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<!doctype html>
<html lang="en">
<head>
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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> |
master
</nav>
<nav aria-labelledby="table-of-contents">
<h2 id="table-of-contents">Table of Contents</h2>
<ul>
<li><a id="toc-Introduction" href="#Introduction">Introduction</a></li>
<li><a id="toc-Zig-Standard-Library" href="#Zig-Standard-Library">Zig Standard Library</a></li>
<li><a id="toc-Hello-World" href="#Hello-World">Hello World</a></li>
<li><a id="toc-Zig-Test" href="#Zig-Test">Zig Test</a>
<ul>
<li><a id="toc-Test-Declarations" href="#Test-Declarations">Test Declarations</a></li>
<li><a id="toc-Test-Failure" href="#Test-Failure">Test Failure</a></li>
<li><a id="toc-Skip-Tests" href="#Skip-Tests">Skip Tests</a></li>
<li><a id="toc-Test-Memory-Leaks" href="#Test-Memory-Leaks">Test Memory Leaks</a></li>
<li><a id="toc-Detecting-Test-Build" href="#Detecting-Test-Build">Detecting Test Build</a></li>
<li><a id="toc-Test-Output-and-Logging" href="#Test-Output-and-Logging">Test Output and Logging</a></li>
<li><a id="toc-The-Testing-Namespace" href="#The-Testing-Namespace">The Testing Namespace</a></li>
<li><a id="toc-Test-Tool-Documentation" href="#Test-Tool-Documentation">Test Tool Documentation</a></li>
</ul></li>
<li><a id="toc-Comments" href="#Comments">Comments</a>
<ul>
<li><a id="toc-Doc-comments" href="#Doc-comments">Doc comments</a></li>
<li><a id="toc-Container-Doc-Comments" href="#Container-Doc-Comments">Container Doc Comments</a></li>
</ul></li>
<li><a id="toc-Values" href="#Values">Values</a>
<ul>
<li><a id="toc-Primitive-Types" href="#Primitive-Types">Primitive Types</a></li>
<li><a id="toc-Primitive-Values" href="#Primitive-Values">Primitive Values</a></li>
<li><a id="toc-String-Literals-and-Unicode-Code-Point-Literals" href="#String-Literals-and-Unicode-Code-Point-Literals">String Literals and Unicode Code Point Literals</a>
<ul>
<li><a id="toc-Escape-Sequences" href="#Escape-Sequences">Escape Sequences</a></li>
<li><a id="toc-Multiline-String-Literals" href="#Multiline-String-Literals">Multiline String Literals</a></li>
</ul></li>
<li><a id="toc-Assignment" href="#Assignment">Assignment</a>
<ul>
<li><a id="toc-undefined" href="#undefined">undefined</a></li>
</ul></li>
</ul></li>
<li><a id="toc-Variables" href="#Variables">Variables</a>
<ul>
<li><a id="toc-Container-Level-Variables" href="#Container-Level-Variables">Container Level Variables</a></li>
<li><a id="toc-Static-Local-Variables" href="#Static-Local-Variables">Static Local Variables</a></li>
<li><a id="toc-Thread-Local-Variables" href="#Thread-Local-Variables">Thread Local Variables</a></li>
<li><a id="toc-Local-Variables" href="#Local-Variables">Local Variables</a></li>
</ul></li>
<li><a id="toc-Integers" href="#Integers">Integers</a>
<ul>
<li><a id="toc-Integer-Literals" href="#Integer-Literals">Integer Literals</a></li>
<li><a id="toc-Runtime-Integer-Values" href="#Runtime-Integer-Values">Runtime Integer Values</a></li>
</ul></li>
<li><a id="toc-Floats" href="#Floats">Floats</a>
<ul>
<li><a id="toc-Float-Literals" href="#Float-Literals">Float Literals</a></li>
<li><a id="toc-Floating-Point-Operations" href="#Floating-Point-Operations">Floating Point Operations</a></li>
</ul></li>
<li><a id="toc-Operators" href="#Operators">Operators</a>
<ul>
<li><a id="toc-Table-of-Operators" href="#Table-of-Operators">Table of Operators</a></li>
<li><a id="toc-Precedence" href="#Precedence">Precedence</a></li>
</ul></li>
<li><a id="toc-Arrays" href="#Arrays">Arrays</a>
<ul>
<li><a id="toc-Anonymous-List-Literals" href="#Anonymous-List-Literals">Anonymous List Literals</a></li>
<li><a id="toc-Multidimensional-Arrays" href="#Multidimensional-Arrays">Multidimensional Arrays</a></li>
<li><a id="toc-Sentinel-Terminated-Arrays" href="#Sentinel-Terminated-Arrays">Sentinel-Terminated Arrays</a></li>
</ul></li>
<li><a id="toc-Vectors" href="#Vectors">Vectors</a>
<ul>
<li><a id="toc-SIMD" href="#SIMD">SIMD</a></li>
</ul></li>
<li><a id="toc-Pointers" href="#Pointers">Pointers</a>
<ul>
<li><a id="toc-volatile" href="#volatile">volatile</a></li>
<li><a id="toc-Alignment" href="#Alignment">Alignment</a></li>
<li><a id="toc-allowzero" href="#allowzero">allowzero</a></li>
<li><a id="toc-Sentinel-Terminated-Pointers" href="#Sentinel-Terminated-Pointers">Sentinel-Terminated Pointers</a></li>
</ul></li>
<li><a id="toc-Slices" href="#Slices">Slices</a>
<ul>
<li><a id="toc-Sentinel-Terminated-Slices" href="#Sentinel-Terminated-Slices">Sentinel-Terminated Slices</a></li>
</ul></li>
<li><a id="toc-Containers" href="#Containers">Containers</a>
<ul>
<li><a id="toc-Types-of-Containers" href="#Types-of-Containers">Types of Containers</a></li>
<li><a id="toc-Container-Memory-Layout" href="#Container-Memory-Layout">Container Memory Layout</a></li>
<li><a id="toc-Nesting-Containers" href="#Nesting-Containers">Nesting Containers</a></li>
<li><a id="toc-Container-Namespace" href="#Container-Namespace">Container Namespace</a>
<ul>
<li><a id="toc-usingnamespace" href="#usingnamespace">usingnamespace</a></li>
</ul></li>
<li><a id="toc-Top-Level" href="#Top-Level">Top-Level</a>
<ul>
<li><a id="toc-Types-of-Top-Level" href="#Types-of-Top-Level">Types of Top-Level</a>
<ul>
<li><a id="toc-Top-Level-Declarations" href="#Top-Level-Declarations">Top-Level Declarations</a></li>
<li><a id="toc-Container-Fields" href="#Container-Fields">Container Fields</a></li>
</ul></li>
<li><a id="toc-Top-Level-Order-Independent-Declarations" href="#Top-Level-Order-Independent-Declarations">Top-Level Order Independent Declarations</a></li>
<li><a id="toc-Top-Level-Compile-Time-Resolution" href="#Top-Level-Compile-Time-Resolution">Top-Level Compile-Time Resolution</a></li>
<li><a id="toc-Lazy-Top-Level-Declaration-Analysis" href="#Lazy-Top-Level-Declaration-Analysis">Lazy Top-Level Declaration Analysis</a></li>
<li><a id="toc-Self-Referential-Containers" href="#Self-Referential-Containers">Self-Referential Containers</a></li>
</ul></li>
</ul></li>
<li><a id="toc-struct" href="#struct">struct</a>
<ul>
<li><a id="toc-Default-Field-Values" href="#Default-Field-Values">Default Field Values</a></li>
<li><a id="toc-extern-struct" href="#extern-struct">extern struct</a></li>
<li><a id="toc-packed-struct" href="#packed-struct">packed struct</a></li>
<li><a id="toc-Struct-Naming" href="#Struct-Naming">Struct Naming</a></li>
<li><a id="toc-Anonymous-Struct-Literals" href="#Anonymous-Struct-Literals">Anonymous Struct Literals</a></li>
</ul></li>
<li><a id="toc-enum" href="#enum">enum</a>
<ul>
<li><a id="toc-extern-enum" href="#extern-enum">extern enum</a></li>
<li><a id="toc-Enum-Literals" href="#Enum-Literals">Enum Literals</a></li>
<li><a id="toc-Non-exhaustive-enum" href="#Non-exhaustive-enum">Non-exhaustive enum</a></li>
</ul></li>
<li><a id="toc-union" href="#union">union</a>
<ul>
<li><a id="toc-Tagged-union" href="#Tagged-union">Tagged union</a></li>
<li><a id="toc-extern-union" href="#extern-union">extern union</a></li>
<li><a id="toc-packed-union" href="#packed-union">packed union</a></li>
<li><a id="toc-Anonymous-Union-Literals" href="#Anonymous-Union-Literals">Anonymous Union Literals</a></li>
</ul></li>
<li><a id="toc-opaque" href="#opaque">opaque</a></li>
<li><a id="toc-blocks" href="#blocks">blocks</a>
<ul>
<li><a id="toc-Shadowing" href="#Shadowing">Shadowing</a></li>
</ul></li>
<li><a id="toc-switch" href="#switch">switch</a>
<ul>
<li><a id="toc-Exhaustive-Switching" href="#Exhaustive-Switching">Exhaustive Switching</a></li>
<li><a id="toc-Switching-with-Enum-Literals" href="#Switching-with-Enum-Literals">Switching with Enum Literals</a></li>
</ul></li>
<li><a id="toc-while" href="#while">while</a>
<ul>
<li><a id="toc-Labeled-while" href="#Labeled-while">Labeled while</a></li>
<li><a id="toc-while-with-Optionals" href="#while-with-Optionals">while with Optionals</a></li>
<li><a id="toc-while-with-Error-Unions" href="#while-with-Error-Unions">while with Error Unions</a></li>
<li><a id="toc-inline-while" href="#inline-while">inline while</a></li>
</ul></li>
<li><a id="toc-for" href="#for">for</a>
<ul>
<li><a id="toc-Labeled-for" href="#Labeled-for">Labeled for</a></li>
<li><a id="toc-inline-for" href="#inline-for">inline for</a></li>
</ul></li>
<li><a id="toc-if" href="#if">if</a></li>
<li><a id="toc-defer" href="#defer">defer</a></li>
<li><a id="toc-unreachable" href="#unreachable">unreachable</a>
<ul>
<li><a id="toc-Basics" href="#Basics">Basics</a></li>
<li><a id="toc-At-Compile-Time" href="#At-Compile-Time">At Compile-Time</a></li>
</ul></li>
<li><a id="toc-noreturn" href="#noreturn">noreturn</a></li>
<li><a id="toc-Functions" href="#Functions">Functions</a>
<ul>
<li><a id="toc-Pass-by-value-Parameters" href="#Pass-by-value-Parameters">Pass-by-value Parameters</a></li>
<li><a id="toc-Function-Parameter-Type-Inference" href="#Function-Parameter-Type-Inference">Function Parameter Type Inference</a></li>
<li><a id="toc-Function-Reflection" href="#Function-Reflection">Function Reflection</a></li>
</ul></li>
<li><a id="toc-Errors" href="#Errors">Errors</a>
<ul>
<li><a id="toc-Error-Set-Type" href="#Error-Set-Type">Error Set Type</a>
<ul>
<li><a id="toc-The-Global-Error-Set" href="#The-Global-Error-Set">The Global Error Set</a></li>
</ul></li>
<li><a id="toc-Error-Union-Type" href="#Error-Union-Type">Error Union Type</a>
<ul>
<li><a id="toc-catch" href="#catch">catch</a></li>
<li><a id="toc-try" href="#try">try</a></li>
<li><a id="toc-errdefer" href="#errdefer">errdefer</a></li>
<li><a id="toc-Merging-Error-Sets" href="#Merging-Error-Sets">Merging Error Sets</a></li>
<li><a id="toc-Inferred-Error-Sets" href="#Inferred-Error-Sets">Inferred Error Sets</a></li>
</ul></li>
<li><a id="toc-Error-Return-Traces" href="#Error-Return-Traces">Error Return Traces</a>
<ul>
<li><a id="toc-Implementation-Details" href="#Implementation-Details">Implementation Details</a></li>
</ul></li>
</ul></li>
<li><a id="toc-Optionals" href="#Optionals">Optionals</a>
<ul>
<li><a id="toc-Optional-Type" href="#Optional-Type">Optional Type</a></li>
<li><a id="toc-null" href="#null">null</a></li>
<li><a id="toc-Optional-Pointers" href="#Optional-Pointers">Optional Pointers</a></li>
</ul></li>
<li><a id="toc-Casting" href="#Casting">Casting</a>
<ul>
<li><a id="toc-Type-Coercion" href="#Type-Coercion">Type Coercion</a>
<ul>
<li><a id="toc-Type-Coercion-Stricter-Qualification" href="#Type-Coercion-Stricter-Qualification">Type Coercion: Stricter Qualification</a></li>
<li><a id="toc-Type-Coercion-Integer-and-Float-Widening" href="#Type-Coercion-Integer-and-Float-Widening">Type Coercion: Integer and Float Widening</a></li>
<li><a id="toc-Type-Coercion-Coercion-Float-to-Int" href="#Type-Coercion-Coercion-Float-to-Int">Type Coercion: Coercion Float to Int</a></li>
<li><a id="toc-Type-Coercion-Slices-Arrays-and-Pointers" href="#Type-Coercion-Slices-Arrays-and-Pointers">Type Coercion: Slices, Arrays and Pointers</a></li>
<li><a id="toc-Type-Coercion-Optionals" href="#Type-Coercion-Optionals">Type Coercion: Optionals</a></li>
<li><a id="toc-Type-Coercion-Error-Unions" href="#Type-Coercion-Error-Unions">Type Coercion: Error Unions</a></li>
<li><a id="toc-Type-Coercion-Compile-Time-Known-Numbers" href="#Type-Coercion-Compile-Time-Known-Numbers">Type Coercion: Compile-Time Known Numbers</a></li>
<li><a id="toc-Type-Coercion-unions-and-enums" href="#Type-Coercion-unions-and-enums">Type Coercion: unions and enums</a></li>
<li><a id="toc-Type-Coercion-Zero-Bit-Types" href="#Type-Coercion-Zero-Bit-Types">Type Coercion: Zero Bit Types</a></li>
<li><a id="toc-Type-Coercion-undefined" href="#Type-Coercion-undefined">Type Coercion: undefined</a></li>
</ul></li>
<li><a id="toc-Explicit-Casts" href="#Explicit-Casts">Explicit Casts</a></li>
<li><a id="toc-Peer-Type-Resolution" href="#Peer-Type-Resolution">Peer Type Resolution</a></li>
</ul></li>
<li><a id="toc-Zero-Bit-Types" href="#Zero-Bit-Types">Zero Bit Types</a>
<ul>
<li><a id="toc-void" href="#void">void</a></li>
<li><a id="toc-Pointers-to-Zero-Bit-Types" href="#Pointers-to-Zero-Bit-Types">Pointers to Zero Bit Types</a></li>
</ul></li>
<li><a id="toc-Result-Location-Semantics" href="#Result-Location-Semantics">Result Location Semantics</a></li>
<li><a id="toc-comptime" href="#comptime">comptime</a>
<ul>
<li><a id="toc-Introducing-the-Compile-Time-Concept" href="#Introducing-the-Compile-Time-Concept">Introducing the Compile-Time Concept</a>
<ul>
<li><a id="toc-Compile-Time-Parameters" href="#Compile-Time-Parameters">Compile-Time Parameters</a></li>
<li><a id="toc-Compile-Time-Variables" href="#Compile-Time-Variables">Compile-Time Variables</a></li>
<li><a id="toc-Compile-Time-Expressions" href="#Compile-Time-Expressions">Compile-Time Expressions</a></li>
</ul></li>
<li><a id="toc-Generic-Data-Structures" href="#Generic-Data-Structures">Generic Data Structures</a></li>
<li><a id="toc-Case-Study-print-in-Zig" href="#Case-Study-print-in-Zig">Case Study: print in Zig</a></li>
</ul></li>
<li><a id="toc-Assembly" href="#Assembly">Assembly</a>
<ul>
<li><a id="toc-Output-Constraints" href="#Output-Constraints">Output Constraints</a></li>
<li><a id="toc-Input-Constraints" href="#Input-Constraints">Input Constraints</a></li>
<li><a id="toc-Clobbers" href="#Clobbers">Clobbers</a></li>
<li><a id="toc-Global-Assembly" href="#Global-Assembly">Global Assembly</a></li>
</ul></li>
<li><a id="toc-Atomics" href="#Atomics">Atomics</a></li>
<li><a id="toc-Async-Functions" href="#Async-Functions">Async Functions</a>
<ul>
<li><a id="toc-Suspend-and-Resume" href="#Suspend-and-Resume">Suspend and Resume</a>
<ul>
<li><a id="toc-Resuming-from-Suspend-Blocks" href="#Resuming-from-Suspend-Blocks">Resuming from Suspend Blocks</a></li>
</ul></li>
<li><a id="toc-Async-and-Await" href="#Async-and-Await">Async and Await</a></li>
<li><a id="toc-Async-Function-Example" href="#Async-Function-Example">Async Function Example</a></li>
</ul></li>
<li><a id="toc-Builtin-Functions" href="#Builtin-Functions">Builtin Functions</a>
<ul style="columns: 2">
<li><a id="toc-addWithOverflow" href="#addWithOverflow">@addWithOverflow</a></li>
<li><a id="toc-addWithSaturation" href="#addWithSaturation">@addWithSaturation</a></li>
<li><a id="toc-alignCast" href="#alignCast">@alignCast</a></li>
<li><a id="toc-alignOf" href="#alignOf">@alignOf</a></li>
<li><a id="toc-as" href="#as">@as</a></li>
<li><a id="toc-asyncCall" href="#asyncCall">@asyncCall</a></li>
<li><a id="toc-atomicLoad" href="#atomicLoad">@atomicLoad</a></li>
<li><a id="toc-atomicRmw" href="#atomicRmw">@atomicRmw</a></li>
<li><a id="toc-atomicStore" href="#atomicStore">@atomicStore</a></li>
<li><a id="toc-bitCast" href="#bitCast">@bitCast</a></li>
<li><a id="toc-bitOffsetOf" href="#bitOffsetOf">@bitOffsetOf</a></li>
<li><a id="toc-boolToInt" href="#boolToInt">@boolToInt</a></li>
<li><a id="toc-bitSizeOf" href="#bitSizeOf">@bitSizeOf</a></li>
<li><a id="toc-breakpoint" href="#breakpoint">@breakpoint</a></li>
<li><a id="toc-mulAdd" href="#mulAdd">@mulAdd</a></li>
<li><a id="toc-byteSwap" href="#byteSwap">@byteSwap</a></li>
<li><a id="toc-bitReverse" href="#bitReverse">@bitReverse</a></li>
<li><a id="toc-offsetOf" href="#offsetOf">@offsetOf</a></li>
<li><a id="toc-call" href="#call">@call</a></li>
<li><a id="toc-cDefine" href="#cDefine">@cDefine</a></li>
<li><a id="toc-cImport" href="#cImport">@cImport</a></li>
<li><a id="toc-cInclude" href="#cInclude">@cInclude</a></li>
<li><a id="toc-clz" href="#clz">@clz</a></li>
<li><a id="toc-cmpxchgStrong" href="#cmpxchgStrong">@cmpxchgStrong</a></li>
<li><a id="toc-cmpxchgWeak" href="#cmpxchgWeak">@cmpxchgWeak</a></li>
<li><a id="toc-compileError" href="#compileError">@compileError</a></li>
<li><a id="toc-compileLog" href="#compileLog">@compileLog</a></li>
<li><a id="toc-ctz" href="#ctz">@ctz</a></li>
<li><a id="toc-cUndef" href="#cUndef">@cUndef</a></li>
<li><a id="toc-divExact" href="#divExact">@divExact</a></li>
<li><a id="toc-divFloor" href="#divFloor">@divFloor</a></li>
<li><a id="toc-divTrunc" href="#divTrunc">@divTrunc</a></li>
<li><a id="toc-embedFile" href="#embedFile">@embedFile</a></li>
<li><a id="toc-enumToInt" href="#enumToInt">@enumToInt</a></li>
<li><a id="toc-errorName" href="#errorName">@errorName</a></li>
<li><a id="toc-errorReturnTrace" href="#errorReturnTrace">@errorReturnTrace</a></li>
<li><a id="toc-errorToInt" href="#errorToInt">@errorToInt</a></li>
<li><a id="toc-errSetCast" href="#errSetCast">@errSetCast</a></li>
<li><a id="toc-export" href="#export">@export</a></li>
<li><a id="toc-extern" href="#extern">@extern</a></li>
<li><a id="toc-fence" href="#fence">@fence</a></li>
<li><a id="toc-field" href="#field">@field</a></li>
<li><a id="toc-fieldParentPtr" href="#fieldParentPtr">@fieldParentPtr</a></li>
<li><a id="toc-floatCast" href="#floatCast">@floatCast</a></li>
<li><a id="toc-floatToInt" href="#floatToInt">@floatToInt</a></li>
<li><a id="toc-frame" href="#frame">@frame</a></li>
<li><a id="toc-Frame" href="#Frame">@Frame</a></li>
<li><a id="toc-frameAddress" href="#frameAddress">@frameAddress</a></li>
<li><a id="toc-frameSize" href="#frameSize">@frameSize</a></li>
<li><a id="toc-hasDecl" href="#hasDecl">@hasDecl</a></li>
<li><a id="toc-hasField" href="#hasField">@hasField</a></li>
<li><a id="toc-import" href="#import">@import</a></li>
<li><a id="toc-intCast" href="#intCast">@intCast</a></li>
<li><a id="toc-intToEnum" href="#intToEnum">@intToEnum</a></li>
<li><a id="toc-intToError" href="#intToError">@intToError</a></li>
<li><a id="toc-intToFloat" href="#intToFloat">@intToFloat</a></li>
<li><a id="toc-intToPtr" href="#intToPtr">@intToPtr</a></li>
<li><a id="toc-maximum" href="#maximum">@maximum</a></li>
<li><a id="toc-memcpy" href="#memcpy">@memcpy</a></li>
<li><a id="toc-memset" href="#memset">@memset</a></li>
<li><a id="toc-minimum" href="#minimum">@minimum</a></li>
<li><a id="toc-wasmMemorySize" href="#wasmMemorySize">@wasmMemorySize</a></li>
<li><a id="toc-wasmMemoryGrow" href="#wasmMemoryGrow">@wasmMemoryGrow</a></li>
<li><a id="toc-mod" href="#mod">@mod</a></li>
<li><a id="toc-mulWithOverflow" href="#mulWithOverflow">@mulWithOverflow</a></li>
<li><a id="toc-mulWithSaturation" href="#mulWithSaturation">@mulWithSaturation</a></li>
<li><a id="toc-panic" href="#panic">@panic</a></li>
<li><a id="toc-popCount" href="#popCount">@popCount</a></li>
<li><a id="toc-ptrCast" href="#ptrCast">@ptrCast</a></li>
<li><a id="toc-ptrToInt" href="#ptrToInt">@ptrToInt</a></li>
<li><a id="toc-rem" href="#rem">@rem</a></li>
<li><a id="toc-returnAddress" href="#returnAddress">@returnAddress</a></li>
<li><a id="toc-select" href="#select">@select</a></li>
<li><a id="toc-setAlignStack" href="#setAlignStack">@setAlignStack</a></li>
<li><a id="toc-setCold" href="#setCold">@setCold</a></li>
<li><a id="toc-setEvalBranchQuota" href="#setEvalBranchQuota">@setEvalBranchQuota</a></li>
<li><a id="toc-setFloatMode" href="#setFloatMode">@setFloatMode</a></li>
<li><a id="toc-setRuntimeSafety" href="#setRuntimeSafety">@setRuntimeSafety</a></li>
<li><a id="toc-shlExact" href="#shlExact">@shlExact</a></li>
<li><a id="toc-shlWithOverflow" href="#shlWithOverflow">@shlWithOverflow</a></li>
<li><a id="toc-shlWithSaturation" href="#shlWithSaturation">@shlWithSaturation</a></li>
<li><a id="toc-shrExact" href="#shrExact">@shrExact</a></li>
<li><a id="toc-shuffle" href="#shuffle">@shuffle</a></li>
<li><a id="toc-sizeOf" href="#sizeOf">@sizeOf</a></li>
<li><a id="toc-splat" href="#splat">@splat</a></li>
<li><a id="toc-reduce" href="#reduce">@reduce</a></li>
<li><a id="toc-src" href="#src">@src</a></li>
<li><a id="toc-sqrt" href="#sqrt">@sqrt</a></li>
<li><a id="toc-sin" href="#sin">@sin</a></li>
<li><a id="toc-cos" href="#cos">@cos</a></li>
<li><a id="toc-exp" href="#exp">@exp</a></li>
<li><a id="toc-exp2" href="#exp2">@exp2</a></li>
<li><a id="toc-log" href="#log">@log</a></li>
<li><a id="toc-log2" href="#log2">@log2</a></li>
<li><a id="toc-log10" href="#log10">@log10</a></li>
<li><a id="toc-fabs" href="#fabs">@fabs</a></li>
<li><a id="toc-floor" href="#floor">@floor</a></li>
<li><a id="toc-ceil" href="#ceil">@ceil</a></li>
<li><a id="toc-trunc" href="#trunc">@trunc</a></li>
<li><a id="toc-round" href="#round">@round</a></li>
<li><a id="toc-subWithOverflow" href="#subWithOverflow">@subWithOverflow</a></li>
<li><a id="toc-subWithSaturation" href="#subWithSaturation">@subWithSaturation</a></li>
<li><a id="toc-tagName" href="#tagName">@tagName</a></li>
<li><a id="toc-This" href="#This">@This</a></li>
<li><a id="toc-truncate" href="#truncate">@truncate</a></li>
<li><a id="toc-Type" href="#Type">@Type</a></li>
<li><a id="toc-typeInfo" href="#typeInfo">@typeInfo</a></li>
<li><a id="toc-typeName" href="#typeName">@typeName</a></li>
<li><a id="toc-TypeOf" href="#TypeOf">@TypeOf</a></li>
<li><a id="toc-unionInit" href="#unionInit">@unionInit</a></li>
</ul></li>
<li><a id="toc-Build-Mode" href="#Build-Mode">Build Mode</a>
<ul>
<li><a id="toc-Debug" href="#Debug">Debug</a></li>
<li><a id="toc-ReleaseFast" href="#ReleaseFast">ReleaseFast</a></li>
<li><a id="toc-ReleaseSafe" href="#ReleaseSafe">ReleaseSafe</a></li>
<li><a id="toc-ReleaseSmall" href="#ReleaseSmall">ReleaseSmall</a></li>
</ul></li>
<li><a id="toc-Single-Threaded-Builds" href="#Single-Threaded-Builds">Single Threaded Builds</a></li>
<li><a id="toc-Undefined-Behavior" href="#Undefined-Behavior">Undefined Behavior</a>
<ul>
<li><a id="toc-Reaching-Unreachable-Code" href="#Reaching-Unreachable-Code">Reaching Unreachable Code</a></li>
<li><a id="toc-Index-out-of-Bounds" href="#Index-out-of-Bounds">Index out of Bounds</a></li>
<li><a id="toc-Cast-Negative-Number-to-Unsigned-Integer" href="#Cast-Negative-Number-to-Unsigned-Integer">Cast Negative Number to Unsigned Integer</a></li>
<li><a id="toc-Cast-Truncates-Data" href="#Cast-Truncates-Data">Cast Truncates Data</a></li>
<li><a id="toc-Integer-Overflow" href="#Integer-Overflow">Integer Overflow</a>
<ul>
<li><a id="toc-Default-Operations" href="#Default-Operations">Default Operations</a></li>
<li><a id="toc-Standard-Library-Math-Functions" href="#Standard-Library-Math-Functions">Standard Library Math Functions</a></li>
<li><a id="toc-Builtin-Overflow-Functions" href="#Builtin-Overflow-Functions">Builtin Overflow Functions</a></li>
<li><a id="toc-Wrapping-Operations" href="#Wrapping-Operations">Wrapping Operations</a></li>
</ul></li>
<li><a id="toc-Exact-Left-Shift-Overflow" href="#Exact-Left-Shift-Overflow">Exact Left Shift Overflow</a></li>
<li><a id="toc-Exact-Right-Shift-Overflow" href="#Exact-Right-Shift-Overflow">Exact Right Shift Overflow</a></li>
<li><a id="toc-Division-by-Zero" href="#Division-by-Zero">Division by Zero</a></li>
<li><a id="toc-Remainder-Division-by-Zero" href="#Remainder-Division-by-Zero">Remainder Division by Zero</a></li>
<li><a id="toc-Exact-Division-Remainder" href="#Exact-Division-Remainder">Exact Division Remainder</a></li>
<li><a id="toc-Attempt-to-Unwrap-Null" href="#Attempt-to-Unwrap-Null">Attempt to Unwrap Null</a></li>
<li><a id="toc-Attempt-to-Unwrap-Error" href="#Attempt-to-Unwrap-Error">Attempt to Unwrap Error</a></li>
<li><a id="toc-Invalid-Error-Code" href="#Invalid-Error-Code">Invalid Error Code</a></li>
<li><a id="toc-Invalid-Enum-Cast" href="#Invalid-Enum-Cast">Invalid Enum Cast</a></li>
<li><a id="toc-Invalid-Error-Set-Cast" href="#Invalid-Error-Set-Cast">Invalid Error Set Cast</a></li>
<li><a id="toc-Incorrect-Pointer-Alignment" href="#Incorrect-Pointer-Alignment">Incorrect Pointer Alignment</a></li>
<li><a id="toc-Wrong-Union-Field-Access" href="#Wrong-Union-Field-Access">Wrong Union Field Access</a></li>
<li><a id="toc-Out-of-Bounds-Float-to-Integer-Cast" href="#Out-of-Bounds-Float-to-Integer-Cast">Out of Bounds Float to Integer Cast</a></li>
<li><a id="toc-Pointer-Cast-Invalid-Null" href="#Pointer-Cast-Invalid-Null">Pointer Cast Invalid Null</a></li>
</ul></li>
<li><a id="toc-Memory" href="#Memory">Memory</a>
<ul>
<li><a id="toc-Choosing-an-Allocator" href="#Choosing-an-Allocator">Choosing an Allocator</a></li>
<li><a id="toc-Where-are-the-bytes" href="#Where-are-the-bytes">Where are the bytes?</a></li>
<li><a id="toc-Implementing-an-Allocator" href="#Implementing-an-Allocator">Implementing an Allocator</a></li>
<li><a id="toc-Heap-Allocation-Failure" href="#Heap-Allocation-Failure">Heap Allocation Failure</a></li>
<li><a id="toc-Recursion" href="#Recursion">Recursion</a></li>
<li><a id="toc-Lifetime-and-Ownership" href="#Lifetime-and-Ownership">Lifetime and Ownership</a></li>
</ul></li>
<li><a id="toc-Compile-Variables" href="#Compile-Variables">Compile Variables</a></li>
<li><a id="toc-Root-Source-File" href="#Root-Source-File">Root Source File</a></li>
<li><a id="toc-Zig-Build-System" href="#Zig-Build-System">Zig Build System</a>
<ul>
<li><a id="toc-Building-an-Executable" href="#Building-an-Executable">Building an Executable</a></li>
<li><a id="toc-Building-a-Library" href="#Building-a-Library">Building a Library</a></li>
<li><a id="toc-Compiling-C-Source-Code" href="#Compiling-C-Source-Code">Compiling C Source Code</a></li>
</ul></li>
<li><a id="toc-C" href="#C">C</a>
<ul>
<li><a id="toc-C-Type-Primitives" href="#C-Type-Primitives">C Type Primitives</a></li>
<li><a id="toc-Import-from-C-Header-File" href="#Import-from-C-Header-File">Import from C Header File</a></li>
<li><a id="toc-C-Translation-CLI" href="#C-Translation-CLI">C Translation CLI</a>
<ul>
<li><a id="toc-Command-line-flags" href="#Command-line-flags">Command line flags</a></li>
<li><a id="toc-Using--target-and--cflags" href="#Using--target-and--cflags">Using -target and -cflags</a></li>
<li><a id="toc-cImport-vs-translate-c" href="#cImport-vs-translate-c">@cImport vs translate-c</a></li>
</ul></li>
<li><a id="toc-C-Translation-Caching" href="#C-Translation-Caching">C Translation Caching</a></li>
<li><a id="toc-Translation-failures" href="#Translation-failures">Translation failures</a></li>
<li><a id="toc-C-Macros" href="#C-Macros">C Macros</a></li>
<li><a id="toc-C-Pointers" href="#C-Pointers">C Pointers</a></li>
<li><a id="toc-Exporting-a-C-Library" href="#Exporting-a-C-Library">Exporting a C Library</a></li>
<li><a id="toc-Mixing-Object-Files" href="#Mixing-Object-Files">Mixing Object Files</a></li>
</ul></li>
<li><a id="toc-WebAssembly" href="#WebAssembly">WebAssembly</a>
<ul>
<li><a id="toc-Freestanding" href="#Freestanding">Freestanding</a></li>
<li><a id="toc-WASI" href="#WASI">WASI</a></li>
</ul></li>
<li><a id="toc-Targets" href="#Targets">Targets</a></li>
<li><a id="toc-Style-Guide" href="#Style-Guide">Style Guide</a>
<ul>
<li><a id="toc-Whitespace" href="#Whitespace">Whitespace</a></li>
<li><a id="toc-Names" href="#Names">Names</a></li>
<li><a id="toc-Examples" href="#Examples">Examples</a></li>
<li><a id="toc-Doc-Comment-Guidance" href="#Doc-Comment-Guidance">Doc Comment Guidance</a></li>
</ul></li>
<li><a id="toc-Source-Encoding" href="#Source-Encoding">Source Encoding</a></li>
<li><a id="toc-Keyword-Reference" href="#Keyword-Reference">Keyword Reference</a></li>
<li><a id="toc-Grammar" href="#Grammar">Grammar</a></li>
<li><a id="toc-Zen" href="#Zen">Zen</a></li>
</ul>
</nav>
</div>
<div id="contents-wrapper"><main id="contents">
<h2 id="Introduction"><a href="#toc-Introduction">Introduction</a> <a class="hdr" href="#Introduction">§</a></h2>
<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>
<h2 id="Zig-Standard-Library"><a href="#toc-Zig-Standard-Library">Zig Standard Library</a> <a class="hdr" href="#Zig-Standard-Library">§</a></h2>
<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>
<h2 id="Hello-World"><a href="#toc-Hello-World">Hello World</a> <a class="hdr" href="#Hello-World">§</a></h2>
<figure><figcaption class="zig-cap"><cite class="file">hello.zig</cite></figcaption><pre><code><span class="tok-kw">const</span> std = <span class="tok-builtin">@import</span>(<span class="tok-str">"std"</span>);
<span class="tok-kw">pub</span> <span class="tok-kw">fn</span> <span class="tok-fn">main</span>() !<span class="tok-type">void</span> {
<span class="tok-kw">const</span> stdout = std.io.getStdOut().writer();
<span class="tok-kw">try</span> stdout.print(<span class="tok-str">"Hello, {s}!\n"</span>, .{<span class="tok-str">"world"</span>});
}</code></pre></figure><figure><figcaption class="shell-cap">Shell</figcaption><pre><samp>$ <kbd>zig build-exe hello.zig</kbd>
$ <kbd>./hello</kbd>
Hello, world!
</samp></pre></figure>
<p>
The Zig code sample above demonstrates one way to create a program that will output: <samp>Hello, world!</samp>.
</p>
<p>
The code sample shows the contents of a file named <code class="file">hello.zig</code>. Files storing Zig
source code are <a href="#Source-Encoding">UTF-8 encoded</a> text files. The files storing
Zig source code are usually named with the <code class="file"><em>.zig</em></code> extension.
</p>
<p>
Following the <code class="file">hello.zig</code> Zig code sample, the <a href="#Zig-Build-System">Zig Build System</a> is used
to build an executable program from the <code class="file">hello.zig</code> source code. Then, the
<code class="file">hello</code> program is executed showing its output <samp>Hello, world!</samp>. The
lines beginning with <samp>$</samp> represent command line prompts and a command.
Everything else is program output.
</p>
<p>
The code sample begins by adding the <a href="#Zig-Standard-Library">Zig Standard Library</a> to the build using the <a href="#import">@import</a> builtin function.
The <code><span class="tok-builtin">@import</span>(<span class="tok-str">"std"</span>)</code> function call creates a structure that represents the Zig Standard Library.
The code then <a href="#Container-Level-Variables">declares</a> a
<a href="#Assignment">constant identifier</a>, named <code>std</code>, that gives access the features of the Zig Standard Library.
</p>
<p>
Next, a <a href="#Functions">public function</a>, <code><span class="tok-kw">pub</span> <span class="tok-kw">fn</span></code>, named <code>main</code>
is declared. The <code>main</code> function is necessary because it tells the Zig compiler where the start of
the program exists. Programs designed to be executed will need a <code><span class="tok-kw">pub</span> <span class="tok-kw">fn</span> <span class="tok-fn">main</span></code> function.
</p>
<aside role="note" aria-label="Note about main function">
<p>
For more advanced use cases, Zig offers other features to inform the compiler where the start of
the program exists. Also, libraries do not need a <code><span class="tok-kw">pub</span> <span class="tok-kw">fn</span> <span class="tok-fn">main</span></code> function because
library code is called by other programs or libraries.
</p>
</aside>
<p>
A function is a block of any number of statements and expressions that, as a whole, perform a task.
Functions may or may not return data after they are done performing their task. If a function
cannot perform its task, it might return an error. Zig makes all of this explicit.
</p>
<p>
In the <code class="file">hello.zig</code> code sample, the <code>main</code> function is declared
with the <code>!<span class="tok-type">void</span></code> return type. This return type is known as an <a href="#Error-Union-Type">Error Union Type</a>.
This syntax tells the Zig compiler that the function will either return an
error or a value. An error union type combines an <a href="#Error-Set-Type">Error Set Type</a> and any other data type
(e.g. a <a href="#Primitive-Types">Primitive Type</a> or a <a href="#Containers">user-defined type</a> such as a
<a href="#struct">struct</a>, <a href="#enum">enum</a>, or <a href="#union">union</a>). The full form of an error union type is
<code><error set type></code><code>!</code><code><any data type></code>. In the code
sample, the error set type is not explicitly written on the left side of the <code>!</code> operator.
When written this way, the error set type is an <a href="#Inferred-Error-Sets">inferred error set type</a>. The
<code><span class="tok-type">void</span></code> after the <code>!</code> operator
tells the compiler that the function will not return a value under normal circumstances (i.e. when no errors occur).
</p>
<aside role="note" aria-label="Note to disambiguate exclamation mark operator">
<p>
Note to experienced programmers: Zig also has the boolean <a href="#Operators">operator</a> <code>!a</code>
where <code>a</code> is a value of type <code><span class="tok-type">bool</span></code>. Error union types contain the
name of the type in the syntax: <code>!</code><code><any data type></code>.
</p>
</aside>
<p>
In Zig, a function's block of statements and expressions are surrounded by an open curly-brace <code>{</code> and
close curly-brace <code>}</code>. Inside of the <code>main</code> function are expressions that perform
the task of outputting <samp>Hello, world!</samp> to standard output.
</p>
<p>
First, a constant identifier, <code>stdout</code>, is initialized to represent standard output's
writer. Then, the program tries to print the <samp>Hello, world!</samp>
message to standard output.
</p>
<p>
Functions sometimes need information to perform their task. In Zig, information is passed
to functions between an open parenthesis <code>(</code> and a close parenthesis <code>)</code> placed after
the function's name. This information is also known as arguments. When there are
multiple arguments passed to a function, they are separated by commas <code>,</code>.
</p>
<p>
The two arguments passed to the <code>stdout.print()</code> function, <code><span class="tok-str">"Hello, {s}!\n"</span></code>
and <code>.{<span class="tok-str">"world"</span>}</code>, are evaluated at <a href="#comptime">compile-time</a>. The code sample is
purposely written to show how to perform <a href="#String-Literals-and-Unicode-Code-Point-Literals">string</a>
substitution in the <code>print</code> function. The curly-braces inside of the first argument
are substituted with the compile-time known value inside of the second argument
(known as a tuple which is type of an <a href="#Anonymous-Struct-Literals">anonymous struct literal</a>). The <code>\n</code>
inside of the double-quotes of the first argument is the <a href="#Escape-Sequences">escape sequence</a> for the
newline character. The <a href="#try">try</a> expression evaluates the result of <code>stdout.print</code>.
If the result is an error, then the <code><span class="tok-kw">try</span></code> expression will return from
<code>main</code> with the error. Otherwise, the program will continue. In this case, there are no
more statements or expressions left to execute in the <code>main</code> function, so the program exits.
</p>
<p>
In Zig, the standard output writer's <code>print</code> function is allowed to fail because
it is actually a function defined as part of a generic Writer. Consider a generic Writer that
represents writing data to a file. When the disk is full, a write to the file will fail.
However, we typically do not expect writing text to the standard output to fail. To avoid having
to handle the failure case of printing to standard output, you can use alternate functions: the
functions in <code>std.log</code> for proper logging or the <code>std.debug.print</code> function.
This documentation will use the latter option to print to standard error (stderr) and silently return
on failure. The next code sample, <code class="file">hello_again.zig</code> demonstrates the use of
<code>std.debug.print</code>.
</p>
<figure><figcaption class="zig-cap"><cite class="file">hello_again.zig</cite></figcaption><pre><code><span class="tok-kw">const</span> print = <span class="tok-builtin">@import</span>(<span class="tok-str">"std"</span>).debug.print;
<span class="tok-kw">pub</span> <span class="tok-kw">fn</span> <span class="tok-fn">main</span>() <span class="tok-type">void</span> {
print(<span class="tok-str">"Hello, world!\n"</span>, .{});
}</code></pre></figure><figure><figcaption class="shell-cap">Shell</figcaption><pre><samp>$ <kbd>zig build-exe hello_again.zig</kbd>
$ <kbd>./hello_again</kbd>
Hello, world!
</samp></pre></figure>
<p>
Note that you can leave off the <code>!</code> from the return type because <code>std.debug.print</code> cannot fail.
</p>
<p>See also:</p><ul>
<li><a href="#Values">Values</a></li>
<li><a href="#import">@import</a></li>
<li><a href="#Errors">Errors</a></li>
<li><a href="#Root-Source-File">Root Source File</a></li>
<li><a href="#Source-Encoding">Source Encoding</a></li>
</ul>
<h2 id="Zig-Test"><a href="#toc-Zig-Test">Zig Test</a> <a class="hdr" href="#Zig-Test">§</a></h2>
<p>
In addition to writing Zig code to create programs or libraries, Zig code can also be written to verify that the
code used for making programs or libraries are functioning correctly. Test code is written within one or more
<code><span class="tok-kw">test</span></code> declarations as demonstrated in the <code class="file">introducing_zig_test.zig</code>
code sample below.
</p>
<figure><figcaption class="zig-cap"><cite class="file">introducing_zig_test.zig</cite></figcaption><pre><code><span class="tok-kw">const</span> std = <span class="tok-builtin">@import</span>(<span class="tok-str">"std"</span>);
<span class="tok-kw">test</span> <span class="tok-str">"expect addOne adds one to 41 correctly"</span> {
<span class="tok-kw">try</span> std.testing.expect(addOne(<span class="tok-number">41</span>) == <span class="tok-number">42</span>);
}
<span class="tok-kw">fn</span> <span class="tok-fn">addOne</span>(number: <span class="tok-type">i32</span>) <span class="tok-type">i32</span> {
<span class="tok-kw">return</span> number + <span class="tok-number">1</span>;
}</code></pre></figure>
<p>
In the code sample below, the <code class="file">introducing_zig_test.zig</code> is shown again with comments
to explain parts of the code. The lines starting with <code><span class="tok-comment">//</span></code> and <code><span class="tok-comment">///</span></code> are
<a href="#Comments">Comments</a> that are useful for sharing information with other people.
</p>
<figure><figcaption class="zig-cap"><cite class="file">introducing_zig_test.zig</cite></figcaption><pre><code><span class="tok-comment">// Using @import("std") causes the Standard Library to be represented as a structure (struct).</span>
<span class="tok-comment">// The constant identifier `std` provides access to the structure's data and methods.</span>
<span class="tok-kw">const</span> std = <span class="tok-builtin">@import</span>(<span class="tok-str">"std"</span>);
<span class="tok-comment">// Tests can only be declared at the top level of a container type (see Containers).</span>
<span class="tok-comment">// Tests can be given an optional name or description written between double-quotes.</span>
<span class="tok-comment">// Test code is written between the required open-curly brace and close-curly brace.</span>
<span class="tok-comment">// Any valid Zig code can be written inside of a test.</span>
<span class="tok-kw">test</span> <span class="tok-str">"expect addOne adds one to 41 correctly"</span> {
<span class="tok-comment">// The Standard Library contains useful functions to help create tests.</span>
<span class="tok-comment">// `expect` is a function that verifies its argument is true.</span>
<span class="tok-comment">// It will return an error if its argument is false to indicate a failure.</span>
<span class="tok-comment">// `try` is used to return an error to the test runner to notify it that the test failed.</span>
<span class="tok-comment">// Errors and `try` are explained in another section of this documentation.</span>
<span class="tok-comment">// The code below demonstrates a common way to verify something is true.</span>
<span class="tok-kw">try</span> std.testing.expect(addOne(<span class="tok-number">41</span>) == <span class="tok-number">42</span>);
}
<span class="tok-comment">/// The function `addOne` adds one to the number given as its argument.</span>
<span class="tok-kw">fn</span> <span class="tok-fn">addOne</span>(number: <span class="tok-type">i32</span>) <span class="tok-type">i32</span> {
<span class="tok-kw">return</span> number + <span class="tok-number">1</span>;
}</code></pre></figure><figure><figcaption class="shell-cap">Shell</figcaption><pre><samp>$ <kbd>zig test introducing_zig_test.zig</kbd>
Test [1/1] test "expect addOne adds one to 41 correctly"...
All 1 tests passed.
</samp></pre></figure>
<p>
<kbd>zig test</kbd> is a tool that runs test code. It packages the given Zig source file into the default test runner,
builds a test runner executable program, and then runs the test runner's executable program. The default test
runner is part of the <a href="#Zig-Standard-Library">Zig Standard Library</a>. When the test runner is being built, it is provided with
all of the tests that the Zig compiler found while <a href="#Root-Source-File">resolving</a> the given Zig source file.
</p>
<aside>This documentation discusses the features of the default test runner as provided by the Zig Standard Library.</aside>
<p>
The shell output shown above displays the line <samp>Test [1/1] test "expect addOne adds one to 41 correctly"...</samp>.
This line was printed to standard error (stderr) by the test runner program. (When the test runner program's
standard error is redirected to the terminal, these lines are cleared when a test succeeds.)
Lines like this indicate which test, out of the total number of tests, is being run. In this case, <samp>[1/1]</samp>
indicates that the first test, out of a total of one test, is being run. The next line, <samp>All 1 tests passed.</samp>,
is also printed to standard error. It indicates the total number of tests that have passed.
</p>
<p>
The code sample in <code class="file">introducing_zig_test.zig</code> tests the <a href="#Functions">function</a>
<code>addOne</code> to verify that it correctly adds one to <code><span class="tok-number">41</span></code> to
demonstrate the use of a Zig <code><span class="tok-kw">test</span></code> declaration. From this test's perspective, the
<code>addOne</code> function is said to be the program/library's "code under test".
More comprehensive tests ought to be written to verify that the <code>addOne</code> function behaves as expected.
</p>
<h3 id="Test-Declarations"><a href="#toc-Test-Declarations">Test Declarations</a> <a class="hdr" href="#Test-Declarations">§</a></h3>
<p>
Test declarations contain the <a href="#Keyword-Reference">keyword</a> <code><span class="tok-kw">test</span></code> followed by an
optional description written between double-quotation marks followed by a <a href="#blocks">block</a> containing test code.
Test declarations can be written in the same files where code under test is written or in a separate Zig source file.
If the test declarations are in the same file as the code under test, then the location of the test declarations can
go before or after the code under test. Concretely, test declarations can only be declared at the top-level of a
<a href="#Containers">Container</a>; hence, they are called <em>top-level test declarations</em>.
</p>
<p>
Tests are similar to <a href="#Functions">Functions</a>, they have a return type and a block of code. The implicit
return type of <code><span class="tok-kw">test</span></code> is the <a href="#Error-Union-Type">Error Union Type</a> <code><span class="tok-type">anyerror</span>!<span class="tok-type">void</span></code>.
(See <a href="#The-Global-Error-Set">The Global Error Set</a> to learn more about <code><span class="tok-type">anyerror</span></code>.) When a
Zig source file is not built using the <kbd>zig test</kbd> tool, the tests are omitted from the program/library.
</p>
<p>
<kbd>zig test</kbd> will only pass the given source file's top-level test declarations to the test runner.
To run <a href="#Nesting-Containers">nested container</a> tests, the nested containers need to
be referenced from any of the given file's top-level test declarations. The code sample below uses
<code>std.testing.refAllDecls(<span class="tok-builtin">@This</span>())</code> function call that will reference all of the