中文版

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

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

• syscall.ForkExec
• os.StartProcess
• exec.Cmd

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：

• fork() has been invented at the time when no threads were used at all, and a process had always had just a single thread of execution in it, and hence forking it was safe.
• In C, you control all the threads by your hand, but in Go, you cannot, so threads will be out of control after call fork without exec, so Go provides fork + exec only.

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).