c - 如何在传递子控件之前列出所有子pid？

Question

我有一个作业，教授要求父进程在将控制权传递给子进程之前，先打印每个子进程的进程ID。

我试图做这样的事情:

pid_t cpids[5]; //I want to create 5 child processes
int n = 0;

do {
    cpids[n++] = fork();
} while(cpids[n] > 0 && n < 5) ; //This doesn't fork 5 times

//print all the child pids
printf("The child processes are: %d, %d, %d, %d, %d\n", cpids[0]..);
//(I know this would be printed multiple times, I'm just trying to explain what I need)

//then after printing the ids, tell child processes what to do
for(int i = 0; i < 5; i++) {

    //error
    if(cpids[i] < 0) {
        printf("There was an error with fork()\n");
        exit(1);
    }

    //child process
    else if(cpids[i] == 0) {
    //...reads from pipe sent from parent process
    }

    //parent process
    //sends message through pipe to child process
    //waits for child to terminate
}

所以这绝对是行不通的:)。有没有更简单的方法来 fork 进程而无需立即给出指示？谢谢!

//更新

所以我知道我做错了fork()事情。这是我的原始代码:

for(int i = 0; i < 5; i++) {
    //error
    if((pid = fork()) < 0) {
        printf("There was an error with fork()\n");
        exit(1);
    }

    //child process
    else if(pid == 0) {
        pid = getpid();
        close(fd[i][1]);

        //read starting position from parent process
        len = read(fd[i][0], &fpos, sizeof(fpos));

        if(len > 0) {
            doChild(numArray, fpos, i);
        }

        printf("id: %d\n", pid);
        _exit(1);
    } 

    //parent process
    else {
        close(fd[i][0]);
        fpos = (SIZE/NUM_CHILD) * i;
        write(fd[i][1], &fpos, sizeof(fpos));

        if ( waitpid(-getpid(), &status, 0) != -1 ) {
            if ( WIFEXITED(status) ) {
                int returned = WEXITSTATUS(status);
                printf("child id: %d ended with status %d\n\n", pid, returned);
            }
        }

        else {
            perror("waitpid() failed");
            exit(EXIT_FAILURE);
        }
    }

}

但是，父进程必须等待子进程终止，然后才能启动另一个子进程。我无法在所有子项开始运行之前获取它们。我基本上是在寻找一种方法来创建我需要的所有子进程，也许让它们在创建后立即进入休眠状态，然后在继续之前从父进程中打印出所有pid。

Answer 1

我并不是很想为您完成任务，但是我建议从类似以下内容开始:

void child_func(int readfd) {
    // read from readfd and do whatever
}

void do_fork(pid_t *childpid, int *pipefd) {
    int pipefds[2];
    pid_t pid;
    pipe(pipefds, 0);
    if ((pid = fork()) == 0) {
        child_func(pipefds[0]);
        exit(0);
    } else {
        *childpid = pid;
        *pipefd = pipefds[1];
    }
}

int main() {
    int i;
    pid_t children[5];
    int pipes[5];
    for (i = 0; i < 5; ++i) {
        do_fork(&children[i], &pipes[i]);
    }
    // whatever
    return 0;
}

如果您的child_func在开始时从文件描述符读取，它将阻塞直到父对象写入其他5个描述符。这将使您有机会打印出pid或其他内容，然后告诉他们该怎么做。如果您不完全了解手册，请查看“管道”和“ fork ”的手册页。我希望这足以让您入门。