Go 语言规范学习（7）

文章目录

• • [Built-in functions](#Built-in functions)
  • • [Appending to and copying slices](#Appending to and copying slices)
    • Clear
    • Close
    • [Manipulating complex numbers](#Manipulating complex numbers)
    • [Deletion of map elements](#Deletion of map elements)
    • [Length and capacity](#Length and capacity)
    • [Making slices, maps and channels](#Making slices, maps and channels)
    • [Min and max](#Min and max)
    • Allocation
    • [Handling panics](#Handling panics)
    • Bootstrapping
  • Packages
  • • [Source file organization](#Source file organization)
    • [Package clause](#Package clause)
    • [Import declarations](#Import declarations)
    • [An example package](#An example package)
  • [Program initialization and execution](#Program initialization and execution)
  • • [The zero value](#The zero value)
    • [Package initialization](#Package initialization)
    • [Program initialization](#Program initialization)
    • [Program execution](#Program execution)
  • Errors
  • [Run-time panics](#Run-time panics)
  • [System considerations](#System considerations)
  • • [Package `unsafe`](#Package unsafe)
    • [Size and alignment guarantees](#Size and alignment guarantees)

Built-in functions

Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.

Appending to and copying slices

The built-in functions append and copy assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps.

The variadic function append appends zero or more values x to a slice s and returns the resulting slice of the same type as s. The core type of s must be a slice of type []E. The values x are passed to a parameter of type ...E and the respective parameter passing rules apply. As a special case, if the core type of s is []byte, append also accepts a second argument with core type bytestring followed by .... This form appends the bytes of the byte slice or string.

append(s S, x ...E) S  // core type of S is []E

If the capacity of s is not large enough to fit the additional values, append allocates a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, append re-uses the underlying array.

s0 := []int{0, 0}
s1 := append(s0, 2)                // append a single element     s1 is []int{0, 0, 2}
s2 := append(s1, 3, 5, 7)          // append multiple elements    s2 is []int{0, 0, 2, 3, 5, 7}
s3 := append(s2, s0...)            // append a slice              s3 is []int{0, 0, 2, 3, 5, 7, 0, 0}
s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 is []int{3, 5, 7, 2, 3, 5, 7, 0, 0}

var t []interface{}
t = append(t, 42, 3.1415, "foo")   //                             t is []interface{}{42, 3.1415, "foo"}

var b []byte
b = append(b, "bar"...)            // append string contents      b is []byte{'b', 'a', 'r' }

The function copy copies slice elements from a source src to a destination dst and returns the number of elements copied. The core types of both arguments must be slices with identical element type. The number of elements copied is the minimum of len(src) and len(dst). As a special case, if the destination's core type is []byte, copy also accepts a source argument with core type bytestring. This form copies the bytes from the byte slice or string into the byte slice.

copy(dst, src []T) int
copy(dst []byte, src string) int

Examples:

var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
var s = make([]int, 6)
var b = make([]byte, 5)
n1 := copy(s, a[0:])            // n1 == 6, s is []int{0, 1, 2, 3, 4, 5}
n2 := copy(s, s[2:])            // n2 == 4, s is []int{2, 3, 4, 5, 4, 5}
n3 := copy(b, "Hello, World!")  // n3 == 5, b is []byte("Hello")

Clear

The built-in function clear takes an argument of map, slice, or type parameter type, and deletes or zeroes out all elements [Go 1.21].

Call        Argument type     Result

clear(m)    map[K]T           deletes all entries, resulting in an
                              empty map (len(m) == 0)

clear(s)    []T               sets all elements up to the length of
                              s to the zero value of T

clear(t)    type parameter    see below

If the type of the argument to clear is a type parameter, all types in its type set must be maps or slices, and clear performs the operation corresponding to the actual type argument.

If the map or slice is nil, clear is a no-op.

Close

For an argument ch with a core type that is a channel, the built-in function close records that no more values will be sent on the channel. It is an error if ch is a receive-only channel. Sending to or closing a closed channel causes a run-time panic. Closing the nil channel also causes a run-time panic. After calling close, and after any previously sent values have been received, receive operations will return the zero value for the channel's type without blocking. The multi-valued receive operation returns a received value along with an indication of whether the channel is closed.

Manipulating complex numbers

Three functions assemble and disassemble complex numbers. The built-in function complex constructs a complex value from a floating-point real and imaginary part, while real and imag extract the real and imaginary parts of a complex value.

complex(realPart, imaginaryPart floatT) complexT
real(complexT) floatT
imag(complexT) floatT

The type of the arguments and return value correspond. For complex, the two arguments must be of the same floating-point type and the return type is the complex type with the corresponding floating-point constituents: complex64 for float32 arguments, and complex128 for float64 arguments. If one of the arguments evaluates to an untyped constant, it is first implicitly converted to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant.

For real and imag, the argument must be of complex type, and the return type is the corresponding floating-point type: float32 for a complex64 argument, and float64 for a complex128 argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant.

The real and imag functions together form the inverse of complex, so for a value z of a complex type Z, z == Z(complex(real(z), imag(z))).

If the operands of these functions are all constants, the return value is a constant.

var a = complex(2, -2)             // complex128
const b = complex(1.0, -1.4)       // untyped complex constant 1 - 1.4i
x := float32(math.Cos(math.Pi/2))  // float32
var c64 = complex(5, -x)           // complex64
var s int = complex(1, 0)          // untyped complex constant 1 + 0i can be converted to int
_ = complex(1, 2<<s)               // illegal: 2 assumes floating-point type, cannot shift
var rl = real(c64)                 // float32
var im = imag(a)                   // float64
const c = imag(b)                  // untyped constant -1.4
_ = imag(3 << s)                   // illegal: 3 assumes complex type, cannot shift

Arguments of type parameter type are not permitted.

Deletion of map elements

The built-in function delete removes the element with key k from a map m. The value k must be assignable to the key type of m.

delete(m, k)  // remove element m[k] from map m

If the type of m is a type parameter, all types in that type set must be maps, and they must all have identical key types.

If the map m is nil or the element m[k] does not exist, delete is a no-op.

Length and capacity

The built-in functions len and cap take arguments of various types and return a result of type int. The implementation guarantees that the result always fits into an int.

Call      Argument type    Result

len(s)    string type      string length in bytes
          [n]T, *[n]T      array length (== n)
          []T              slice length
          map[K]T          map length (number of defined keys)
          chan T           number of elements queued in channel buffer
          type parameter   see below

cap(s)    [n]T, *[n]T      array length (== n)
          []T              slice capacity
          chan T           channel buffer capacity
          type parameter   see below

If the argument type is a type parameter P, the call len(e) (or cap(e) respectively) must be valid for each type in P's type set. The result is the length (or capacity, respectively) of the argument whose type corresponds to the type argument with which P was instantiated.

The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds:

0 <= len(s) <= cap(s)

The length of a nil slice, map or channel is 0. The capacity of a nil slice or channel is 0.

The expression len(s) is constant if s is a string constant. The expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain channel receives or (non-constant) function calls; in this case s is not evaluated. Otherwise, invocations of len and cap are not constant and s is evaluated.

const (
	c1 = imag(2i)                    // imag(2i) = 2.0 is a constant
	c2 = len([10]float64{2})         // [10]float64{2} contains no function calls
	c3 = len([10]float64{c1})        // [10]float64{c1} contains no function calls
	c4 = len([10]float64{imag(2i)})  // imag(2i) is a constant and no function call is issued
	c5 = len([10]float64{imag(z)})   // invalid: imag(z) is a (non-constant) function call
)
var z complex128

Making slices, maps and channels

The built-in function make takes a type T, optionally followed by a type-specific list of expressions. The core type of T must be a slice, map or channel. It returns a value of type T (not *T). The memory is initialized as described in the section on initial values.

Call             Core type    Result

make(T, n)       slice        slice of type T with length n and capacity n
make(T, n, m)    slice        slice of type T with length n and capacity m

make(T)          map          map of type T
make(T, n)       map          map of type T with initial space for approximately n elements

make(T)          channel      unbuffered channel of type T
make(T, n)       channel      buffered channel of type T, buffer size n

Each of the size arguments n and m must be of integer type, have a type set containing only integer types, or be an untyped constant. A constant size argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. If both n and m are provided and are constant, then n must be no larger than m. For slices and channels, if n is negative or larger than m at run time, a run-time panic occurs.

s := make([]int, 10, 100)       // slice with len(s) == 10, cap(s) == 100
s := make([]int, 1e3)           // slice with len(s) == cap(s) == 1000
s := make([]int, 1<<63)         // illegal: len(s) is not representable by a value of type int
s := make([]int, 10, 0)         // illegal: len(s) > cap(s)
c := make(chan int, 10)         // channel with a buffer size of 10
m := make(map[string]int, 100)  // map with initial space for approximately 100 elements

Calling make with a map type and size hint n will create a map with initial space to hold n map elements. The precise behavior is implementation-dependent.

Min and max

The built-in functions min and max compute the smallest---or largest, respectively---value of a fixed number of arguments of ordered types. There must be at least one argument [Go 1.21].

The same type rules as for operators apply: for ordered arguments x and y, min(x, y) is valid if x + y is valid, and the type of min(x, y) is the type of x + y (and similarly for max). If all arguments are constant, the result is constant.

var x, y int
m := min(x)                 // m == x
m := min(x, y)              // m is the smaller of x and y
m := max(x, y, 10)          // m is the larger of x and y but at least 10
c := max(1, 2.0, 10)        // c == 10.0 (floating-point kind)
f := max(0, float32(x))     // type of f is float32
var s []string
_ = min(s...)               // invalid: slice arguments are not permitted
t := max("", "foo", "bar")  // t == "foo" (string kind)

For numeric arguments, assuming all NaNs are equal, min and max are commutative and associative:

min(x, y)    == min(y, x)
min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))

For floating-point arguments negative zero, NaN, and infinity the following rules apply:

   x        y    min(x, y)    max(x, y)

  -0.0    0.0         -0.0          0.0    // negative zero is smaller than (non-negative) zero
  -Inf      y         -Inf            y    // negative infinity is smaller than any other number
  +Inf      y            y         +Inf    // positive infinity is larger than any other number
   NaN      y          NaN          NaN    // if any argument is a NaN, the result is a NaN

For string arguments the result for min is the first argument with the smallest (or for max, largest) value, compared lexically byte-wise:

min(x, y)    == if x <= y then x else y
min(x, y, z) == min(min(x, y), z)

Allocation

The built-in function new takes a type T, allocates storage for a variable of that type at run time, and returns a value of type *T pointing to it. The variable is initialized as described in the section on initial values.

new(T)

For instance

type S struct { a int; b float64 }
new(S)

allocates storage for a variable of type S, initializes it (a=0, b=0.0), and returns a value of type *S containing the address of the location.

Handling panics

Two built-in functions, panic and recover, assist in reporting and handling run-time panics and program-defined error conditions.

func panic(interface{})
func recover() interface{}

While executing a function F, an explicit call to panic or a run-time panic terminates the execution of F. Any functions deferred by F are then executed as usual. Next, any deferred functions run by F's caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to panic. This termination sequence is called panicking.

---

1. 触发 panic 的情况

• 显式调用 ：在函数 F 中直接调用 panic(value)。
• 运行时 panic：例如数组越界、空指针解引用等运行时错误。

---

2. 函数 F 的执行立即停止

• 当 panic 发生时，函数 F 的当前代码会立刻终止执行，后续代码不再运行。

---

3. 执行 F 中的 deferred 函数

• Go 会按 后进先出（LIFO） 的顺序执行 F 中所有通过 defer 注册的函数。
• 这些 defer 函数通常用于释放资源（如关闭文件、解锁互斥锁等），即使发生 panic 也会确保执行。

---

4. 向上传递 panic 到调用栈

• 如果 F 的调用者（比如函数 G）也有 defer 函数，这些 defer 函数会接着执行。
• 这一过程会递归向上 传递，直到当前 goroutine 的最顶层函数 （如 main 函数）。

---

5. 程序终止并报错

• 当所有 defer 函数执行完毕后，程序会崩溃退出。
• 错误信息会包含 panic 的参数值（例如 panic("file not found") 中的字符串）。

---

6. 关键术语：Panicking（恐慌过程）

• 整个从 panic 触发到程序终止的流程被称为 panicking。

---

panic(42)
panic("unreachable")
panic(Error("cannot parse"))

The recover function allows a program to manage behavior of a panicking goroutine. Suppose a function G defers a function D that calls recover and a panic occurs in a function on the same goroutine in which G is executing. When the running of deferred functions reaches D, the return value of D's call to recover will be the value passed to the call of panic. If D returns normally, without starting a new panic, the panicking sequence stops. In that case, the state of functions called between G and the call to panic is discarded, and normal execution resumes. Any functions deferred by G before D are then run and G's execution terminates by returning to its caller.

---

1. 核心场景描述

• 函数 G 中通过 defer 注册了一个函数 D。
• D 内部调用了 recover。
• 同一 goroutine 中发生了 panic（例如在 G 或其调用的函数中触发了 panic）。

---

2. recover 的触发时机

• 当 panic 发生后，Go 会按顺序执行当前 goroutine 的 defer 函数。
• 当执行到 D 时，D 中的 recover() 会捕获 panic 的值 ，并返回该值（即 panic(value) 中的 value）。

---

3. recover 如何停止 panic 的传播

• 如果 D 正常执行完毕（没有触发新的 panic）：
  1. panic 的连锁反应会立即停止，程序不再继续向上崩溃。
  2. 丢弃 panic 后的堆栈状态 ：在 G 和发生 panic 的函数之间的所有函数调用状态会被丢弃（这些函数的剩余代码不会执行）。
  3. 程序恢复正常执行 ：从 G 中尚未执行的 defer 开始继续运行。

---

4. 后续流程

• 执行 G 中剩余的 defer ：在 D 之前注册的其他 defer 函数会按顺序执行。
• G 正常返回 ：G 会像没有发生过 panic 一样返回给它的调用者。

---

5. 关键点总结

• recover 必须通过 defer 调用 ，且仅在 panic 后的 defer 函数中生效。
• recover 会终止 panic 的传播 ，但不会恢复 panic 发生点之后的代码（那些代码已被跳过）。
• 如果 recover 后程序继续运行，资源清理依赖 defer（例如释放锁、关闭文件等）。

例子：

func main() {
    fmt.Println("Start")
    G() // 调用函数 G
    fmt.Println("End") // 会正常执行，因为 panic 被 recover 捕获
}

func G() {
    defer fmt.Println("Defer in G") // (3)
    defer D()                      // (2) 先注册的 D 会后执行
    panic("something went wrong")  // (1) 触发 panic
}

func D() {
    if r := recover(); r != nil {
        fmt.Println("Recovered:", r) // 捕获 panic 的值
    }
}

输出

Start
Recovered: something went wrong
Defer in G
End

---

The return value of recover is nil when the goroutine is not panicking or recover was not called directly by a deferred function. Conversely, if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic with a nil interface value (or an untyped nil) causes a run-time panic.

The protect function in the example below invokes the function argument g and protects callers from run-time panics raised by g.

func protect(g func()) {
	defer func() {
		log.Println("done")  // Println executes normally even if there is a panic
		if x := recover(); x != nil {
			log.Printf("run time panic: %v", x)
		}
	}()
	log.Println("start")
	g()
}

Bootstrapping

Current implementations provide several built-in functions useful during bootstrapping. These functions are documented for completeness but are not guaranteed to stay in the language. They do not return a result.

Function   Behavior

print      prints all arguments; formatting of arguments is implementation-specific
println    like print but prints spaces between arguments and a newline at the end

Implementation restriction: print and println need not accept arbitrary argument types, but printing of boolean, numeric, and string types must be supported.

Packages

Go programs are constructed by linking together packages . A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be exported and used in another package.

Source file organization

Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.

SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .

Package clause

A package clause begins each source file and defines the package to which the file belongs.

PackageClause = "package" PackageName .
PackageName   = identifier .

The PackageName must not be the blank identifier.

package math

A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory.

Import declarations

An import declaration states that the source file containing the declaration depends on functionality of the imported package (§Program initialization and execution) and enables access to exported identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.

ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec = [ "." | PackageName ] ImportPath .
ImportPath = string_lit .

The PackageName is used in qualified identifiers to access exported identifiers of the package within the importing source file. It is declared in the file block.

If the PackageName is omitted, it defaults to the identifier specified in the package clause of the imported package.

If an explicit period (.) appears instead of a name, all the package's exported identifiers declared in that package's package block will be declared in the importing source file's file block and must be accessed without a qualifier.

The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages.

Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to Unicode's L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters !"#$%&'()*,:;<=>?[\]^{|}` and the Unicode replacement character U+FFFD.

Consider a compiled a package containing the package clause package math, which exports function Sin, and installed the compiled package in the file identified by "lib/math". This table illustrates how Sin is accessed in files that import the package after the various types of import declaration.

Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import m "lib/math"         m.Sin
import . "lib/math"         Sin

An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name:

import _ "lib/math"

---

1. 核心概念：包的初始化（Side Effects）

• Go 的包可以包含 init() 函数，它会在包被导入时自动执行 ，通常用于：
  • 注册驱动（如数据库驱动 import _ "github.com/go-sql-driver/mysql"）。
  • 初始化全局配置或资源。
  • 执行一些只需运行一次的代码（如日志系统初始化）。
• 这种初始化行为被称为包的 "Side Effects"（副作用）。

---

2. 空白标识符 _ 的作用

• 正常情况下，如果导入一个包但未使用它，Go 编译器会报错（避免无用导入）。
• 通过 _ 显式忽略包名，告诉编译器：
  "我只需要执行这个包的 init 函数，不需要调用它的其他功能。"

---

An example package

Here is a complete Go package that implements a concurrent prime sieve.

package main

import "fmt"

// Send the sequence 2, 3, 4, ... to channel 'ch'.
func generate(ch chan<- int) {
	for i := 2; ; i++ {
		ch <- i  // Send 'i' to channel 'ch'.
	}
}

// Copy the values from channel 'src' to channel 'dst',
// removing those divisible by 'prime'.
func filter(src <-chan int, dst chan<- int, prime int) {
	for i := range src {  // Loop over values received from 'src'.
		if i%prime != 0 {
			dst <- i  // Send 'i' to channel 'dst'.
		}
	}
}

// The prime sieve: Daisy-chain filter processes together.
func sieve() {
	ch := make(chan int)  // Create a new channel.
	go generate(ch)       // Start generate() as a subprocess.
	for {
		prime := <-ch
		fmt.Print(prime, "\n")
		ch1 := make(chan int)
		go filter(ch, ch1, prime)
		ch = ch1
	}
}

func main() {
	sieve()
}

Program initialization and execution

The zero value

When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: false for booleans, 0 for numeric types, "" for strings, and nil for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.

These two simple declarations are equivalent:

var i int
var i int = 0

After

type T struct { i int; f float64; next *T }
t := new(T)

the following holds:

t.i == 0
t.f == 0.0
t.next == nil

The same would also be true after

var t T

Package initialization

Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in declaration order which has no dependencies on uninitialized variables.

在包内部，包级变量的初始化是分步进行的。每一步会选择声明顺序中最早且不依赖于任何未初始化变量的变量进行初始化。

More precisely, a package-level variable is considered ready for initialization if it is not yet initialized and either has no initialization expression or its initialization expression has no dependencies on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization.

更准确地说，一个包级变量在以下情况下被认为"已准备好初始化"：它尚未被初始化，并且（1）没有初始化表达式，或（2）其初始化表达式不依赖于任何未初始化的变量。初始化过程会反复选择声明顺序中最早且"已准备好初始化"的包级变量进行初始化，直到没有变量满足这一条件为止。

If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid.

Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step.

var x = a
var a, b = f() // a and b are initialized together, before x is initialized

For the purpose of package initialization, blank variables are treated like any other variables in declarations.

The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on. To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.

Dependency analysis does not rely on the actual values of the variables, only on lexical references to them in the source, analyzed transitively.

依赖分析不依赖于变量的实际值，而仅依赖于源代码中对它们的词法引用（且会进行传递性分析）。

For instance, if a variable x's initialization expression refers to a function whose body refers to variable y then x depends on y. Specifically:

• A reference to a variable or function is an identifier denoting that variable or function.
• A reference to a method m is a method value or method expression of the form t.m, where the (static) type of t is not an interface type, and the method m is in the method set of t. It is immaterial【不重要】 whether the resulting function value t.m is invoked.
• A variable, function, or method x depends on a variable y if x's initialization expression or body (for functions and methods) contains a reference to y or to a function or method that depends on y.

For example, given the declarations

var (
	a = c + b  // == 9
	b = f()    // == 4
	c = f()    // == 5
	d = 3      // == 5 after initialization has finished
)

func f() int {
	d++
	return d
}

the initialization order is d, b, c, a. Note that the order of subexpressions in initialization expressions is irrelevant: a = c + b and a = b + c result in the same initialization order in this example.

Dependency analysis is performed per package; only references referring to variables, functions, and (non-interface) methods declared in the current package are considered. If other, hidden, data dependencies exists between variables, the initialization order between those variables is unspecified.

For instance, given the declarations

var x = I(T{}).ab()   // x has an undetected, hidden dependency on a and b
var _ = sideEffect()  // unrelated to x, a, or b
var a = b
var b = 42

type I interface      { ab() []int }
type T struct{}
func (T) ab() []int   { return []int{a, b} }

the variable a will be initialized after b but whether x is initialized before b, between b and a, or after a, and thus also the moment at which sideEffect() is called (before or after x is initialized) is not specified.

Variables may also be initialized using functions named init declared in the package block, with no arguments and no result parameters.

func init() { ... }

Multiple such functions may be defined per package, even within a single source file. In the package block, the init identifier can be used only to declare init functions, yet the identifier itself is not declared. Thus init functions cannot be referred to from anywhere in a program.

The entire package is initialized by assigning initial values to all its package-level variables followed by calling all init functions in the order they appear in the source, possibly in multiple files, as presented to the compiler.

Program initialization

The packages of a complete program are initialized stepwise, one package at a time. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. More precisely:

Given the list of all packages, sorted by import path, in each step the first uninitialized package in the list for which all imported packages (if any) are already initialized is initialized. This step is repeated until all packages are initialized.

Package initialization---variable initialization and the invocation of init functions---happens in a single goroutine, sequentially, one package at a time. An init function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the init functions: it will not invoke the next one until the previous one has returned.

Program execution

A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.

func main() { ... }

Program execution begins by initializing the program and then invoking the function main in package main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.

Errors

The predeclared type error is defined as

type error interface {
	Error() string
}

It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined:

func Read(f *File, b []byte) (n int, err error)

Run-time panics

Execution errors such as attempting to index an array out of bounds trigger a run-time panic equivalent to a call of the built-in function panic with a value of the implementation-defined interface type runtime.Error. That type satisfies the predeclared interface type error. The exact error values that represent distinct run-time error conditions are unspecified.

package runtime

type Error interface {
	error
	// and perhaps other methods
}

System considerations

Package unsafe

The built-in package unsafe, known to the compiler and accessible through the import path "unsafe", provides facilities for low-level programming including operations that violate the type system. A package using unsafe must be vetted manually for type safety and may not be portable. The package provides the following interface:

package unsafe

type ArbitraryType int  // shorthand for an arbitrary Go type; it is not a real type
type Pointer *ArbitraryType

func Alignof(variable ArbitraryType) uintptr
func Offsetof(selector ArbitraryType) uintptr
func Sizeof(variable ArbitraryType) uintptr

type IntegerType int  // shorthand for an integer type; it is not a real type
func Add(ptr Pointer, len IntegerType) Pointer
func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
func SliceData(slice []ArbitraryType) *ArbitraryType
func String(ptr *byte, len IntegerType) string
func StringData(str string) *byte

A Pointer is a pointer type but a Pointer value may not be dereferenced. Any pointer or value of core type uintptr can be converted to a type of core type Pointer and vice versa. The effect of converting between Pointer and uintptr is implementation-defined.

var f float64
bits = *(*uint64)(unsafe.Pointer(&f))

type ptr unsafe.Pointer
bits = *(*uint64)(ptr(&f))

func f[P ~*B, B any](p P) uintptr {
	return uintptr(unsafe.Pointer(p))
}

var p ptr = nil

The functions Alignof and Sizeof take an expression x of any type and return the alignment or size, respectively, of a hypothetical variable v as if v were declared via var v = x.

The function Offsetof takes a (possibly parenthesized) selector s.f, denoting a field f of the struct denoted by s or *s, and returns the field offset in bytes relative to the struct's address. If f is an embedded field, it must be reachable without pointer indirections through fields of the struct. For a struct s with field f:

uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))

Computer architectures may require memory addresses to be aligned ; that is, for addresses of a variable to be a multiple of a factor, the variable's type's alignment . The function Alignof takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable x:

uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0

A (variable of) type T has variable size if T is a type parameter, or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is constant . Calls to Alignof, Offsetof, and Sizeof are compile-time constant expressions of type uintptr if their arguments (or the struct s in the selector expression s.f for Offsetof) are types of constant size.

The function Add adds len to ptr and returns the updated pointer unsafe.Pointer(uintptr(ptr) + uintptr(len)) [Go 1.17]. The len argument must be of integer type or an untyped constant. A constant len argument must be representable by a value of type int; if it is an untyped constant it is given type int. The rules for valid uses of Pointer still apply.

The function Slice returns a slice whose underlying array starts at ptr and whose length and capacity are len. Slice(ptr, len) is equivalent to

(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]

except that, as a special case, if ptr is nil and len is zero, Slice returns nil [Go 1.17].

The len argument must be of integer type or an untyped constant. A constant len argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. At run time, if len is negative, or if ptr is nil and len is not zero, a run-time panic occurs [Go 1.17].

The function SliceData returns a pointer to the underlying array of the slice argument. If the slice's capacity cap(slice) is not zero, that pointer is &slice[:1][0]. If slice is nil, the result is nil. Otherwise it is a non-nil pointer to an unspecified memory address [Go 1.20].

The function String returns a string value whose underlying bytes start at ptr and whose length is len. The same requirements apply to the ptr and len argument as in the function Slice. If len is zero, the result is the empty string "". Since Go strings are immutable, the bytes passed to String must not be modified afterwards. [Go 1.20]

The function StringData returns a pointer to the underlying bytes of the str argument. For an empty string the return value is unspecified, and may be nil. Since Go strings are immutable, the bytes returned by StringData must not be modified [Go 1.20].

Size and alignment guarantees

For the numeric types, the following sizes are guaranteed:

type                                 size in bytes

byte, uint8, int8                     1
uint16, int16                         2
uint32, int32, float32                4
uint64, int64, float64, complex64     8
complex128                           16

The following minimal alignment properties are guaranteed:

1. For a variable x of any type: unsafe.Alignof(x) is at least 1.
2. For a variable x of struct type: unsafe.Alignof(x) is the largest of all the values unsafe.Alignof(x.f) for each field f of x, but at least 1.
3. For a variable x of array type: unsafe.Alignof(x) is the same as the alignment of a variable of the array's element type.

A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.