父/子fork（）和C中的管道

Question

I need to implement unix codes into my C code, such as PS, WC, and grep. I have pipes and forks that create a child to do each part of the code, however, I am getting a bit confused about what number processes need to be closed each time. Here is my code so far, but obviously it doesn't work because the rights things aren't being closed.

int main() {
int pfd[2];
int pid;

if (pipe(pfd) == -1) {
    perror("pipe failed");
    exit(-1);
}
if ((pid = fork()) < 0) {
    perror("fork failed");
    exit(-2);
}
if (pid == 0) {
    close(pfd[1]);
    dup2(pfd[0], 0);
    close(pfd[0]);
    if (pipe(pdf) == -1) {
        perror("pipe failed");
        exit(-1);
    }
    if (pid ==0) {
        close(pfd[1]);
        dup2(pfd[0], 0);
        close(pfd[0]);
        execlp("wc", "wc", (char *) 0);
        perror("wc failed");
        exit(-3);
    }
    else 
        close(pfd[1]);
        dup2(pfd[0], 0);
        close(pfd[0]);
        execlp("grep", "grep" "1111", (char *) 0);
        perror("grep failed");
        exit(-3);
} 
else {
    close(pfd[0]);
    dup2(pfd[1], 1);
    close(pfd[1]);
    execlp("ps", "ps", "auxj", (char *) 0);
    perror("ps failed");
    exit(-4);
}
exit(0);
}

any help would be great

Answer 1

There's no guarantee in the order of execution of the fork'ed processes you are creating. This means you can't really close the pipes. In order to to so, you would need to add some synchronization, to assure you will have to wait for all the processes that use the pipes to finish and just then close them in the last process alive.