zig/doc/langref.md

Language Reference

Grammar

Root = many(TopLevelItem) "EOF"

TopLevelItem = ErrorValueDecl | CompTimeExpression | TopLevelDecl | TestDecl

TestDecl = "test" String Block

TopLevelDecl = option(VisibleMod) (FnDef | ExternDecl | GlobalVarDecl | TypeDecl | UseDecl)

TypeDecl = "type" Symbol "=" TypeExpr ";"

ErrorValueDecl = "error" Symbol ";"

GlobalVarDecl = VariableDeclaration ";"

VariableDeclaration = option("comptime") ("var" | "const") Symbol option(":" TypeExpr) "=" Expression

ContainerMember = (ContainerField | FnDef | GlobalVarDecl)

ContainerField = Symbol option(":" Expression) ",")

UseDecl = "use" Expression ";"

ExternDecl = "extern" (FnProto | VariableDeclaration) ";"

FnProto = option("coldcc" | "nakedcc") "fn" option(Symbol) ParamDeclList option("->" TypeExpr)

VisibleMod = "pub" | "export"

FnDef = option("inline" | "extern") FnProto Block

ParamDeclList = "(" list(ParamDecl, ",") ")"

ParamDecl = option("noalias" | "comptime") option(Symbol ":") (TypeExpr | "...")

Block = "{" list(option(Statement), ";") "}"

Statement = Label | VariableDeclaration ";" | Defer ";" | NonBlockExpression ";" | BlockExpression

Label = Symbol ":"

Expression = BlockExpression | NonBlockExpression

TypeExpr = PrefixOpExpression | "var"

NonBlockExpression = ReturnExpression | AssignmentExpression

AsmExpression = "asm" option("volatile") "(" String option(AsmOutput) ")"

AsmOutput = ":" list(AsmOutputItem, ",") option(AsmInput)

AsmInput = ":" list(AsmInputItem, ",") option(AsmClobbers)

AsmOutputItem = "[" Symbol "]" String "(" (Symbol | "->" TypeExpr) ")"

AsmInputItem = "[" Symbol "]" String "(" Expression ")"

AsmClobbers= ":" list(String, ",")

UnwrapExpression = BoolOrExpression (UnwrapMaybe | UnwrapError) | BoolOrExpression

UnwrapMaybe = "??" Expression

UnwrapError = "%%" option("|" Symbol "|") Expression

AssignmentExpression = UnwrapExpression AssignmentOperator UnwrapExpression | UnwrapExpression

AssignmentOperator = "=" | "*=" | "/=" | "%=" | "+=" | "-=" | "<<=" | ">>=" | "&=" | "^=" | "|=" | "&&=" | "||=" | "*%=" | "+%=" | "-%=" | "<<%="

BlockExpression = IfExpression | Block | WhileExpression | ForExpression | SwitchExpression | CompTimeExpression | TryExpression

CompTimeExpression = option("comptime") Expression

SwitchExpression = "switch" "(" Expression ")" "{" many(SwitchProng) "}"

SwitchProng = (list(SwitchItem, ",") | "else") "=>" option("|" option("*") Symbol "|") Expression ","

SwitchItem = Expression | (Expression "..." Expression)

WhileExpression = option("inline") "while" "(" Expression option(";" Expression) ")" Expression

ForExpression = option("inline") "for" "(" Expression ")" option("|" option("*") Symbol option("," Symbol) "|") Expression

BoolOrExpression = BoolAndExpression "||" BoolOrExpression | BoolAndExpression

ReturnExpression = option("%" | "?") "return" option(Expression)

Defer = option("%" | "?") "defer" Expression

IfExpression = IfVarExpression | IfBoolExpression

IfBoolExpression = "if" "(" Expression ")" Expression option(Else)

TryExpression = "try" "(" option(("const" | "var") option("*") Symbol "=") Expression  ")" Expression option("else" option("|" Symbol "|") Expression)

IfVarExpression = "if" "(" ("const" | "var") option("*") Symbol option(":" TypeExpr) "?=" Expression ")" Expression Option(Else)

Else = "else" Expression

BoolAndExpression = ComparisonExpression "&&" BoolAndExpression | ComparisonExpression

ComparisonExpression = BinaryOrExpression ComparisonOperator BinaryOrExpression | BinaryOrExpression

ComparisonOperator = "==" | "!=" | "<" | ">" | "<=" | ">="

BinaryOrExpression = BinaryXorExpression "|" BinaryOrExpression | BinaryXorExpression

BinaryXorExpression = BinaryAndExpression "^" BinaryXorExpression | BinaryAndExpression

BinaryAndExpression = BitShiftExpression "&" BinaryAndExpression | BitShiftExpression

BitShiftExpression = AdditionExpression BitShiftOperator BitShiftExpression | AdditionExpression

BitShiftOperator = "<<" | ">>" | "<<%"

AdditionExpression = MultiplyExpression AdditionOperator AdditionExpression | MultiplyExpression

AdditionOperator = "+" | "-" | "++" | "+%" | "-%"

MultiplyExpression = CurlySuffixExpression MultiplyOperator MultiplyExpression | CurlySuffixExpression

CurlySuffixExpression = TypeExpr option(ContainerInitExpression)

MultiplyOperator = "*" | "/" | "%" | "**" | "*%"

PrefixOpExpression = PrefixOp PrefixOpExpression | SuffixOpExpression

SuffixOpExpression = PrimaryExpression option(FnCallExpression | ArrayAccessExpression | FieldAccessExpression | SliceExpression)

FieldAccessExpression = "." Symbol

FnCallExpression = "(" list(Expression, ",") ")"

ArrayAccessExpression = "[" Expression "]"

SliceExpression = "[" Expression "..." option(Expression) "]" option("const")

ContainerInitExpression = "{" ContainerInitBody "}"

ContainerInitBody = list(StructLiteralField, ",") | list(Expression, ",")

StructLiteralField = "." Symbol "=" Expression

PrefixOp = "!" | "-" | "~" | "*" | ("&" option("const") option("volatile")) | "?" | "%" | "%%" | "??" | "-%"

PrimaryExpression = Number | String | CharLiteral | KeywordLiteral | GroupedExpression | GotoExpression | BlockExpression | Symbol | ("@" Symbol FnCallExpression) | ArrayType | (option("extern") FnProto) | AsmExpression | ("error" "." Symbol) | ContainerDecl

ArrayType = "[" option(Expression) "]" option("const") TypeExpr

GotoExpression = "goto" Symbol

GroupedExpression = "(" Expression ")"

KeywordLiteral = "true" | "false" | "null" | "break" | "continue" | "undefined" | "error" | "type" | "this" | "unreachable"

ContainerDecl = option("extern" | "packed") ("struct" | "enum" | "union") "{" many(ContainerMember) "}"

Operator Precedence

x() x[] x.y
!x -x ~x *x &x ?x %x %%x
x{}
* / %
+ - ++
<< >>
&
^
|
== != < > <= >=
&&
||
?? %%
= *= /= %= += -= <<= >>= &= ^= |= &&= ||=

Types

Numeric Types

Type name       C equivalent        Description

i8              int8_t              signed 8-bit integer
u8              uint8_t             unsigned 8-bit integer
i16             int16_t             signed 16-bit integer
u16             uint16_t            unsigned 16-bit integer
i32             int32_t             signed 32-bit integer
u32             uint32_t            unsigned 32-bit integer
i64             int64_t             signed 64-bit integer
u64             uint64_t            unsigned 64-bit integer
isize           intptr_t            signed pointer sized integer
usize           uintptr_t           unsigned pointer sized integer

c_short         short               for ABI compatibility with C
c_ushort        unsigned short      for ABI compatibility with C
c_int           int                 for ABI compatibility with C
c_uint          unsigned int        for ABI compatibility with C
c_long          long                for ABI compatibility with C
c_ulong         unsigned long       for ABI compatibility with C
c_longlong      long long           for ABI compatibility with C
c_ulonglong     unsigned long long  for ABI compatibility with C
c_long_double   long double         for ABI compatibility with C
c_void          void                for ABI compatibility with C

f32             float               32-bit floating point
f64             double              64-bit floating point

Expressions

Literals

Character and String Literals

Literal            Example       Characters   Escapes         Null Term  Type

Byte               'H'           All ASCII    Byte            No         u8
UTF-8 Bytes        "hello"       All Unicode  Byte & Unicode  No         [5]u8
UTF-8 C string     c"hello"      All Unicode  Byte & Unicode  Yes        &const u8

Escapes

Escape	Name
\n	Newline
\r	Carriage Return
\t	Tab
\	Backslash
'	Single Quote
"	Double Quote
\xNN	hexadecimal 8-bit character code (2 digits)
\uNNNN	hexadecimal 16-bit Unicode character code UTF-8 encoded (4 digits)
\UNNNNNN	hexadecimal 24-bit Unicode character code UTF-8 encoded (6 digits)

Note that the maximum valid Unicode point is 0x10ffff.

Multiline String Literals

Multiline string literals have no escapes and can span across multiple lines. To start a multiline string literal, use the \\ token. Just like a comment, the string literal goes until the end of the line. The end of the line is not included in the string literal.

However, if the next line begins with \\ then a newline is appended and the string literal continues.

Example:

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

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

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

In this example the variable c_string_literal has type &const char and has a terminating null byte.

Number Literals

Number literals	Example	Exponentiation
Decimal integer	98222	N/A
Hex integer	0xff	N/A
Octal integer	0o77	N/A
Binary integer	0b11110000	N/A
Floating point	123.0E+77	Optional
Hex floating point	0x103.70p-5	Optional

Built-in Functions

Built-in functions are prefixed with @. Remember that the comptime keyword on a parameter means that the parameter must be known at compile time.

@alloca(comptime T: type, count: usize) -> []T

Allocates memory in the stack frame of the caller. This temporary space is automatically freed when the function that called alloca returns to its caller, just like other stack variables.

When using this function to allocate memory, you should know the upper bound of count. Consider putting a constant array on the stack with the upper bound instead of using alloca. If you do use alloca it is to save a few bytes off the memory size given that you didn't actually hit your upper bound.

The allocated memory contents are undefined.

@typeOf(expression) -> type

This function returns a compile-time constant, which is the type of the expression passed as an argument. The expression is not evaluated.

@sizeOf(comptime T: type) -> (number literal)

This function returns the number of bytes it takes to store T in memory.

The result is a target-specific compile time constant.

@alignOf(comptime T: type) -> (number literal)

This function returns the number of bytes that this type should be aligned to for the current target.

The result is a target-specific compile time constant.

Overflow Arithmetic

These functions take an integer type, two variables of the specified type, and a pointer to memory of the specified type where the result is stored.

The functions return a boolean value: true if overflow or underflow occurred, false otherwise.

Function                                                             Operation
@addWithOverflow(comptime T: type, a: T, b: T, result: &T) -> bool   *x = a + b
@subWithOverflow(comptime T: type, a: T, b: T, result: &T) -> bool   *x = a - b
@mulWithOverflow(comptime T: type, a: T, b: T, result: &T) -> bool   *x = a * b
@shlWithOverflow(comptime T: type, a: T, b: T, result: &T) -> bool   *x = a << b

@memset(dest: &u8, c: u8, byte_count: usize)

This function sets a region of memory to c. dest is a pointer.

This function is a low level intrinsic with no safety mechanisms. Most higher level code will not use this function, instead using something like this:

for (destSlice) |*b| *b = c;

The optimizer is intelligent enough to turn the above snippet into a memset.

@memcpy(noalias dest: &u8, noalias source: &const u8, byte_count: usize)

This function copies bytes from one region of memory to another. dest and source are both pointers and must not overlap.

This function is a low level intrinsic with no safety mechanisms. Most higher level code will not use this function, instead using something like this:

const mem = @import("std").mem;
mem.copy(destSlice, sourceSlice);

The optimizer is intelligent enough to turn the above snippet into a memcpy.

@breakpoint()

This function inserts a platform-specific debug trap instruction which causes debuggers to break there.

This function is only valid within function scope.

@returnAddress()

This function returns a pointer to the return address of the current stack frame.

The implications of this are target specific and not consistent across all platforms.

This function is only valid within function scope.

@frameAddress()

This function returns the base pointer of the current stack frame.

The implications of this are target specific and not consistent across all platforms. The frame address may not be available in release mode due to aggressive optimizations.

This function is only valid within function scope.

@maxValue(comptime T: type) -> (number literal)

This function returns the maximum integer value of the integer type T.

The result is a compile time constant. For some types such as c_long, the result is marked as depending on a compile variable.

@minValue(comptime T: type) -> (number literal)

This function returns the minimum integer value of the integer type T.

The result is a compile time constant. For some types such as c_long, the result is marked as depending on a compile variable.

@memberCount(comptime T: type) -> (number literal)

This function returns the number of enum values in an enum type.

The result is a compile time constant.

@import(comptime path: []u8) -> (namespace)

This function finds a zig file corresponding to path and imports all the public top level declarations into the resulting namespace.

path can be a relative or absolute path, or it can be the name of a package, such as "std".

This function is only valid at top level scope.

@cImport(expression) -> (namespace)

This function parses C code and imports the functions, types, variables, and compatible macro definitions into the result namespace.

expression is interpreted at compile time. The builtin functions @c_include, @c_define, and @c_undef work within this expression, appending to a temporary buffer which is then parsed as C code.

This function is only valid at top level scope.

@cInclude(comptime path: []u8)

This function can only occur inside @c_import.

This appends #include <$path>\n to the c_import temporary buffer.

@cDefine(comptime name: []u8, value)

This function can only occur inside @c_import.

This appends #define $name $value to the c_import temporary buffer.

@cUndef(comptime name: []u8)

This function can only occur inside @c_import.

This appends #undef $name to the c_import temporary buffer.

@compileVar(comptime name: []u8) -> (varying type)

This function returns a compile-time variable. There are built in compile variables:

• "is_big_endian" bool - either true for big endian or false for little endian.
• "is_release" bool- either true for release mode builds or false for debug mode builds.
• "is_test" bool- either true for test builds or false otherwise.
• "os" Os - use zig targets to see what enum values are possible here.
• "arch" Arch - use zig targets to see what enum values are possible here.
• "environ" Environ - use zig targets to see what enum values are possible here.

Build scripts can set additional compile variables of any name and type.

The result of this function is a compile time constant that is marked as depending on a compile variable.

@generatedCode(expression) -> @typeOf(expression)

This function wraps an expression and returns the result of the expression unmodified.

Inside the expression, code is considered generated, which means that the following compile errors are disabled:

• unnecessary if statement error

The result of the expression is marked as depending on a compile variable.

@ctz(x: T) -> T

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

@clz(x: T) -> T

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

@errorName(err: error) -> []u8

This function returns the string representation of an error. If an error declaration is:

error OutOfMem;

Then the string representation is "OutOfMem".

If there are no calls to @errorName in an entire application, then no error name table will be generated.

@typeName(T: type) -> []u8

This function returns the string representation of a type.

@embedFile(comptime path: []u8) -> [X]u8

This function returns a compile time constant fixed-size array with length equal to the byte count of the file given by path. The contents of the array are the contents of the file.

@cmpxchg(ptr: &T, cmp: T, new: T, success_order: MemoryOrder, fail_order: MemoryOrder) -> bool

This function performs an atomic compare exchange operation.

@fence(order: MemoryOrder)

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

@divExact(a: T, b: T) -> T

This function performs integer division a / b and returns the result.

The caller guarantees that this operation will have no remainder.

In debug mode, a remainder causes a panic. In release mode, a remainder is undefined behavior.

@truncate(comptime T: type, integer) -> T

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

The following produces a crash in debug mode and undefined behavior in release mode:

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

However this is well defined and working code:

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

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

This function also performs a twos complement cast. For example, the following produces a crash in debug mode and undefined behavior in release mode:

const a = i16(-1);
const b = u16(a);

However this is well defined and working code:

const a = i16(-1);
const b = @truncate(u16, a);
// b is now 0xffff

@compileError(comptime msg: []u8)

This function, when semantically analyzed, causes a compile error with the message msg.

There are several ways that code avoids being semantically checked, such as using if or switch with compile time constants, and comptime functions.

@compileLog(args: ...)

This function, when semantically analyzed, causes a compile error, but it does not prevent compile-time code from continuing to run, and it otherwise does not interfere with analysis.

Each of the arguments will be serialized to a printable debug value and output to stderr, and then a newline at the end.

This function can be used to do "printf debugging" on compile-time executing code.

@intType(comptime is_signed: bool, comptime bit_count: u8) -> type

This function returns an integer type with the given signness and bit count.

@setDebugSafety(scope, safety_on: bool)

Sets a whether we want debug safety checks on for a given scope.

@isInteger(comptime T: type) -> bool

Returns whether a given type is an integer.

@isFloat(comptime T: type) -> bool

Returns whether a given type is a float.

@canImplicitCast(comptime T: type, value) -> bool

Returns whether a value can be implicitly casted to a given type.

@setGlobalAlign(global_variable_name, byte_count: usize) -> bool

Sets the alignment property of a global variable.

@setGlobalSection(global_variable_name, section_name: []u8) -> bool

Puts the global variable in the specified section.