0.3.0 Release Notes

Download & Documentation

Zig is an open-source programming language designed for robustness, optimality, and clarity. Zig is aggressively pursuing its goal of overthrowing C as the de facto language for system programming. Zig intends to be so practical that people find themselves using it even if they dislike it.

This is a massive release, featuring 6 months of work and changes from 36 different contributors.

I tried to give credit where credit is due, but it's inevitable I missed some contributions as I had to go through 1,345 commits to type up these release notes. I apologize in advance for any mistakes.

Special thanks to my patrons who provide financial support. You're making Zig sustainable.

Stack Traces on All Targets

Zig uses LLVM's debug info API to emit native debugging information on all targets. This means that you can use native debugging tools on Zig code, for example:

In addition, Zig's standard library can read its own native debug information. This means that crashes produce stack traces, and errors produce Error Return Traces.


This implementation is able to look at the executable's own memory to find out where the .o files are, which have the DWARF info.


Thanks to Sahnvour for implementing the PE parsing and starting the effort to PDB parsing. I picked up where he left off and finished Windows stack traces.

Thanks to Zachary Turner from the LLVM project for helping me understand the PDB format. I still owe LLVM some PDB documentation patches in return.

Similar to MacOS, a Windows executable in memory has location information pointing to a .pdb file which contains debug information.


Linux stack traces worked in 0.2.0. However now std.debug.dumpStackTrace & friends use ArenaAllocator backed by DirectAllocator. This has the downside of failing to print a stack trace when the system is out of memory, but for the more common use case when the system is not out of memory, but the debug info cannot fit in std.debug.global_allocator, now stack traces will work. This is the case for the self hosted compiler. There is a proposal to mmap() debug info rather than using read().

See also Compatibility with Valgrind.

zig fmt

Thanks to Jimmi Holst Christensen's diligent work, the Zig standard library now supports parsing Zig code. This API is used to implement zig fmt, a tool that reformats code to fit the canonical style.

As an example, zig fmt will change this code:

test "fmt" {
const a = []u8{
    1, 2, //
    4, // foo
    7 };
         switch (0) { 0 => {}, 1 => unreachable, 2,
            3 => {}, 4...7 => {}, 1 + 4 * 3 + 22 => {}, else => { const a = 1; const b = a; }, }

    foo(a, b, c, d, e, f, g,);

...into this code:

test "fmt" {
    const a = []u8{
        1, 2,
        3, 4, // foo
        5, 6,
    switch (0) {
        0 => {},
        1 => unreachable,
        2, 3 => {},
        4...7 => {},
        1 + 4 * 3 + 22 => {},
        else => {
            const a = 1;
            const b = a;


It does not make any decisions about line widths. That is left up to the user. However, it follows certain cues about when to line break. For example, it will put the same number of array items in a line as there are in the first one. And it will put a function call all on one line if there is no trailing comma, but break every parameter into its own line if there is a trailing comma.

Thanks to Marc Tiehuis, there are currently two editor plugins that integrate with zig fmt:

zig fmt is only implemented in the self-hosted compiler, which is not finished yet. To use it, one must follow the README instructions to build the self-hosted compiler from source.

The implementation of the self-hosted parser is an interesting case study of avoiding recursion by using an explicit stack. It is essentially a hand-written recursive descent parser, but with heap allocations instead of recursion. When Jimmi originally implemented the code, we thought that we could not solve the unbounded stack growth problem of recursion. However, since then, I prototyped several solutions that provide the ability to have recursive function calls without giving up statically known upper bound stack growth. See Recursion Status for more details.

Automatic formatting can be disabled in source files with a comment like this:

// zig fmt: off
test     "this is left alone"  {   }
// zig fmt: on

zig fmt is written using the standard library's event-based I/O abstractions and async/await syntax, which means that it is multi-threaded with non-blocking I/O. A debug build of zig fmt on my laptop formats the entire Zig standard library in 2.1 seconds, which is 75,516 lines per second. See Concurrency Status for more details.

zig run

zig run file.zig can now be used to execute a file directly.

Thanks to Marc Tiehuis for the initial implementation of this feature. Marc writes:

On a POSIX system, a shebang can be used to run a zig file directly. An example shebang would be #!/usr/bin/zig run. You may not be able pass extra compile arguments currently as part of the shebang. Linux for example treats all arguments after the first as a single argument which will result in an 'invalid command'.

Note: there is a proposal to change this to zig file.zig to match the interface of other languages, as well as enable the common pattern #!/usr/bin/env zig.

Zig caches the binary generated by zig run so that subsequent invocations have low startup cost. See Build Artifact Caching for more details.

Automated Static Linux x86_64 Builds of Master Branch

Zig now supports building statically against musl libc.

On every master branch push, the continuous integration server creates a static Linux build of zig and updates the URL https://ziglang.org/builds/zig-linux-x86_64-master.tar.xz to redirect to it.

In addition, Zig now looks for libc and the Zig standard library at runtime. This makes static builds the easiest and most reliable way to start using the latest version of Zig immediately.

Windows has automated static builds of master branch via AppVeyor.

MacOS static CI builds are in progress and should be available soon.

Pointer Reform

During this release cycle, two design flaws were fixed, which led to a chain reaction of changes that I called Pointer Reform, resulting in a more consistent syntax with simpler semantics.

The first design flaw was that the syntax for pointers was ambiguous if the pointed to type was a type. Consider this 0.2.0 code:

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

comptime {
    var a: i32 = 1;
    const b = &a;
    *b = 2;
    assert(a == 2);

This works fine. The value printed from the @compileLog statement is &i32. This makes sense because b is a pointer to a.

Now let's do it with a type:

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

comptime {
    var a: type = i32;
    const b = &a;
    *b = f32;
    assert(a == f32);
$ zig build-obj test.zig 
| &i32
test.zig:6:5: error: found compile log statement
test.zig:7:5: error: attempt to dereference non-pointer type 'type'
    *b = f32;

It doesn't work in 0.2.0, because the & operator worked differently for type than other types. Here, b is the type &i32 instead of a pointer to a type which is how we wanted to use it.

This prevented other things from working too; for example if you had a []type{i32, u8, f64} and you tried to use a for loop, it crashed the compiler because internally a for loop uses the & operator on the array element.

The only reasonable solution to this is to have different syntax for the address-of operator and the pointer operator, rather than them both being &.

So pointer syntax becomes *T, matching syntax from most other languages such as C. Address-of syntax remains &foo, again matching common address-of syntax such as in C. This leaves one problem though.

With this modification, the syntax *foo becomes ambiguous with the syntax for dereferencing. And so dereferencing syntax is changed to a postfix operator: foo.*. This matches post-fix indexing syntax: foo[0], and in practice ends up harmonizing nicely with other postfix operators.

The other design flaw is a problem that has plagued C since its creation: the pointer type doesn't tell you how many items there are at the address. This is now fixed by having two kinds of pointers in Zig:

Note that this causes pointers to arrays to fall into place, as a single-item pointer to an array acts as a pointer to a compile-time known number of items:

Consider how slices fit into this picture:

This makes Zig pointers significantly less error prone. For example, it fixed issue #386, which demonstrates how a pointer to an array in Zig 0.2.0 is a footgun when passed as a parameter. Meanwhile in 0.3.0, equivalent code is nearly impossible to get wrong.

For consistency with the postfix pointer dereference operator, optional unwrapping syntax is now postfix as well:

0.2.0: ??x

0.3.0: x.?

And finally, to remove the last inconsistency of optional syntax, the ?? operator is now the keyword orelse. This means that Zig now has the property that all control flow occurs exclusively via keywords.

There is a plan for one more pointer type, which is a pointer that has a null-terminated number of items. This would be the type of the parameter to strlen for example. Although this will make the language bigger by adding a new type, it allows Zig to delete a feature in exchange, since it will make C string literals unnecessary. String literals will both have a compile-time known length and be null-terminated; therefore they will implicitly cast to slices as well as null-terminated pointers.

There is one new issue caused by Pointer Reform. Because C does not have the concept of single-item pointers or unknown-length pointers (or non-null pointers), Zig must translate all C pointers as ?[*]T. That is, a pointer to an unknown number of items that might be null. This can cause some friction when using C APIs, which is unfortunate because Zig's types are perfectly compatible with C's types, but .h files are unable to adequately describe pointers. Although it would be much safer to translate .h files offline and fix their prototypes, there is a proposal to add a C pointer type. This new pointer type should never be used on purpose, but would be used when auto-translating C code. It would simply have C pointer semantics, which means it would be just as much of a footgun as C pointers are. The upside is that it would make interaction with C APIs go back to being perfectly seamless.

Default Float Mode is now Strict

In response to an overwhelming consensus, floating point operations use Strict mode by default. Code can use @setFloatMode to override the mode on a per-scope basis.

Thanks to Marc Tiehuis for implementing the change.

Remove this

this was always a weird language feature. An identifier which referred to the thing in the most immediate scope, which could be a module, a type, a function, or even a block of code.

The main use case for it was for anonymous structs to refer to themselves. This use case is solved with a new builtin function, @This(), which always returns the innermost struct or union that the builtin call is inside.

The "block of code" type is removed from Zig, and the first argument of @setFloatMode is removed. @setFloatMode now always refers to the current scope.

Remove Explicit Casting Syntax

Previously, these two lines would have different meanings:

export fn foo(x: u32) void {
    const a: u8 = x;
    const b = u8(x);

The assignment to a would give error: expected type 'u8', found 'u32', because not all values of u32 can fit in a u8. But the assignment to b was "cast harder" syntax, and Zig would truncate bits, with a safety check to ensure that the mathematical meaning of the integer was preserved.

Now, both lines are identical in semantics. There is no more "cast harder" syntax. Both cause the compile error because 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. For other casts, Zig has builtin functions:

Some are safe; some are not. Some perform language-level assertions; some do not. Some are no-ops at runtime; some are not. Each casting function is documented independently.

Having explicit and fine-grained casting like this is a form of intentional redundancy. Casts are often the source of bugs, and therefore it is worth double-checking a cast to verify that it is still correct when the type of the operand changes. For example, imagine that we have the following code:

fn foo(x: i32) void {
    var i = @intCast(usize, x);

Now consider what happens when the type of x changes to a pointer:

test.zig:2:29: error: expected integer type, found '*i32'
    var i = @intCast(usize, x);
Although we technically know how to convert a pointer to an integer, because we used @intCast, we are forced to inspect the cast and change it appropriately. Perhaps that means changing it to @ptrToInt, or perhaps the entire function needs to be reworked in response to the type change.

Direct Parameter Passing

Previously, it was illegal to pass structs and unions by value in non-extern functions. Instead, one would have to have the function accept a const pointer parameter. This was to avoid the ambiguity that C programs face - having to make the decision about whether by-value or by-reference was better. However, there were some problems with this. For example, when the parameter type is inferred, Zig would automatically convert to a const pointer. This caused problems in generic code, which could not distinguish between a type which is a pointer, and a type which has been automatically converted to a pointer.

Now, parameters can be passed directly:

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

const Foo = struct {
    x: i32,
    y: i32,

fn callee(foo: Foo) void {
    assert(foo.y == 2);

test "pass directly" {
    callee(Foo{ .x = 1, .y = 2 });

I have avoided using the term "by-value" because the semantics of this kind of parameter passing are different:

Because of these semantics, there's a clear flow chart for whether to accept a parameter as T or *const T:

Now that we have this kind of parameter passing, Zig's implicit cast from T to *const T is less important. One might even make the case that such a cast is dangerous. Therefore we have a proposal to remove it.

There is one more area that needs consideration with regards to direct parameter passing, and that is with coroutines. The problem is that if a reference to a stack variable is passed to a coroutine, it may become invalid after the coroutine suspends. This is a design flaw in Zig that will be addressed in a future version. See Concurrency Status for more details.

Note that extern functions are bound by the C ABI, and therefore none of this applies to them.

Rewrite Rand Functions

Marc Tiehuis writes:

We now use a generic Rand structure which abstracts the core functions from the backing engine.

The old Mersenne Twister engine is removed and replaced instead with three alternatives:

These should provide sufficient coverage for most purposes, including a CSPRNG using Isaac64. Consumers of the library that do not care about the actual engine implementation should use DefaultPrng and DefaultCsprng.

Error Return Traces across async/await

One of the problems with non-blocking programming is that stack traces and exceptions are less useful, because the actual stack trace points back to the event loop code.

In Zig 0.3.0, Error Return Traces work across suspend points. This means you can use try as the main error handling strategy, and when an error bubbles up all the way, you'll still be able to find out where it came from:

const std = @import("std");
const event = std.event;
const fs = event.fs;

test "unwrap error in async fn" {
    var da = std.heap.DirectAllocator.init();
    defer da.deinit();
    const allocator = &da.allocator;

    var loop: event.Loop = undefined;
    try loop.initMultiThreaded(allocator);
    defer loop.deinit();

    const handle = try async<allocator> openTheFile(&loop);
    defer cancel handle;


async fn openTheFile(loop: *event.Loop) void {
    const future = (async fs.openRead(loop, "does_not_exist.txt") catch unreachable);
    const fd = (await future) catch unreachable;
$ zig test test.zig
Test 1/1 unwrap error in async fn...attempt to unwrap error: FileNotFound
std/event/fs.zig:367:5: 0x22cb15 in ??? (test)
    return req_node.data.msg.Open.result;
std/event/fs.zig:374:13: 0x22e5fc in ??? (test)
            return await (async openPosix(loop, path, flags, os.File.default_mode) catch unreachable);
test.zig:22:31: 0x22f34b in ??? (test)
    const fd = (await future) catch unreachable;
std/event/loop.zig:664:25: 0x20c147 in ??? (test)
                        resume handle;
std/event/loop.zig:543:23: 0x206dee in ??? (test)
test.zig:17:13: 0x206178 in ??? (test)

Tests failed. Use the following command to reproduce the failure:

Note that this output contains 3 components:

It is important to note in this example, that the error return trace survived despite the fact that the event loop is multi-threaded, and any one of those threads could be the worker thread that resumes an async function at the await point.

This feature is enabled by default for Debug and ReleaseSafe builds, and disabled for ReleaseFast and ReleaseSmall builds.

This is just the beginning of an exploration of what debugging non-blocking behavior could look like in the future of Zig. See Concurrency Status for more details.

New Async Call Syntax

Instead of async(allocator) call(), now it is async<allocator> call().

This fixes syntax ambiguity when leaving off the allocator, and fixes parse failure when call is a field access.

This sets a precedent for using < > to pass arguments to a keyword. This will affect enum, union, fn, and align (see #661).

ReleaseSmall Mode

Alexandros Naskos contributed a new build mode.

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

New builtins: @typeInfo and @field

Alexandros Naskos bravely dove head-first into the deepest, darkest parts of the Zig compiler and implemented an incredibly useful builtin function: @typeInfo.

This function accepts a type as a parameter, and returns a compile-time known value of this type:

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

    pub const Int = struct {
        is_signed: bool,
        bits: u8,

    pub const Float = struct {
        bits: u8,

    pub const Pointer = struct {
        size: Size,
        is_const: bool,
        is_volatile: bool,
        alignment: u32,
        child: type,

        pub const Size = enum {

    pub const Array = struct {
        len: usize,
        child: type,

    pub const ContainerLayout = enum {

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

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

    pub const Optional = struct {
        child: type,

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

    pub const Error = struct {
        name: []const u8,
        value: usize,

    pub const ErrorSet = struct {
        errors: []Error,

    pub const EnumField = struct {
        name: []const u8,
        value: usize,

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

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

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

    pub const CallingConvention = enum {

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

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

    pub const Promise = struct {
        child: ?type,

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

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

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

                pub const Inline = enum {

This kicks open the door for compile-time reflection, especially when combined with the fact that Jimmi Holst Christensen implemented @field, which performs field access with a compile-time known name:

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

test "@field" {
    const Foo = struct {
        one: i32,
        two: bool,
    var f = Foo{
        .one = 42,
        .two = true,

    const names = [][]const u8{ "two", "one" };

    assert(@field(f, names[0]) == true);
    assert(@field(f, names[1]) == 42);

    @field(f, "one") += 1;
    assert(@field(f, "on" ++ "e") == 43);

This has the potential to be abused, and so the feature should be used carefully.

After Jimmi implemented @field, he improved the implementation of @typeInfo and fixed several bugs. And now, the combination of these builtins is used to implement struct printing in userland:

const std = @import("std");

const Foo = struct {
    one: i32,
    two: *u64,
    three: bool,

pub fn main() void {
    var x: u64 = 1234;
    var f = Foo{
        .one = 42,
        .two = &x,
        .three = false,

    std.debug.warn("here it is: {}\n", f);


here it is: Foo{ .one = 42, .two = u64@7ffdda208cf0, .three = false }

See std/fmt/index.zig:15 for the implementation.

Now that we have @typeInfo, there is one more question to answer: should there be a function which accepts a TypeInfo value, and makes a type out of it?

This hypothetical feature is called @reify, and it's a hot topic. Although undeniably powerful and useful, there is concern that it would be too powerful, leading to complex meta-programming that goes against the spirit of simplicity that Zig stands for.

Improve cmpxchg

@cmpxchg is removed. @cmpxchgStrong and @cmpxchgWeak are added.

The functions have operand type as the first parameter.

The return type is ?T where T is the operand type.

New Type: f16

Ben Noordhuis implemented f16. This is guaranteed to be IEEE-754-2008 binary16 format, even on systems that have no hardware support, thanks to the additions to compiler_rt that Ben contributed. He also added support for f16 to std.math functions such as isnormal and fabs.

All Integer Sizes are Primitives

Zig 0.2.0 had primitive types for integer bit widths of 2-8, 16, 29, 32, 64, 128. Any number other than that, and you had to use @IntType to create the type. But you would get a compile error if you shadowed one of the above bit widths that already existed, for example with

const u29 = @IntType(false, 29); // error: u29 shadows primitive type

Needless to say, this situation was unnecessarily troublesome (#745). And so now 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.

u0 is a 0-bit type, which means:

i0 doesn't make sense and will probably crash the compiler.

Although Zig defines arbitrary integer sizes to support all primitive operations, if you try to use, for example, multiplication on 256-bit integers:

test "large multiplication" {
    var x: u256 = 0xabcd;
    var y: u256 = 0xefef;
    var z = x * y;

Then you'll get an error like this:

LLVM ERROR: Unsupported library call operation!

Zig isn't supposed to be letting LLVM leak through here, but that's a separate issue. What's happening is that normally if a primitive operation such as multiplication of integers cannot be lowered to a machine instruction, LLVM will emit a library call to compiler_rt to perform the operation. This works for up to 128-bit multiplication, for example. However compiler_rt does not define an arbitrary precision multiplication library function, and so LLVM is not able to generate code.

It is planned to submit a patch to LLVM which adds the ability to emit a lib call for situations like this, and then Zig will include the arbitrary precision multiplication function in Zig's compiler_rt.

In addition to this, Zig 0.3.0 fixes a bug where @IntType was silently wrapping the bit count parameter if it was greater than pow(2, 32).

Improved f128 Support

Marc Tiehuis & Ben Noordhuis solved the various issues that prevented f128 from being generally useful.

Build Artifact Caching

Zig now supports global build artifact caching. This feature is one of those things that you can generally ignore, because it "just works" without any babysitting.

By default, compilations are not cached. You can enable the global cache for a compilation by using --cache on:

andy@xps:~/tmp$ time zig build-exe hello.zig 

real	0m0.414s
user	0m0.369s
sys	0m0.049s
andy@xps:~/tmp$ time zig build-exe hello.zig --cache on

real	0m0.412s
user	0m0.377s
sys	0m0.038s
andy@xps:~/tmp$ time zig build-exe hello.zig --cache on

real	0m0.012s
user	0m0.009s
sys	0m0.003s

When the cache is on, the output is not written to the current directory. Instead, the output is kept in the cache directory, and the path to it is printed to stdout.

This is off by default, because this is an uncommon use case. The real benefit of build artifact caching comes in 3 places:

The cache is perfect; there are no false positives. You could even fix a bug in memcpy in the system's libc, and Zig will detect that its own code has (indirectly) been updated, and invalidate the cache entry.

If you use zig build-exe, Zig will still create a zig-cache directory in the current working directory in order to store an intermediate .o file. This is because on MacOS, the intermediate .o file stores the debug information, and therefore it needs to stick around somewhere sensible for Stack Traces to work.

Likewise, if you use zig test, Zig will put the test binary in the zig-cache directory in the current working directory. It's useful to leave the test binary here so that the programmer can use a debugger on it or otherwise inspect it.

The zig-cache directory is cleaner than before, however. For example, the builtin.zig file is no longer created there. It participates in the global caching system, just like compiler_rt.o. You can use zig builtin to see the contents of @import("builtin").

Compatibility with Valgrind

I noticed that valgrind does not see Zig's debug symbols (#896):

pub fn main() void {
    foo().* += 1;

fn foo() *i32 {
    return @intToPtr(*i32, 10000000);
==24133== Invalid read of size 4
==24133==    at 0x2226D5: ??? (in /home/andy/downloads/zig/build/test)
==24133==    by 0x2226A8: ??? (in /home/andy/downloads/zig/build/test)
==24133==    by 0x222654: ??? (in /home/andy/downloads/zig/build/test)
==24133==    by 0x2224B7: ??? (in /home/andy/downloads/zig/build/test)
==24133==    by 0x22236F: ??? (in /home/andy/downloads/zig/build/test)

After digging around, I was able to reproduce the problem using only Clang and LLD:

static int *foo(void) {
    return (int *)10000000;

int main(void) {
    int *x = foo();
    *x += 1;

If this C code is built with Clang and linked with LLD, Valgrind has the same issue as with the Zig code.

I sent a message to the Valgrind mailing list, and they suggested submitting a bug fix to Valgrind. That's a good idea. I'm a little busy with Zig development though - anybody else want to take a crack at it?

In the meantime, Zig now has a --no-rosegment flag, which works around the bug. It should only be used for this purpose; the flag will likely be removed once Valgrind fixes the issue upstream and enough time passes that the new version becomes generally available.

$ zig build-exe test.zig --no-rosegment
$ valgrind ./test
==24241== Invalid read of size 4
==24241==    at 0x221FE5: main (test.zig:2)

Zig is now on Godbolt Compiler Explorer

Marc Tiehuis added Zig support, and then worked with the Compiler Explorer team to get it merged upstream and deployed.

The command line API that Compiler Explorer uses is covered by Zig's main test suite to ensure that it continues working as the language evolves.

zig init-lib and init-exe

zig init-lib can be used to initialize a zig build project in the current directory which will create a simple library:

$ zig init-lib
Created build.zig
Created src/main.zig

Next, try `zig build --help` or `zig build test`
$ zig build test
Test 1/1 basic add functionality...OK
All tests passed.

Likewise, zig init-exe initializes a simple application:

$ zig init-exe
Created build.zig
Created src/main.zig

Next, try `zig build --help` or `zig build run`
$ zig build run
All your base are belong to us.

The main Zig test suite tests this functionality so that it will not regress as Zig continues to evolve.

Concurrency Status

Concurrency is now solved. That is, there is a concrete plan for how concurrency will work in Zig, and now it's a matter of implementing all the pieces.

First and foremost, Zig supports low-level control over hardware. That means that it has atomic primitives:

...and it means that you can directly spawn kernel threads using standard library functions:

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

test "spawn threads" {
    var shared_ctx: i32 = 1;

    const thread1 = try std.os.spawnThread({}, start1);
    const thread2 = try std.os.spawnThread(&shared_ctx, start2);
    const thread3 = try std.os.spawnThread(&shared_ctx, start2);
    const thread4 = try std.os.spawnThread(&shared_ctx, start2);


    assert(shared_ctx == 4);

fn start1(ctx: void) u8 {
    return 0;

fn start2(ctx: *i32) u8 {
    _ = @atomicRmw(i32, ctx, AtomicRmwOp.Add, 1, AtomicOrder.SeqCst);
    return 0;

On POSIX targets, when you link against libc, the standard library uses pthreads; otherwise it uses its own lightweight kernel thread implementation.

You can use mutexes, signals, condition variables, and all those things. Anything you can accomplish in C, you can accomplish in Zig.

However, the standard library provides a higher level concurrency abstraction, designed for optimal performance, debuggability, and structuring code to closely model the problems that concurrency presents.

The abstraction is built on two language features: stackless coroutines and async/await syntax. Everything else is implemented in userland.

std.event.Loop creates a kernel thread pool matching the number of logical CPUs. It can then be used for non-blocking I/O that will be dispatched across the thread pool, using the platform-native API:

This is a competitor to libuv, except multi-threaded.

Once you have an event loop, all of the std.event API becomes available to use:

All of these abstractions provide convenient APIs based on async/await syntax, making it practical for API users to model their code with maximally efficient concurrency. None of these abstractions block or use mutexes; when an API user must suspend, control flow goes to the next coroutine waiting to run, if any. If no coroutines are waiting to run, the application will sit idly, waiting for an event from the respective platform-native API (e.g. epoll on Linux).

As an example, here is a snippet from a test in the standard library:

async fn testFsWatch(loop: *Loop) !void {
    const file_path = try os.path.join(loop.allocator, test_tmp_dir, "file.txt");
    defer loop.allocator.free(file_path);

    const contents =
        \\line 1
        \\line 2
    const line2_offset = 7;

    // first just write then read the file
    try await try async fs.writeFile(loop, file_path, contents);

    const read_contents = try await try async fs.readFile(loop, file_path, 1024 * 1024);
    assert(mem.eql(u8, read_contents, contents));

    // now watch the file
    var watch = try fs.Watch(void).create(loop, 0);
    defer watch.destroy();

    assert((try await try async watch.addFile(file_path, {})) == null);

    const ev = try async watch.channel.get();
    var ev_consumed = false;
    defer if (!ev_consumed) cancel ev;

    // overwrite line 2
    const fd = try await try async fs.openReadWrite(loop, file_path, os.File.default_mode);
        defer os.close(fd);

        try await try async fs.pwritev(loop, fd, [][]const u8{"lorem ipsum"}, line2_offset);

    ev_consumed = true;
    switch ((try await ev).id) {
        WatchEventId.CloseWrite => {},
        WatchEventId.Delete => @panic("wrong event"),
    const contents_updated = try await try async fs.readFile(loop, file_path, 1024 * 1024);
    assert(mem.eql(u8, contents_updated,
        \\line 1
        \\lorem ipsum

You can see that even though Zig is a language with manual memory management that insists on handling every possible error, it manages to be quite high level using these event-based APIs.

Now, there are some problems to solve:

And so, the plan is to rework coroutines, without using any of LLVM's coroutines API. Zig will implement coroutines in the frontend, and LLVM will see only functions and structs. This is how Rust does it, and I think it was a strong choice.

The coroutine frame will be in a struct, and so Zig will know the size of it at compile-time, and it will solve the problem of guaranteeing allocation elision - the async callsite will simply have to provide the coroutine frame pointer in order to create the promise.

This will also be relevant for recursion; stackless function calls do not count against the static stack size upper bound calculation. See Recursion Status for more details.

Self-Hosted Compiler Status

The self-hosted compiler is well underway. Here's a 1 minute demo of the self-hosted compiler watching source files and rebuilding.

The self-hosted compiler cannot do much more than Hello World at the moment, but it's being constructed from the ground up to fully take advantage of multiple cores and in-memory caching. In addition, Zig's error system and other safety features are making it easy to write reliable, robust code. Between stack traces, error return traces, and runtime safety checks, I barely even need a debugger.

Marc Tiehuis contributed a Big Integer library, which the self-hosted compiler is using for integer literals and compile-time math operations.

Writing the self-hosted compiler code revealed to me how coroutines should work in Zig. All the little details and ergonomics are clear to me now. And so before I continue any further on the self-hosted compiler, I will use this knowledge to rework coroutines and solve the problems with them. See Concurrency Status for more details.

As a reminder, even when the self-hosted compiler is complete, Zig will forever be stuck with the stage1 C++ compiler code. See The Grand Bootstrapping Plan for more details.

The self-hosted compiler is successfully sharing some C++ code with the stage1 compiler. For example the libLLVM C++ API wrapper is built into a static library, which then exports a C API wrapper. The self-hosted compiler links against this static library in order to make libLLVM C++ API calls via the C API wrapper. In addition, the Microsoft Visual Studio detection code requires the Windows COM API, which is also C++, and so a similar strategy is used. I think it's pretty neat that the build system builds a static library once and then ends up linking against it twice - one for each of the two compiler stages!

Recursion Status

I've said before that recursion is one of the enemies of perfect software, because it represents a way that a program can fail with no foolproof way of preventing it. With recursion, pick any stack size and I'll give you an input that will crash your program. Embedded developers are all too familiar with this problem.

It's always possible to rewrite code using an explicit stack using heap allocations, and that's exactly what Jimmi did in the self-hosted parser.

On the other hand, when recursion fits the problem, it's significantly more clear and maintainable. It would be a real shame to have to give it up.

I researched different ways that Zig could keep recursion, even when we introduce statically known stack upper bound size. I came up with a proof of concept for @newStackCall, a builtin function that calls a function using an explicitly provided new stack. You can find a usage example in the documentation by following that link.

This works, and it does break call graph cycles, but it would be a little bit awkward to use. Because if you allocate an entire new stack, it has to be big enough for the rest of the stack upper bound size, but in a recursive call, which should be only one stack frame, it would overallocate every time.

So that's why I think that the actual solution to this problem is Zig's stackless coroutines. Because Zig's coroutines are stackless, they are the perfect solution for recursion (direct or indirect). With the reworking of coroutines, it will be possible to put the coroutine frame of an async function anywhere - in a struct, in the stack, in a global variable - as long as it outlives the duration of the coroutine. See Concurrency for more details.

Although recursion is not yet solved, we know enough to know that recursion is OK to use in Zig. It does suffer from the stack overflow issue today, but in the future we will have a compile error to prevent call graph cycles. And then this hypothetical compile error will be solved by using @newStackCall or stackless functions (but probably stackless functions). Once recursion is solved, if stackless functions turn out to be the better solution, Zig will remove @newStackCall from the language, unless someone demonstrates a compelling use case for it.

For now, use recursion whenever you want; you'll know when it's time to update your code.

WebAssembly Status

The pieces for web assembly are starting to come together.

Ben Noordhuis fixed support for --target-arch wasm32 (#1094).

LLVM merged my patch to make WebAssembly a normal (non-experimental) target. But they didn't do it before the LLVM 7 release. So Zig 0.3.0 will not have WebAssembly support by default, but 0.4.0 will.

That being said, the static builds of Zig provided by ziglang.org have the WebAssembly target enabled.

Apart from this, there appears to be an issue with Zig's WebAssembly linker. Once this is solved, all that is left is to use WebAssembly in real life use cases, to work out the ergonomics, and solve the inevitable issues that arise.


The language reference documentation now contains no JavaScript. The code blocks are pre-formatted with std.zig.Tokenizer. The same is true for these release notes.

The builtin.zig example code in the documentation is now automatically updated from the output of Zig, so the docs can't get out of date for this.

In addition to the above, the following improvements were made to the documentation:

Standard Library API Changes

Thank you contributors!

Miscellaneous Improvements

Bug Fixes

This Release Contains Bugs

Zig has known bugs.

The first release that will ship with no known bugs will be 1.0.0.


Active External Projects Using Zig

Thank you financial supporters!

Special thanks to those who donate monthly. We're now at $1,349 of the $3,000 goal. I hope this release helps to show how much time I've been able to dedicate to the project thanks to your support.

Thank you Andrea Orru for sending me a giant box of Turkish Delight