Golang 中的 unsafe.Pointer 和 uintptr详解

// uintptr is an integer type that is large enough to hold the bit pattern of
// any pointer.
type uintptr uintptr

func Sizeof(x ArbitraryType) uintptr
func Offsetof(x ArbitraryType) uintptr
func Alignof(x ArbitraryType) uintptr

// ArbitraryType is here for the purposes of documentation only and is not actually
// part of the unsafe package. It represents the type of an arbitrary Go expression.
type ArbitraryType int
// Pointer represents a pointer to an arbitrary type. There are four special operations
// available for type Pointer that are not available for other types:
//	- A pointer value of any type can be converted to a Pointer.
//	- A Pointer can be converted to a pointer value of any type.
//	- A uintptr can be converted to a Pointer.
//	- A Pointer can be converted to a uintptr.
// Pointer therefore allows a program to defeat the type system and read and write
// arbitrary memory. It should be used with extreme care.
type Pointer *ArbitraryType

func Float64bits(f float64) uint64 {
    return *(*uint64)(unsafe.Pointer(&f))
}

package main
import (
	"fmt"
	"unsafe"
)
func main() {
	fmt.Println("int8 => int64", Int8To64(5))
	fmt.Println("int64 => int8", Int64To8(5))
}
func Int64To8(f int64) int8 {
	return *(*int8)(unsafe.Pointer(&f))
}
func Int8To64(f int8) int64 {
	return *(*int64)(unsafe.Pointer(&f))
}

int8 => int64 1079252997
int64 => int8 5

Program exited.

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

e := unsafe.Pointer(&x[i])
e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0]))

u := uintptr(p)
p = unsafe.Pointer(u + offset)

package main
import (
	"fmt"
	"unsafe"
)
func main() {
	length := 6
	arr := make([]int, length)
	for i := 0; i < length; i++ {
		arr[i] = i
	}
	fmt.Println(arr)
	// [0 1 2 3 4 5]
	// 取slice的第5个元素：通过计算第1个元素 + 4 个元素的size 得出
	end := unsafe.Pointer(uintptr(unsafe.Pointer(&arr[0])) + 4*unsafe.Sizeof(arr[0]))

	fmt.Println(*(*int)(end)) // 4
	fmt.Println(arr[4]) // 4
	
}

p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer()))

u := reflect.ValueOf(new(int)).Pointer()
p := (*int)(unsafe.Pointer(u))

import (
	"unsafe"
)
func BytesToString(b []byte) string {
	return *(*string)(unsafe.Pointer(&b))
}
func StringToBytes(s string) []byte {
	return *(*[]byte)(unsafe.Pointer(&s))
}

// A Builder is used to efficiently build a string using Write methods.
// It minimizes memory copying. The zero value is ready to use.
// Do not copy a non-zero Builder.
type Builder struct {
	addr *Builder // of receiver, to detect copies by value
	buf  []byte
}
// String returns the accumulated string.
func (b *Builder) String() string {
	return *(*string)(unsafe.Pointer(&b.buf))
}

// Reset resets the Builder to be empty.
func (b *Builder) Reset() {
	b.addr = nil
	b.buf = nil
}

// Write appends the contents of p to b's buffer.
// Write always returns len(p), nil.
func (b *Builder) Write(p []byte) (int, error) {
	b.copyCheck()
	b.buf = append(b.buf, p...)
	return len(p), nil
}

// WriteString appends the contents of s to b's buffer.
// It returns the length of s and a nil error.
func (b *Builder) WriteString(s string) (int, error) {
	b.copyCheck()
	b.buf = append(b.buf, s...)
	return len(s), nil
}

func indexLocal(l unsafe.Pointer, i int) *poolLocal {
	lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
	return (*poolLocal)(lp)
}