c - 为什么我的进程没有同时运行？

Question

我的问题是:

1.) 我怎样才能让父进程总是最后死掉？我知道这没有完成，因为父级是第一个运行的 pid，但我不知道如何更改它。

2.) 我如何让我的子进程像他的一样同时执行？我什至把这个数字调高了很多，看看这是否只是巧合，但它似乎不是。

编辑:解决方案

1.) 在内部 Default 中添加了两次 wait(NULL)
2.) 正在发生。用 sleep(1) 来证明。

我的代码如下

#include <stdio.h>
int main() {
    int pid, i;
    pid = fork();
    switch(pid) {
        case -1:
            // error
            printf("Fork error");
            break;
        case 0:
            // child process
            printf("First child is born, my pid is %d\n", getpid());
            for(i=1; i<10; i++)
                printf("First child executes iteration %d\n", i);
            printf("First child dies quietly.\n");
            break;
        default:
            // parent process
            printf("Parent process is born, my pid is %d\n", getpid());
            pid = fork();
            switch(pid) {
                case -1:
                    // error
                    printf("Fork error");
                    break;
                case 0:
                    // child process
                    printf("Second child is born, my pid is %d\n", getpid());
                    for(i=1; i<10; i++)
                        printf("Second child executes iteration %d\n", i);
                    printf("Second child dies quietly.\n");
                    break;
                default:
                    // parent process
                    printf("Parent process dies quietly.");
        }
    }
    return 0;
}

我的输出总是这样的:

Parent process is born, my pid is 7847  First child is born, my pid is 7848  First child executes iteration: 1  First child executes iteration: 2  First child executes iteration: 3  First child executes iteration: 4  First child executes iteration: 5  First child executes iteration: 6  First child executes iteration: 7  First child executes iteration: 8  First child executes iteration: 9  First child executes iteration: 10  First child dies quietly.  Parent process dies quietly.  Second child is born, my pid is 7849  Second child executes iteration 1  Second child executes iteration 2  Second child executes iteration 3  Second child executes iteration 4  Second child executes iteration 5  Second child executes iteration 6  Second child executes iteration 7  Second child executes iteration 8  Second child executes iteration 9  Second child executes iteration 10  Second child dies quietly.  

我的任务是:

编写一个创建三个进程的 C 程序(“procs.c”):一个创建两个子进程的父进程。

第一个 child 应该做以下事情:

• 显示“第一个 child 出生，我的pid是……”

• 显示十次消息“第一个 child 执行迭代 X”，其中 X 是迭代次数

• 显示“第一个 child 安静地死去。”

第二个 child 应该做到以下几点:

• 显示“二胎出生，我的pid是……”

• 显示十次消息“Second child executes iteration X”，其中X是迭代次数

• 显示“第二个 child 安静地死去。”

父进程应该做以下事情:

• 显示“父进程诞生，我的pid是……”

• 创建第一个 child

• 创建第二个 child

• 显示“父进程安静地死去。”

使用 gcc 编译程序并将可执行文件命名为“procs”。多次执行程序，注意两个 child 的输出如何交错。

这个程序可能的输出是:

nova> ./procsParent process is born, my pid is 7847  First child is born, my pid is 7848  First child executes iteration: 1  First child executes iteration: 2  First child executes iteration: 3  First child executes iteration: 4  First child executes iteration: 5  Second child is born, my pid is 7849  Second child executes iteration 1  Second child executes iteration 2  Second child executes iteration 3  First child executes iteration: 6  Second child executes iteration 4  Second child executes iteration 5  Second child executes iteration 6  First child executes iteration: 7  Second child executes iteration 7  Second child executes iteration 8  Second child executes iteration 9  Second child executes iteration 10  Second child dies quietly.  First child executes iteration: 8  First child executes iteration: 9  First child executes iteration: 10  First child dies quietly.  Parent process dies quietly.  

Answer 1

1. 适当的父进程应该等待(使用 wait() or waitpid() 或特定于平台的变体)其子进程在退出前死亡。

2. 并发执行需要调度程序获得运行的机会。您可以通过适当的 sleep (微 sleep ，nano-sleep)操作强制解决问题。一些系统调用会有所帮助(因此 fflush(0) 可能会有所帮助)。

---

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

int main(void)
{
    int pid, i;
    struct timespec tw = { .tv_sec = 0, .tv_nsec = 10000000 };
    pid = fork();
    switch(pid)
    {
        case -1:
            printf("Fork error");
            break;
        case 0:
            printf("First child is born, my pid is %d\n", getpid());
            for(i=1; i<10; i++)
            {
                printf("First child executes iteration %d\n", i);
                nanosleep(&tw, 0);
            }
            printf("First child dies quietly.\n");
            break;
        default:
            printf("Parent process is born, my pid is %d\n", getpid());
            pid = fork();
            switch(pid)
            {
                case -1:
                    printf("Fork error");
                    break;
                case 0:
                    printf("Second child is born, my pid is %d\n", getpid());
                    for(i=1; i<10; i++)
                    {
                        printf("Second child executes iteration %d\n", i);
                        nanosleep(&tw, 0);
                    }
                    printf("Second child dies quietly.\n");
                    break;
                default:
                    printf("Parent process waiting for children.\n");
                    int corpse;
                    int status;
                    while ((corpse = waitpid(0, &status, 0)) > 0)
                        printf("Child %d died with exit status 0x%.4X\n", corpse, status);
                    printf("Parent process dies quietly.\n");
                    break;
            }
    }
    return 0;
}

示例输出:

Parent process is born, my pid is 46624
First child is born, my pid is 46625
First child executes iteration 1
Parent process waiting for children.
Second child is born, my pid is 46626
Second child executes iteration 1
First child executes iteration 2
Second child executes iteration 2
First child executes iteration 3
Second child executes iteration 3
First child executes iteration 4
Second child executes iteration 4
Second child executes iteration 5
First child executes iteration 5
Second child executes iteration 6
First child executes iteration 6
Second child executes iteration 7
First child executes iteration 7
Second child executes iteration 8
First child executes iteration 8
Second child executes iteration 9
First child executes iteration 9
First child dies quietly.
Second child dies quietly.
Child 46625 died with exit status 0x0000
Child 46626 died with exit status 0x0000
Parent process dies quietly.

请注意，10 毫秒的延迟几乎强制交替执行。如果没有类似的东西，您将受制于系统及其调度程序的特性，而且许多现代机器实在是太快了!