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How to implement fork syscall in Golang?

中文版

ref: https://github.com/moby/moby/tree/master/pkg/reexec

We don't have a fork syscall in Golang, we have:

All those three functions is like a combination of fork + exec, but there has no pure fork syscall just like in C programming language(after syscall, which will return pid in caller). Reasons can be found in here:

It mainly says:

Let's have a look at how we use pure fork in C:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>

void child() {
    printf("child process\n");
}

int main() {
    printf("main process\n");
    pid_t pid = fork();
    int wstatus;

    if (pid == 0) {
        child();
    } else {
        printf("main exit\n");
        waitpid(pid, &wstatus, 0);
    }
}

Run it:

$ gcc main.c && ./a.out
main process
main exit
child process

Let's look how can we implements this in Go:

package main

import (
    "log"
    "os"

    "github.com/docker/docker/pkg/reexec"
)

func init() {
    log.Printf("init start, os.Args = %+v\n", os.Args)
    reexec.Register("childProcess", childProcess)
    if reexec.Init() {
        os.Exit(0)
    }
}

func childProcess() {
    log.Println("childProcess")
}

func main() {
    log.Printf("main start, os.Args = %+v\n", os.Args)
    cmd := reexec.Command("childProcess")
    cmd.Stdin = os.Stdin
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr
    if err := cmd.Start(); err != nil {
        log.Panicf("failed to run command: %s", err)
    }
    if err := cmd.Wait(); err != nil {
        log.Panicf("failed to wait command: %s", err)
    }
    log.Println("main exit")
}

Run it:

$ go run main.go
2018/03/08 19:52:39 init start, os.Args = [/tmp/go-build209640177/b001/exe/main]
2018/03/08 19:52:39 main start, os.Args = [/tmp/go-build209640177/b001/exe/main]
2018/03/08 19:52:39 init start, os.Args = [childProcess]
2018/03/08 19:52:39 childProcess
2018/03/08 19:52:39 main exit

Explanation:

init will be execute before main function. When you execute the binary executable file from command line, os.Args[0] will be the name of binary executable file, but, reexec.Command will change os.Args[0], so child process will find function registed by reexec.Register, and execute it, return true, then call os.Exit(0).