c - 带 fork 的管道和子创建

Question

这是我正在尝试解决的问题的代码。如果有人知道如何解决这个问题，请帮忙。我试图用与我们将创建的 child 一样多的管道来创建无限个 child ，但我失去了一些带有管道的 child ，我不知道为什么。

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <sys/wait.h>
void main() {
    int n;
    pid_t p;
    int pfd[2];
    int val=0;
    int i;

    printf("Enter the number of child you want:");
    scanf("%d",&n);

    pipe(pfd);

    while(1) {
        for(i=1;i<=n;i++) {

            p=fork();
            if(p>0){
                close(pfd[0]);
                sleep(5);
                val++;
                printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getppid(),getpid());
                write(pfd[0],&val,sizeof(val));
                close(pfd[1]);
                //wait(NULL);
                //exit(EXIT_SUCCESS);
            }
            else {
                close(pfd[1]);
                val++;
                //printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getpid(),getppid());
                //while(read(pfd[0],&val,sizeof(val))>0)
                read(pfd[0],&val,sizeof(val));
                //printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getpid(),getppid());
                write(pfd[0],&val,sizeof(val));
                close(pfd[0]);
                _exit(EXIT_SUCCESS);
            }
        }
    }
    printf("Message %d: last child's pid is %d and my father is : %d \n", n,getpid(),getppid());    
} 

输出是:输入您想要的 child 数量:3

Message 1: Process 1 has process id 678 and his child's id is : 15041

Message 2: Process 2 has process id 678 and his child's id is : 15041

Message 3: Process 3 has process id 678 and his child's id is : 15041

Message 4: Process 1 has process id 678 and his child's id is : 15041

Message 5: Process 2 has process id 678 and his child's id is : 15041

Message 6: Process 3 has process id 678 and his child's id is : 15041

Message 7: Process 1 has process id 678 and his child's id is : 15041

Message 8: Process 2 has process id 678 and his child's id is : 15041

Message 9: Process 3 has process id 678 and his child's id is : 15041

Message 10: Process 1 has process id 678 and his child's id is : 15041

Message 11: Process 2 has process id 678 and his child's id is : 15041

Message 12: Process 3 has process id 678 and his child's id is : 15041

所以我的问题是进程ID没有改变，我找不到原因

Answer 1

So my problem is that the process id's are not changing and I can't find why

因为

printf("Message %d: Process %d has process id %d and its child has process id %d \n",val,i,getppid(),getpid());

是由父进程执行的，所以getppid(父进程的父进程的pid)是始终相同，getpid 也保持不变。唯一改变的是 val 和 `i'。

您只需创建一次管道，然后将其用于所有 fork ，同时关闭唯一创建的管道然后再次 fork ，这不会有好结果。

您必须在进行 fork 之前创建管道。另外，您正在关闭父进程上管道的读取端，以及关闭端的写入阅读结束。你必须在书写端写。

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

int main(void)
{
    pid_t p;
    int pfd[2];
    int i;
    int val = 0;
    int n = 3;

    for(i=1;i<=n;i++) {
        if(pipe(pfd) < 0)
        {
            perror("pipe");
            continue;
        }

        p = fork();

        if(p < 0)
        {
            perror("fork");
            continue;
        }

        if(p)
        {
            // PARENT PROC
            close(pfd[0]);
            val++;
            printf("Message %d: Parent process has process id %d and its child has process id %d\n", val, getpid(), p);
            write(pfd[1], &val, sizeof val);
            close(pfd[1]);
            int status;
            waitpid(p, &status, 0);

            if(WIFEXITED(status))
                printf("Child %d exit status: %d\n", i, WEXITSTATUS(status));
            else
                printf("Child %d did not exit normally\n", i);
        } else {
            // CHILD PROC

            close(pfd[1]);
            if(read(pfd[0], &val, sizeof val) < 0)
            {
                printf("Child %d: could not read from pipe\n", i);
                _exit(EXIT_FAILURE);
            }

            printf("Child %d, message: %d. Child pid: %d, parent pid: %d\n", i, val, getpid(), getppid());

            close(pfd[0]);
            _exit(EXIT_SUCCESS);
        }
    }

    return 0;
}

这会打印

Message 1: Parent process has process id 9829 and its child has process id 9830
Child 1, message: 1. Child pid: 9830, parent pid: 9829
Child 1 exit status: 0
Message 2: Parent process has process id 9829 and its child has process id 9831
Child 2, message: 2. Child pid: 9831, parent pid: 9829
Child 2 exit status: 0
Message 3: Parent process has process id 9829 and its child has process id 9832
Child 3, message: 3. Child pid: 9832, parent pid: 9829
Child 3 exit status: 0

最后一件事:main 的正确原型(prototype)是:

• int main(void);
• int main(int argc, char *argv[]);
• int main(int argc, char **argv);

您的void main()不正确。