
// A union has only 1 active field at a time.
const Payload = union {
Int: i64,
Float: f64,
Bool: bool,
};
test "simple union" {
var payload = Payload {.Int = 1234};
// payload.Float = 12.34; // ERROR! field not active
assert(payload.Int == 1234);
// You can activate another field by assigning the entire union.
payload = Payload {.Float = 12.34};
assert(payload.Float == 12.34);
}
// Unions can be given an enum tag type:
const ComplexTypeTag = enum { Ok, NotOk };
const ComplexType = union(ComplexTypeTag) {
Ok: u8,
NotOk: void,
};
// Declare a specific instance of the union variant.
test "declare union value" {
const c = ComplexType { .Ok = 0 };
assert(ComplexTypeTag(c) == ComplexTypeTag.Ok);
}
// @TagType can be used to access the enum tag type of a union.
test "@TagType" {
assert(@TagType(ComplexType) == ComplexTypeTag);
}
// Unions can be made to infer the enum tag type.
const Foo = union(enum) {
String: []const u8,
Number: u64,
// void can be omitted when inferring enum tag type.
None,
};
test "union variant switch" {
const p = Foo { .Number = 54 };
const what_is_it = switch (p) {
// Capture by reference
Foo.String => |*x| blk: {
break :blk "this is a string";
},
// Capture by value
Foo.Number => |x| blk: {
assert(x == 54);
break :blk "this is a number";
},
Foo.None => blk: {
break :blk "this is a none";
},
};
assert(mem.eql(u8, what_is_it, "this is a number"));
}
// Unions can have methods just like structs and enums:
const Variant = union(enum) {
Int: i32,
Bool: bool,
fn truthy(self: *const Variant) bool {
return switch (self.*) {
Variant.Int => |x_int| x_int != 0,
Variant.Bool => |x_bool| x_bool,
};
}
};
test "union method" {
var v1 = Variant { .Int = 1 };
var v2 = Variant { .Bool = false };
assert(v1.truthy());
assert(!v2.truthy());
}
const Small = union {
A: i32,
B: bool,
C: u8,
};
// @memberCount tells how many fields a union has:
test "@memberCount" {
assert(@memberCount(Small) == 3);
}
// @memberName tells the name of a field in an enum:
test "@memberName" {
assert(mem.eql(u8, @memberName(Small, 1), "B"));
}
// @tagName gives a []const u8 representation of an enum value,
// but only if the union has an enum tag type.
const Small2 = union(enum) {
A: i32,
B: bool,
C: u8,
};
test "@tagName" {
assert(mem.eql(u8, @tagName(Small2.C), "C"));
}
$ zig test union.zig
Test 1/8 simple union...OK
Test 2/8 declare union value...OK
Test 3/8 @TagType...OK
Test 4/8 union variant switch...OK
Test 5/8 union method...OK
Test 6/8 @memberCount...OK
Test 7/8 @memberName...OK
Test 8/8 @tagName...OK
All tests passed.
Unions with an enum tag are generated as a struct with a tag field and union field. Zig sorts the order of the tag and union field by the largest alignment.
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:
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);
}
test "switch enum" {
const Item = union(enum) {
A: u32,
C: struct { x: u8, y: u8 },
D,
};
var a = Item { .A = 3 };
// 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 == 3);
}
// 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 1/3 switch simple...OK
Test 2/3 switch enum...OK
Test 3/3 switch inside function...OK
All tests passed.
See also:
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.
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.
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.
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() error!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.
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:
See also:
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.
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.
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:
See also:
if.zig
// If expressions have three uses, corresponding to the three types:
// * bool
// * ?T
// * error!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: error!u32 = 0;
if (a) |value| {
assert(value == 0);
} else |err| {
unreachable;
}
const b: error!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: error!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:
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);
_ = deferErrorExample(true);
}
$ 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:
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
.
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: 0x2051c9 in ??? (test)
if (!ok) unreachable; // assertion failure
^
/home/andy/dev/zig/docgen_tmp/test.zig:7:11: 0x20504b in ??? (test)
assert(false);
^
/home/andy/dev/zig/build/lib/zig/std/special/test_runner.zig:13:25: 0x22297a in ??? (test)
if (test_fn.func()) |_| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:96:22: 0x22272b in ??? (test)
root.main() catch |err| {
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:70:20: 0x2226a5 in ??? (test)
return callMain();
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:64:39: 0x222508 in ??? (test)
std.os.posix.exit(callMainWithArgs(argc, argv, envp));
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:37:5: 0x2223c0 in ??? (test)
@noInlineCall(posixCallMainAndExit);
^
Tests failed. Use the following command to reproduce the failure:
/home/andy/dev/zig/docgen_tmp/test
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:
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() error!u32 {
return 1234;
}
const assert = @import("std").debug.assert;
$ zig test test.zig
Created /home/andy/dev/zig/docgen_tmp/test but skipping execution because it is non-native.
functions.zig
const assert = @import("std").debug.assert;
// Functions are declared like this
fn add(a: i8, b: i8) i8 {
if (a == 0) {
// You can still return manually if needed.
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) {}
}
// nakedcc makes a function not have any function prologue or epilogue.
// This can be useful when integrating with assembly.
nakedcc fn _start() noreturn {
abort();
}
// 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
In Zig, structs, unions, and enums with payloads can be passed directly to a function:
test.zig
const Point = struct {
x: i32,
y: i32,
};
fn foo(point: Point) i32 {
return point.x + point.y;
}
const assert = @import("std").debug.assert;
test "pass aggregate type by non-copy value to function" {
assert(foo(Point{ .x = 1, .y = 2 }) == 3);
}
$ zig test test.zig
Test 1/1 pass aggregate type by non-copy value to function...OK
All tests passed.
In this case, the value may be passed by reference, or by value, whichever way Zig decides will be faster.
For extern functions, Zig follows the C ABI for passing structs and unions by value.
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.
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,
^
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.
error
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 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.
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
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 => @maxValue(u8),
};
}
test "parse u64" {
const result = try parseU64("1234", 10);
@import("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 error!u64
.
What it looks like to use this function varies depending on what you're trying to do. One of the following:
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
.
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,
}
}
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:
catch unreachable
and
get the added benefit of crashing in Debug and ReleaseSafe modes if your assumption was wrong.
See also:
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: error!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 == error);
}
$ zig test test.zig
Test 1/1 error union...OK
All tests passed.
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.
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 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: 0x222bc9 in ??? (test)
return error.FileNotFound;
^
/home/andy/dev/zig/docgen_tmp/test.zig:23:5: 0x222aad in ??? (test)
try bang1();
^
/home/andy/dev/zig/docgen_tmp/test.zig:43:5: 0x222a79 in ??? (test)
return error.PermissionDenied;
^
/home/andy/dev/zig/docgen_tmp/test.zig:31:5: 0x222b9d in ??? (test)
try bang2();
^
/home/andy/dev/zig/docgen_tmp/test.zig:17:31: 0x222a49 in ??? (test)
error.FileNotFound => try hello(),
^
/home/andy/dev/zig/docgen_tmp/test.zig:7:9: 0x22293a in ??? (test)
try bar();
^
/home/andy/dev/zig/docgen_tmp/test.zig:2:5: 0x2227e2 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: 0x222734 in ??? (test)
@panic("PermissionDenied");
^
/home/andy/dev/zig/docgen_tmp/test.zig:30:10: 0x222779 in ??? (test)
bang2();
^
/home/andy/dev/zig/docgen_tmp/test.zig:17:14: 0x22271b in ??? (test)
hello();
^
/home/andy/dev/zig/docgen_tmp/test.zig:7:12: 0x2226e6 in ??? (test)
bar();
^
/home/andy/dev/zig/docgen_tmp/test.zig:2:8: 0x2226ce in ??? (test)
foo(12);
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:86:22: 0x2226a9 in ??? (test)
root.main();
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:70:20: 0x222655 in ??? (test)
return callMain();
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:64:39: 0x2224b8 in ??? (test)
std.os.posix.exit(callMainWithArgs(argc, argv, envp));
^
/home/andy/dev/zig/build/lib/zig/std/special/bootstrap.zig:37:5: 0x222370 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:
catch unreachable
and you have not overridden the default panic handlerstd.debug.dumpStackTrace
to print it. This function returns comptime-known null when building without error return tracing support.To analyze performance cost, there are two cases:
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.
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.
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.
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;
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.
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.
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 allowedvolatile
- non-volatile to volatile is allowedalign
- bigger to smaller alignment 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.
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.
TODO: [N]T to []const T
TODO: *const [N]T to []const T
TODO: [N]T to *const []const T
TODO: [N]T to ?[]const T
TODO: *[N]T to []T
TODO: *[N]T to [*]T
TODO: *[N]T to ?[*]T
TODO: *T to *[1]T
TODO: [N]T to E![]const T
TODO: T to ?T
TODO: T to E!?T
TODO: null to ?T
TODO
TODO
TODO
TODO
TODO
TODO
TODO
TODO
TODO
TODO
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.
Peer Type Resolution occurs in these places:
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 error![]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) error![]u8 {
if (a) {
return []u8{};
}
return slice[0..1];
}
$ zig test test.zig
Test 1/6 peer resolve int widening...OK
Test 2/6 peer resolve arrays of different size to const slice...OK
Test 3/6 peer resolve array and const slice...OK
Test 4/6 peer type resolution: ?T and T...OK
Test 5/6 peer type resolution: [0]u8 and []const u8...OK
Test 6/6 peer type resolution: [0]u8, []const u8, and error![]u8...OK
All tests passed.
void
represents a type that has no value. Code that makes use of void values is
not included in the final generated code:
export fn entry() void {
var x: void = {};
var y: void = {};
x = y;
}
When this turns into LLVM IR, 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
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
:
test.zig
test "ignoring expression value" {
foo();
}
fn foo() void {}
$ zig test test.zig
Test 1/1 ignoring expression value...OK
All tests passed.
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.
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:
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:1:29: error: unable to evaluate constant expression
fn max(comptime T: type, a: T, b: T) T {
^
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.
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.
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:
comptime
variables.if
, while
, for
, and switch
expressions are evaluated at compile-time, or emit a compile error if this is not possible.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/index.zig:118:13: error: encountered @panic at compile-time
@panic("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.
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.
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: ...) error!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/index.zig:95:13: error: Unused arguments
@compileError("Unused arguments");
^
/home/andy/dev/zig/build/lib/zig/std/io.zig:227:34: note: called from here
return std.fmt.format(self, Error, self.writeFn, format, args);
^
/home/andy/dev/zig/build/lib/zig/std/debug/index.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:
TODO: example of inline assembly
TODO: example of module level assembly
TODO: example of using inline assembly return value
TODO: example of using inline assembly assigning values to variables
TODO: @fence()
TODO: @atomic rmw
TODO: builtin atomic memory ordering enum
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.
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.
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.
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.
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.
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:
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(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
.
@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(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:
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(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:
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(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:
f32
to u32
bitsi32
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(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:
@breakpoint()
This function inserts a platform-specific debug trap instruction which causes debuggers to break there.
This function is only valid within function scope.
@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:
@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(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:
@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
.
@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(comptime name: []u8, value)
This function can only occur inside @cImport
.
This appends #define $name $value
to the @cImport
temporary buffer.
To define without a value, like this:
#define _GNU_SOURCE
Use the void value, like this:
@cDefine("_GNU_SOURCE", {})
See also:
@cImport(expression) (namespace)
This function parses C code and imports the functions, types, variables, and compatible macro definitions into the result namespace.
expression
is interpreted at compile time. The builtin functions
@cInclude
, @cDefine
, and @cUndef
work
within this expression, appending to a temporary buffer which is then parsed as C code.
Usually you should only have one @cImport
in your entire application, because it saves the compiler
from invoking clang multiple times, and prevents inline functions from being duplicated.
Reasons for having multiple @cImport
expressions would be:
#define CONNECTION_COUNT
See also:
@cInclude(comptime path: []u8)
This function can only occur inside @cImport
.
This appends #include <$path>\n
to the c_import
temporary buffer.
See also:
@cUndef(comptime name: []u8)
This function can only occur inside @cImport
.
This appends #undef $name
to the @cImport
temporary buffer.
See also:
@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:
@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:
@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:
@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(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(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:
@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:
@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 == @minValue(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(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 == @minValue(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:
@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:
@enumToInt(enum_value: var) var
Converts an enumeration value into its integer tag type.
If the enum has only 1 possible value, the resut is a comptime_int
known at comptime.
See also:
@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.
@errorName(err: error) []u8
This function returns the string representation of an error. If an error declaration is:
error OutOfMem
Then the string representation 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() ?*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(err: var) @IntType(false, @sizeOf(error) * 8)
Supports the following types:
E!void
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:
@export(comptime name: []const u8, target: var, linkage: builtin.GlobalLinkage) []const u8
Creates a symbol in the output object file.
@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:
@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(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(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(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:
@frameAddress()
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()
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(comptime path: []u8) (namespace)
This function finds a zig file corresponding to path
and imports all the
public top level declarations into the resulting namespace.
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 variablesSee also:
@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:
@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(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:
@intToError(value: @IntType(false, @sizeOf(error) * 8)) error
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:
@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(comptime DestType: type, int: usize) DestType
Converts an integer to a pointer. To convert the other way, use @ptrToInt.
@IntType(comptime is_signed: bool, comptime bit_count: u32) type
This function returns an integer type with the given signness and bit count.
@maxValue(comptime T: type) comptime_int
This function returns the maximum value of the integer type T
.
The result is a compile time constant.
@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(comptime T: type, comptime index: usize) [N]u8