C/C++ launch a subprocess and wait it finish

最新推荐文章于 2025-04-24 12:20:58 发布

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本文介绍了一个使用C++实现的进程间通信案例，包括父进程通过fork创建子进程、执行子进程并等待其结束，以及检查返回码的过程。演示了正常执行、调用不存在的进程及信号终止子进程三种情况。

This is a usage sample of how to fork a sub-process and wait the sub-process ended, and check its return code.

1. main.cpp

2. sub.cpp

main process will fork a process to execute the sub process, and wait until sub process is finished, and print the sub process's return code.

usage steps:

1. g++ main.cpp -o main

2. g++ sub.cpp -o sub

case 1: call sub

$ main sub
main: entry
main: fork subpid=1390
fork: entry, with process=sub
sub: entry
sub: exit
main: waitpid return 512
main: WIFEXITED=2
main: exit

sub-process was normally launched, and exit with a return code 2

case 2: call un-existed sub-process

$ main fake-sub     
main: entry
main: fork subpid=3161
fork: entry, with process=fake-sub
fork: sub execvp failure
main: waitpid return 2304
main: WIFEXITED=6
main: exit

sub-process cannot be launched, the return code is returned from main.cpp

case 3: call sub, and kill the sub-process before it finish

$ main sub
main: entry
main: fork subpid=4170
fork: entry, with process=sub
sub: entry
...

now kill the sub-process in another terminal

$ kill -9 4170

$ main sub
main: entry
main: fork subpid=4170
fork: entry, with process=sub
sub: entry
main: waitpid return 9
main: WIFSIGNALED=9
main: exit

sub-process is terminated by a signal 9.

Appendix:

sub.cpp

$ cat sub.cpp 
#include <string>
#include <iostream>

int main(int argc, char * argv[]) {
    std::cout << "sub: entry" << std::endl;
    system("sleep 10");
    std::cout << "sub: exit" << std::endl;

    if (argc > 1) {
        return atoi(argv[1]);
    }
    return 0;
}

main.cpp

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

#include <string>
#include <iostream>

int main(int argc, char * argv[]) {
    const char * args[3];
    args[0] = argc > 1 ? argv[1] : "sub";
    args[1] = "2";
    args[2] = NULL;

    std::cout << "main: entry" << std::endl;

    pid_t childpid = fork();
    if (childpid == -1) {       // error
        std::cout << "main: cannot fork()" << std::endl;
    }
    else if (childpid == 0) {
        std::cout << "fork: entry, with process=" << args[0] << std::endl;
        if (execvp(args[0], (char **)args) == -1) {   // child process
            std::cout << "fork: sub execvp failure" << std::endl;
            exit (6);
        }
        std::cout << "fork: never come here" << std::endl;
    }
    else {                      // parent process
        std::cout << "main: fork subpid=" << childpid << std::endl;
        
        int status;
        if (childpid == waitpid(childpid, &status, 0)) {
            std::cout << "main: waitpid return " << status << std::endl;
        
            if (WIFEXITED(status)) {
                std::cout << "main: WIFEXITED=" << WEXITSTATUS(status) << std::endl;
            }
            else if (WIFSIGNALED(status)) {
                std::cout << "main: WIFSIGNALED=" << WTERMSIG(status) << std::endl;
            }
            else {
                std::cout << "main: others" << std::endl;
            }
        }
        else {
            std::cout << "main: waitpid return failure" << std::endl;
        }
    }

    std::cout << "main: exit" << std::endl;
    
    return 0;
}

Thanks