C Linux编程-管道使子进程退出

Question

I am having a hard time understanding the behavior of the following code. Closing the file descriptor p[0] makes the program exit (since otherwise the parent process just waits on the child processes in all eternity). It doesn't make sense to me because the child processes are running an infinite while loop, why would they just exit because the read end of the pipe is closed? Sure, you won't be able to write to the pipe but the while loop is not dependent on whether the read end of the parent process is open or not. I tried removing the exit() functions in the child processes and the program exits anyways, so why is the child process killing itself as soon as it notices that the read end is closed?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

main()
{
    int run=10, fd[2]; pipe(fd); srand(time(0)); char ch, x='x', o='o'; 

    if(!fork())
    {
        close(fd[0]);

        while(run)
        {
            sleep(1+rand()%6); write(fd[1],&x,1);
        }

        exit(0); //This exit doesn't happen
    }

    if(!fork())
    {
        close(fd[0]);

        while(run)
        {
            sleep(1+rand()%3); write(fd[1],&o,1);
        }

        exit(0); //This exit doesn't happen
    }

    close(fd[1]);
    while(run--)
    {
        read(fd[0],&ch,1);
        printf("%d %c\n",run,ch);
        sleep(1);
    }

    close(fd[0]); //closing this file descriptor results in that the program can exit
    wait(0);
    wait(0);
    exit(0);

}

Answer 1

That's standard behavior. You close the read-end of the pipe, so there is nowhere to write. That results in a SIGPIPE signal being sent to the writing process.

The default behavior of SIGPIPE is to terminate the process receiving the signal.

If you want your process to survive the signal, you have to catch or ignore it. Then you must check the result of write as it will return with an error when the pipes read-end is closed.

Answer 2

Your problem is not with pipe. Two header files missing in your codes and for which it is crashing when calling wait() function. Add below two header files in your program and it will fix your problem. Also your declaration of main function is old c type declaration make it like void main() .

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