Golang unsafe包使用笔记

unsafe包简介

unsafe包提供了访问底层内存的方法。使用unsafe函数可以提高访问对象的速度。

应用场景

通常用于对大数组的遍历。

访问数组

package main

import (
     "fmt"
     "unsafe"
)

func main() {
     array := []int{1,2,3}
     base := uintptr(unsafe.Pointer(&array[0]))
     size := unsafe.Sizeof(array[0])
     ptr := unsafe.Pointer(base + 2*size)
     element := *(*int)(ptr)

     fmt.Println(element,array[2])
}

访问结构体

package main

import (
     "fmt"
     "unsafe"
)

type Foo struct {
     A int
     B int
}

func main() {
     foo := &Foo{1,2}
     fmt.Println(foo)

     base := uintptr(unsafe.Pointer(foo))
     offset := unsafe.Offsetof(foo.A)

     ptr := unsafe.Pointer(base + offset)
     *(*int)(ptr) = 3

     fmt.Println(foo)
}

类型转换

package main

import (
     "fmt"
     "unsafe"
)

func main() {
     var f float64 = 0.0
     var x uint64 = *(*uint64)(unsafe.Pointer(&f))
     fmt.Println(f,x)
}

注意事项

1. unsafe可能引起兼容性问题。

性能测试

代码：

func sum(arr []int) int {
     sum := 0
     for _,x := range arr {
             sum += x
     }
     return sum
}

func unsafeSum(arr []int) int {
     ptr := unsafe.Pointer(&arr[0])
     count := len(arr)
     step := unsafe.Sizeof(arr[0])

     sum := 0
     for i := 0; i < count; i++ {
             sum += *(*int)(ptr)
             ptr = unsafe.Pointer(uintptr(ptr) + step)
     }

     return sum
}

func unsafeSum2(arr []int) int {
     base := unsafe.Pointer(&arr[0])
     count := len(arr)
     step := unsafe.Sizeof(arr[0])

     sum := 0
     for i := 0; i < count; i++ {
             ptr := unsafe.Pointer(uintptr(base) + uintptr(i)*step)
             sum += *(*int)(ptr)
     }

     return sum
}

func unsafeSum3(arr []int) int {
     beg := unsafe.Pointer(&arr[0])
     size := unsafe.Sizeof(arr[0])
     end := unsafe.Pointer(uintptr(beg) + uintptr(len(arr))*size)

     ptr := beg
     sum := 0

     for ptr != end {
             sum += *(*int)(ptr)
             ptr = unsafe.Pointer(uintptr(ptr) + size)
     }

     return sum
}

测试代码：

package main

import (
     "testing"
)

var (
     array = []int{
             1,3,4,5,6,7,8,9,10,1,}
)

func BenchmarkUnsafeSum(b *testing.B) {
     for i := 0; i < b.N; i++ {
             unsafeSum(array)
     }
}

func BenchmarkSum(b *testing.B) {
     for i := 0; i < b.N; i++ {
             sum(array)
     }
}

func TestUnsafeSum(t *testing.T) {
     if result := unsafeSum(array); result != 550 {
             t.Fatal("expect %v get %v",550,result)
     }
}

func TestUnsafeSum2(t *testing.T) {
     if result := unsafeSum2(array); result != 550 {
             t.Fatal("expect %v get %v",result)
     }
}

func BenchmarkUnsafeSum2(b *testing.B) {
     for i := 0; i < b.N; i++ {
             unsafeSum2(array)
     }
}

func TestUnsafeSum3(t *testing.T) {
     if result := unsafeSum3(array); result != 550 {
             t.Fatal("expect %v get %v",result)
     }
}

func BenchmarkUnsafeSum3(b *testing.B) {
     for i := 0; i < b.N; i++ {
             unsafeSum3(array)
     }
}

结果：

BenchmarkUnsafeSum-2            20000000                94.0 ns/op
BenchmarkSum-2                  10000000               129 ns/op
BenchmarkUnsafeSum2-2           10000000               126 ns/op
BenchmarkUnsafeSum3-2           20000000                84.0 ns/op
PASS
ok      tq/lab/Unsafe   6.926s

参考资料

1. http://golang.org/pkg/unsafe

修订记录

1. 2017年06月15日 建立文档。

（编辑：李大同）

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