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langref.html.in
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langref.html.in
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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<title>Documentation - The Zig Programming Language</title>
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<body>
<header><h1>Zig Language Reference</h1></header>
<div id="main-wrapper">
<div id="navigation">
<nav aria-labelledby="zig-version">
<h2 id="zig-version">Zig Version</h2>
<a href="https://ziglang.org/documentation/0.1.1/">0.1.1</a> |
<a href="https://ziglang.org/documentation/0.2.0/">0.2.0</a> |
<a href="https://ziglang.org/documentation/0.3.0/">0.3.0</a> |
<a href="https://ziglang.org/documentation/0.4.0/">0.4.0</a> |
<a href="https://ziglang.org/documentation/0.5.0/">0.5.0</a> |
<a href="https://ziglang.org/documentation/0.6.0/">0.6.0</a> |
<a href="https://ziglang.org/documentation/0.7.1/">0.7.1</a> |
<a href="https://ziglang.org/documentation/0.8.1/">0.8.1</a> |
<a href="https://ziglang.org/documentation/0.9.1/">0.9.1</a> |
<a href="https://ziglang.org/documentation/0.10.1/">0.10.1</a> |
<a href="https://ziglang.org/documentation/0.11.0/">0.11.0</a> |
<a href="https://ziglang.org/documentation/0.12.0/">0.12.0</a> |
master
</nav>
<nav aria-labelledby="table-of-contents">
<h2 id="table-of-contents">Table of Contents</h2>
{#nav#}
</nav>
</div>
<div id="contents-wrapper"><main id="contents">
{#header_open|Introduction#}
<p>
<a href="https://ziglang.org">Zig</a> is a general-purpose programming language and toolchain for maintaining
<strong>robust</strong>, <strong>optimal</strong>, and <strong>reusable</strong> software.
</p>
<dl>
<dt>Robust</dt><dd>Behavior is correct even for edge cases such as out of memory.</dd>
<dt>Optimal</dt><dd>Write programs the best way they can behave and perform.</dd>
<dt>Reusable</dt><dd>The same code works in many environments which have different
constraints.</dd>
<dt>Maintainable</dt><dd>Precisely communicate intent to the compiler and
other programmers. The language imposes a low overhead to reading code and is
resilient to changing requirements and environments.</dd>
</dl>
<p>
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.
</p>
<p>
The code samples in this document are compiled and tested as part of the main test suite of Zig.
</p>
<p>
This HTML document depends on no external files, so you can use it offline.
</p>
{#header_close#}
{#header_open|Zig Standard Library#}
<p>
The <a href="https://ziglang.org/documentation/master/std/">Zig Standard Library</a> has its own documentation.
</p>
<p>
Zig's Standard Library contains commonly used algorithms, data structures, and definitions to help you build programs or libraries.
You will see many examples of Zig's Standard Library used in this documentation. To learn more about the Zig Standard Library,
visit the link above.
</p>
{#header_close#}
{#header_open|Hello World#}
{#code|hello.zig#}
<p>
Most of the time, it is more appropriate to write to stderr rather than stdout, and
whether or not the message is successfully written to the stream is irrelevant.
For this common case, there is a simpler API:
</p>
{#code|hello_again.zig#}
<p>
In this case, the {#syntax#}!{#endsyntax#} may be omitted from the return
type because no errors are returned from the function.
</p>
{#see_also|Values|@import|Errors|Root Source File|Source Encoding#}
{#header_close#}
{#header_open|Comments#}
<p>
Zig supports 3 types of comments. Normal comments are ignored, but doc comments
and top-level doc comments are used by the compiler to generate the package documentation.
</p>
<p>
The generated documentation is still experimental, and can be produced with:
</p>
{#shell_samp#}zig test -femit-docs main.zig{#end_shell_samp#}
{#code|comments.zig#}
<p>
There are no multiline comments in Zig (e.g. like <code class="c">/* */</code>
comments in C). This allows Zig to have the property that each line
of code can be tokenized out of context.
</p>
{#header_open|Doc Comments#}
<p>
A doc comment is one that begins with exactly three slashes (i.e.
{#syntax#}///{#endsyntax#} but not {#syntax#}////{#endsyntax#});
multiple doc comments in a row are merged together to form a multiline
doc comment. The doc comment documents whatever immediately follows it.
</p>
{#code|doc_comments.zig#}
<p>
Doc comments are only allowed in certain places; it is a compile error to
have a doc comment in an unexpected place, such as in the middle of an expression,
or just before a non-doc comment.
</p>
{#code|invalid_doc-comment.zig#}
{#code|unattached_doc-comment.zig#}
<p>
Doc comments can be interleaved with normal comments. Currently, when producing
the package documentation, normal comments are merged with doc comments.
</p>
{#header_close#}
{#header_open|Top-Level Doc Comments#}
<p>
A top-level doc comment is one that begins with two slashes and an exclamation
point: {#syntax#}//!{#endsyntax#}; it documents the current module.
</p>
<p>
It is a compile error if a top-level doc comment is not placed at the start
of a {#link|container|Containers#}, before any expressions.
</p>
{#code|tldoc_comments.zig#}
{#header_close#}
{#header_close#}
{#header_open|Values#}
{#code|values.zig#}
{#header_open|Primitive Types#}
<div class="table-wrapper">
<table>
<caption>Primitive Types</caption>
<thead>
<tr>
<th scope="col">Type</th>
<th scope="col">C Equivalent</th>
<th scope="col">Description</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}i8{#endsyntax#}</th>
<td><code class="c">int8_t</code></td>
<td>signed 8-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u8{#endsyntax#}</th>
<td><code class="c">uint8_t</code></td>
<td>unsigned 8-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i16{#endsyntax#}</th>
<td><code class="c">int16_t</code></td>
<td>signed 16-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u16{#endsyntax#}</th>
<td><code class="c">uint16_t</code></td>
<td>unsigned 16-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i32{#endsyntax#}</th>
<td><code class="c">int32_t</code></td>
<td>signed 32-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u32{#endsyntax#}</th>
<td><code class="c">uint32_t</code></td>
<td>unsigned 32-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i64{#endsyntax#}</th>
<td><code class="c">int64_t</code></td>
<td>signed 64-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u64{#endsyntax#}</th>
<td><code class="c">uint64_t</code></td>
<td>unsigned 64-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}i128{#endsyntax#}</th>
<td><code class="c">__int128</code></td>
<td>signed 128-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}u128{#endsyntax#}</th>
<td><code class="c">unsigned __int128</code></td>
<td>unsigned 128-bit integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}isize{#endsyntax#}</th>
<td><code class="c">intptr_t</code></td>
<td>signed pointer sized integer</td>
</tr>
<tr>
<th scope="row">{#syntax#}usize{#endsyntax#}</th>
<td><code class="c">uintptr_t</code>, <code class="c">size_t</code></td>
<td>unsigned pointer sized integer. Also see <a href="https://github.com/ziglang/zig/issues/5185">#5185</a></td>
</tr>
<tr>
<th scope="row">{#syntax#}c_char{#endsyntax#}</th>
<td><code class="c">char</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_short{#endsyntax#}</th>
<td><code class="c">short</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ushort{#endsyntax#}</th>
<td><code class="c">unsigned short</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_int{#endsyntax#}</th>
<td><code class="c">int</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_uint{#endsyntax#}</th>
<td><code class="c">unsigned int</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_long{#endsyntax#}</th>
<td><code class="c">long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ulong{#endsyntax#}</th>
<td><code class="c">unsigned long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_longlong{#endsyntax#}</th>
<td><code class="c">long long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_ulonglong{#endsyntax#}</th>
<td><code class="c">unsigned long long</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}c_longdouble{#endsyntax#}</th>
<td><code class="c">long double</code></td>
<td>for ABI compatibility with C</td>
</tr>
<tr>
<th scope="row">{#syntax#}f16{#endsyntax#}</th>
<td><code class="c">_Float16</code></td>
<td>16-bit floating point (10-bit mantissa) IEEE-754-2008 binary16</td>
</tr>
<tr>
<th scope="row">{#syntax#}f32{#endsyntax#}</th>
<td><code class="c">float</code></td>
<td>32-bit floating point (23-bit mantissa) IEEE-754-2008 binary32</td>
</tr>
<tr>
<th scope="row">{#syntax#}f64{#endsyntax#}</th>
<td><code class="c">double</code></td>
<td>64-bit floating point (52-bit mantissa) IEEE-754-2008 binary64</td>
</tr>
<tr>
<th scope="row">{#syntax#}f80{#endsyntax#}</th>
<td><code class="c">double</code></td>
<td>80-bit floating point (64-bit mantissa) IEEE-754-2008 80-bit extended precision</td>
</tr>
<tr>
<th scope="row">{#syntax#}f128{#endsyntax#}</th>
<td><code class="c">_Float128</code></td>
<td>128-bit floating point (112-bit mantissa) IEEE-754-2008 binary128</td>
</tr>
<tr>
<th scope="row">{#syntax#}bool{#endsyntax#}</th>
<td><code class="c">bool</code></td>
<td>{#syntax#}true{#endsyntax#} or {#syntax#}false{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}anyopaque{#endsyntax#}</th>
<td><code class="c">void</code></td>
<td>Used for type-erased pointers.</td>
</tr>
<tr>
<th scope="row">{#syntax#}void{#endsyntax#}</th>
<td>(none)</td>
<td>Always the value {#syntax#}void{}{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}noreturn{#endsyntax#}</th>
<td>(none)</td>
<td>the type of {#syntax#}break{#endsyntax#}, {#syntax#}continue{#endsyntax#}, {#syntax#}return{#endsyntax#}, {#syntax#}unreachable{#endsyntax#}, and {#syntax#}while (true) {}{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}type{#endsyntax#}</th>
<td>(none)</td>
<td>the type of types</td>
</tr>
<tr>
<th scope="row">{#syntax#}anyerror{#endsyntax#}</th>
<td>(none)</td>
<td>an error code</td>
</tr>
<tr>
<th scope="row">{#syntax#}comptime_int{#endsyntax#}</th>
<td>(none)</td>
<td>Only allowed for {#link|comptime#}-known values. The type of integer literals.</td>
</tr>
<tr>
<th scope="row">{#syntax#}comptime_float{#endsyntax#}</th>
<td>(none)</td>
<td>Only allowed for {#link|comptime#}-known values. The type of float literals.</td>
</tr>
</tbody>
</table>
</div>
<p>
In addition to the integer types above, arbitrary bit-width integers can be referenced by using
an identifier of <code>i</code> or <code>u</code> followed by digits. For example, the identifier
{#syntax#}i7{#endsyntax#} refers to a signed 7-bit integer. The maximum allowed bit-width of an
integer type is {#syntax#}65535{#endsyntax#}.
</p>
{#see_also|Integers|Floats|void|Errors|@Type#}
{#header_close#}
{#header_open|Primitive Values#}
<div class="table-wrapper">
<table>
<caption>Primitive Values</caption>
<thead>
<tr>
<th scope="col">Name</th>
<th scope="col">Description</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row">{#syntax#}true{#endsyntax#} and {#syntax#}false{#endsyntax#}</th>
<td>{#syntax#}bool{#endsyntax#} values</td>
</tr>
<tr>
<th scope="row">{#syntax#}null{#endsyntax#}</th>
<td>used to set an optional type to {#syntax#}null{#endsyntax#}</td>
</tr>
<tr>
<th scope="row">{#syntax#}undefined{#endsyntax#}</th>
<td>used to leave a value unspecified</td>
</tr>
</tbody>
</table>
</div>
{#see_also|Optionals|undefined#}
{#header_close#}
{#header_open|String Literals and Unicode Code Point Literals#}
<p>
String literals are constant single-item {#link|Pointers#} to null-terminated byte arrays.
The type of string literals encodes both the length, and the fact that they are null-terminated,
and thus they can be {#link|coerced|Type Coercion#} to both {#link|Slices#} and
{#link|Null-Terminated Pointers|Sentinel-Terminated Pointers#}.
Dereferencing string literals converts them to {#link|Arrays#}.
</p>
<p>
Because Zig source code is {#link|UTF-8 encoded|Source Encoding#}, any
non-ASCII bytes appearing within a string literal in source code carry
their UTF-8 meaning into the content of the string in the Zig program;
the bytes are not modified by the compiler. It is possible to embed
non-UTF-8 bytes into a string literal using <code>\xNN</code> notation.
</p>
<p>Indexing into a string containing non-ASCII bytes returns individual
bytes, whether valid UTF-8 or not.</p>
<p>
Unicode code point literals have type {#syntax#}comptime_int{#endsyntax#}, the same as
{#link|Integer Literals#}. All {#link|Escape Sequences#} are valid in both string literals
and Unicode code point literals.
</p>
{#code|string_literals.zig#}
{#see_also|Arrays|Source Encoding#}
{#header_open|Escape Sequences#}
<div class="table-wrapper">
<table>
<caption>Escape Sequences</caption>
<thead>
<tr>
<th scope="col">Escape Sequence</th>
<th scope="col">Name</th>
</tr>
</thead>
<tbody>
<tr>
<th scope="row"><code>\n</code></th>
<td>Newline</td>
</tr>
<tr>
<th scope="row"><code>\r</code></th>
<td>Carriage Return</td>
</tr>
<tr>
<th scope="row"><code>\t</code></th>
<td>Tab</td>
</tr>
<tr>
<th scope="row"><code>\\</code></th>
<td>Backslash</td>
</tr>
<tr>
<th scope="row"><code>\'</code></th>
<td>Single Quote</td>
</tr>
<tr>
<th scope="row"><code>\"</code></th>
<td>Double Quote</td>
</tr>
<tr>
<th scope="row"><code>\xNN</code></th>
<td>hexadecimal 8-bit byte value (2 digits)</td>
</tr>
<tr>
<th scope="row"><code>\u{NNNNNN}</code></th>
<td>hexadecimal Unicode code point UTF-8 encoded (1 or more digits)</td>
</tr>
</tbody>
</table>
</div>
<p>Note that the maximum valid Unicode point is {#syntax#}0x10ffff{#endsyntax#}.</p>
{#header_close#}
{#header_open|Multiline String Literals#}
<p>
Multiline string literals have no escapes and can span across multiple lines.
To start a multiline string literal, use the {#syntax#}\\{#endsyntax#} 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 {#syntax#}\\{#endsyntax#} then a newline is appended and
the string literal continues.
</p>
{#code|multiline_string_literals.zig#}
{#see_also|@embedFile#}
{#header_close#}
{#header_close#}
{#header_open|Assignment#}
<p>Use the {#syntax#}const{#endsyntax#} keyword to assign a value to an identifier:</p>
{#code|constant_identifier_cannot_change.zig#}
<p>{#syntax#}const{#endsyntax#} applies to all of the bytes that the identifier immediately addresses. {#link|Pointers#} have their own const-ness.</p>
<p>If you need a variable that you can modify, use the {#syntax#}var{#endsyntax#} keyword:</p>
{#code|mutable_var.zig#}
<p>Variables must be initialized:</p>
{#code|var_must_be_initialized.zig#}
{#header_open|undefined#}
<p>Use {#syntax#}undefined{#endsyntax#} to leave variables uninitialized:</p>
{#code|assign_undefined.zig#}
<p>
{#syntax#}undefined{#endsyntax#} can be {#link|coerced|Type Coercion#} to any type.
Once this happens, it is no longer possible to detect that the value is {#syntax#}undefined{#endsyntax#}.
{#syntax#}undefined{#endsyntax#} means the value could be anything, even something that is nonsense
according to the type. Translated into English, {#syntax#}undefined{#endsyntax#} means "Not a meaningful
value. Using this value would be a bug. The value will be unused, or overwritten before being used."
</p>
<p>
In {#link|Debug#} mode, Zig writes {#syntax#}0xaa{#endsyntax#} bytes to undefined memory. This is to catch
bugs early, and to help detect use of undefined memory in a debugger. However, this behavior is only an
implementation feature, not a language semantic, so it is not guaranteed to be observable to code.
</p>
{#header_close#}
{#header_close#}
{#header_close#}
{#header_open|Zig Test#}
<p>
Code written within one or more {#syntax#}test{#endsyntax#} declarations can be used to ensure behavior meets expectations:
</p>
{#code|testing_introduction.zig#}
<p>
The <code class="file">testing_introduction.zig</code> code sample tests the {#link|function|Functions#}
{#syntax#}addOne{#endsyntax#} to ensure that it returns {#syntax#}42{#endsyntax#} given the input
{#syntax#}41{#endsyntax#}. From this test's perspective, the {#syntax#}addOne{#endsyntax#} function is
said to be <em>code under test</em>.
</p>
<p>
<kbd>zig test</kbd> is a tool that creates and runs a test build. By default, it builds and runs an
executable program using the <em>default test runner</em> provided by the {#link|Zig Standard Library#}
as its main entry point. During the build, {#syntax#}test{#endsyntax#} declarations found while
{#link|resolving|Root Source File#} the given Zig source file are included for the default test runner
to run and report on.
</p>
<aside>
This documentation discusses the features of the default test runner as provided by the Zig Standard Library.
Its source code is located in <code class="file">lib/test_runner.zig</code>.
</aside>
<p>
The shell output shown above displays two lines after the <kbd>zig test</kbd> command. These lines are
printed to standard error by the default test runner:
</p>
<dl>
<dt><samp>1/2 testing_introduction.test.expect addOne adds one to 41...</samp></dt>
<dd>Lines like this indicate which test, out of the total number of tests, is being run.
In this case, <samp>1/2</samp> indicates that the first test, out of a total of two tests,
is being run. Note that, when the test runner program's standard error is output
to the terminal, these lines are cleared when a test succeeds.
</dd>
<dt><samp>2/2 testing_introduction.decltest.addOne...</samp></dt>
<dd>When the test name is an identifier, the default test runner uses the text
decltest instead of test.
</dd>
<dt><samp>All 2 tests passed.</samp></dt>
<dd>This line indicates the total number of tests that have passed.</dd>
</dl>
{#header_open|Test Declarations#}
<p>
Test declarations contain the {#link|keyword|Keyword Reference#} {#syntax#}test{#endsyntax#}, followed by an
optional name written as a {#link|string literal|String Literals and Unicode Code Point Literals#} or an
{#link|identifier|Identifiers#}, followed by a {#link|block|Blocks#} containing any valid Zig code that
is allowed in a {#link|function|Functions#}.
</p>
<p>Non-named test blocks always run during test builds and are exempt from
{#link|Skip Tests#}.</p>
<p>
Test declarations are similar to {#link|Functions#}: they have a return type and a block of code. The implicit
return type of {#syntax#}test{#endsyntax#} is the {#link|Error Union Type#} {#syntax#}anyerror!void{#endsyntax#},
and it cannot be changed. When a Zig source file is not built using the <kbd>zig test</kbd> tool, the test
declarations are omitted from the build.
</p>
<p>
Test declarations can be written in the same file, where code under test is written, or in a separate Zig source file.
Since test declarations are top-level declarations, they are order-independent and can
be written before or after the code under test.
</p>
{#see_also|The Global Error Set|Grammar#}
{#header_open|Doctests#}
<p>
Test declarations named using an identifier are <em>doctests</em>. The identifier must refer to another declaration in
scope. A doctest, like a {#link|doc comment|Doc Comments#}, serves as documentation for the associated declaration, and
will appear in the generated documentation for the declaration.
</p>
<p>
An effective doctest should be self-contained and focused on the declaration being tested, answering questions a new
user might have about its interface or intended usage, while avoiding unnecessary or confusing details. A doctest is not
a substitute for a doc comment, but rather a supplement and companion providing a testable, code-driven example, verified
by <kbd>zig test</kbd>.
</p>
{#header_close#}
{#header_close#}
{#header_open|Test Failure#}
<p>
The default test runner checks for an {#link|error|Errors#} returned from a test.
When a test returns an error, the test is considered a failure and its {#link|error return trace|Error Return Traces#}
is output to standard error. The total number of failures will be reported after all tests have run.
</p>
{#code|testing_failure.zig#}
{#header_close#}
{#header_open|Skip Tests#}
<p>
One way to skip tests is to filter them out by using the <kbd>zig test</kbd> command line parameter
<kbd>--test-filter [text]</kbd>. This makes the test build only include tests whose name contains the
supplied filter text. Note that non-named tests are run even when using the <kbd>--test-filter [text]</kbd>
command line parameter.
</p>
<p>
To programmatically skip a test, make a {#syntax#}test{#endsyntax#} return the error
{#syntax#}error.SkipZigTest{#endsyntax#} and the default test runner will consider the test as being skipped.
The total number of skipped tests will be reported after all tests have run.
</p>
{#code|testing_skip.zig#}
{#header_close#}
{#header_open|Report Memory Leaks#}
<p>
When code allocates {#link|Memory#} using the {#link|Zig Standard Library#}'s testing allocator,
{#syntax#}std.testing.allocator{#endsyntax#}, the default test runner will report any leaks that are
found from using the testing allocator:
</p>
{#code|testing_detect_leak.zig#}
{#see_also|defer|Memory#}
{#header_close#}
{#header_open|Detecting Test Build#}
<p>
Use the {#link|compile variable|Compile Variables#} {#syntax#}@import("builtin").is_test{#endsyntax#}
to detect a test build:
</p>
{#code|testing_detect_test.zig#}
{#header_close#}
{#header_open|Test Output and Logging#}
<p>
The default test runner and the Zig Standard Library's testing namespace output messages to standard error.
</p>
{#header_close#}
{#header_open|The Testing Namespace#}
<p>
The Zig Standard Library's <code>testing</code> namespace contains useful functions to help
you create tests. In addition to the <code>expect</code> function, this document uses a couple of more functions
as exemplified here:
</p>
{#code|testing_namespace.zig#}
<p>The Zig Standard Library also contains functions to compare {#link|Slices#}, strings, and more. See the rest of the
{#syntax#}std.testing{#endsyntax#} namespace in the {#link|Zig Standard Library#} for more available functions.</p>
{#header_close#}
{#header_open|Test Tool Documentation#}
<p>
<kbd>zig test</kbd> has a few command line parameters which affect the compilation.
See <kbd>zig test --help</kbd> for a full list.
</p>
{#header_close#}
{#header_close#}
{#header_open|Variables#}
<p>
A variable is a unit of {#link|Memory#} storage.
</p>
<p>
It is generally preferable to use {#syntax#}const{#endsyntax#} rather than
{#syntax#}var{#endsyntax#} when declaring a variable. This causes less work for both
humans and computers to do when reading code, and creates more optimization opportunities.
</p>
<p>
The {#syntax#}extern{#endsyntax#} keyword or {#link|@extern#} builtin function can be used to link against a variable that is exported
from another object. The {#syntax#}export{#endsyntax#} keyword or {#link|@export#} builtin function
can be used to make a variable available to other objects at link time. In both cases,
the type of the variable must be C ABI compatible.
</p>
{#see_also|Exporting a C Library#}
{#header_open|Identifiers#}
<p>
Variable identifiers are never allowed to shadow identifiers from an outer scope.
</p>
<p>
Identifiers must start with an alphabetic character or underscore and may be followed
by any number of alphanumeric characters or underscores.
They must not overlap with any keywords. See {#link|Keyword Reference#}.
</p>
<p>
If a name that does not fit these requirements is needed, such as for linking with external libraries, the {#syntax#}@""{#endsyntax#} syntax may be used.
</p>
{#code|identifiers.zig#}
{#header_close#}
{#header_open|Container Level Variables#}
<p>
{#link|Container|Containers#} level variables have static lifetime and are order-independent and lazily analyzed.
The initialization value of container level variables is implicitly
{#link|comptime#}. If a container level variable is {#syntax#}const{#endsyntax#} then its value is
{#syntax#}comptime{#endsyntax#}-known, otherwise it is runtime-known.
</p>
{#code|test_container_level_variables.zig#}
<p>
Container level variables may be declared inside a {#link|struct#}, {#link|union#}, {#link|enum#}, or {#link|opaque#}:
</p>
{#code|test_namespaced_container_level_variable.zig#}
{#header_close#}
{#header_open|Static Local Variables#}
<p>
It is also possible to have local variables with static lifetime by using containers inside functions.
</p>
{#code|test_static_local_variable.zig#}
{#header_close#}
{#header_open|Thread Local Variables#}
<p>A variable may be specified to be a thread-local variable using the
{#syntax#}threadlocal{#endsyntax#} keyword,
which makes each thread work with a separate instance of the variable:</p>
{#code|test_thread_local_variables.zig#}
<p>
For {#link|Single Threaded Builds#}, all thread local variables are treated as regular {#link|Container Level Variables#}.
</p>
<p>
Thread local variables may not be {#syntax#}const{#endsyntax#}.
</p>
{#header_close#}
{#header_open|Local Variables#}
<p>
Local variables occur inside {#link|Functions#}, {#link|comptime#} blocks, and {#link|@cImport#} blocks.
</p>
<p>
When a local variable is {#syntax#}const{#endsyntax#}, it means that after initialization, the variable's