go mmap example

go_mmap_example.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"syscall"
	"unsafe"
)

func write() {
	const n = 1e3
	t := int(unsafe.Sizeof(int64(0))) * n
	mapFile, err := os.Create("./mmap.dat")
	if err != nil {
		panic(err)
	}
	_, err = mapFile.Seek(int64(t-1), 0)
	if err != nil {
		panic(err)
	}
	_, err = mapFile.Write([]byte(" "))
	if err != nil {
		panic(err)
	}
	// 将文件操作映射为字节数组操作
	mmap, err := syscall.Mmap(int(mapFile.Fd()), 0, t, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
	if err != nil {
		panic(err)
	}
	// 获取第一个字节的位置
	mapArray := (*[n]int)(unsafe.Pointer(&mmap[0]))
	for i := 0; i < n; i++ {
		mapArray[i] = i * i
	}
	// 解除映射？数组和文件底层操作不再映射起来了？
	err = syscall.Munmap(mmap)
	if err != nil {
		panic(err)
	}
	err = mapFile.Close()
	if err != nil {
		panic(err)
	}
}

func read() {
	const n = 1e3
	b, err := ioutil.ReadFile("./mmap.dat")
	if err != nil {
		panic(err)
	}
	intArray := (*[n]int)(unsafe.Pointer(&b[0]))
	for _, v := range *intArray {
		fmt.Printf("%d\n", v)
	}
}

func main() {
	write()
	read()
}