使用pipe（），execlp（）和dup（）在C中使用三重管道

Question

Consider a new operator '|||'. It will take the output of the previous program and pass it on as input to three different programs. Give a program triplepipe.c for implementing the following command: ls –l | uniq ||| grep ^d, grep ^-, grep ^p. Don't use popen() library call, system() library call, or temporary files.

I am using the following logic to solve first part of the question. 'ls -l' is done in child1, 'uniq' in child2 using child1's stdout as stdin and 'grep ^d' in parent using child2's stdout as stdin. I am able to see the output.

But I don't know how to give the output from child2 to three separate grep satisfying the limitations set. Can anyone please help?

Answer 1

Part of this assignment is basically to implement a 3-way tee . After setting up the pipes and child processes, you'll have to enter a loop which reads once and writes 3 times, until the read hits EOF.

In Linux there is also a tee syscall which isn't in your forbidden list. It would be perfect for this program, but it's fairly new, so there's a good chance your teacher doesn't know about it yet.

Answer 2

Quote "Pipes are just files "

Think you dup2'ed and currently have your data in a pipe in one of the childs. Now just read from the pipe and use Write System call and write to 3 different pipes.

From there you can process the data differently in 3 different processes

Think thats what you require ;)

Answer 3

Attaching the answer to my question.

void main() {
    int p1[2]; // pipe between ls | uniq and grep
    int p2[2]; // pipt between ls and uniq
    int p3[2]; // pipt between ls and uniq
    int p4[2]; // pipt between ls and uniq
    int p5[2]; // pipt between ls and uniq
    char buff[200];
    char data[10 * 1024];
    int count;
    int teelen;
    pid_t ret;
    pid_t chd;

    pipe (p1);
    ret = fork();
    if (ret == 0) {
        pipe (p2);
        chd = fork();
        if (chd == 0) {
            close (1); // close stdout
            dup (p2[1]); // dup stdout to p2's out
            close (p2[0]); // close p2's in
            execlp ("ls", "ls", "-l", NULL); // execute command
            exit (0);
        }
        if (chd > 0) {
            close (0); // close stdin
            dup (p2[0]); // dup stdin to p2's in
            close (p2[1]); // close p2's out
            wait(NULL); // wait for child to finish
            close (1); // close stdout
            dup (p1[1]); // dup stdout to p1's out
            close (p1[0]); // close p1's in 
            execlp ("uniq", "uniq", NULL); // command
            exit (0);
        }
    }
    if (ret > 0) {
        close (0); // close stdin
        dup (p1[0]); // dup stdin to p1's in
        close(p1[1]);
        wait(NULL); // wait for child to complete
        memset (data, '\0', sizeof(data));
        do {
            memset(buff, '\0', sizeof(buff));
            count = read(p1[0], buff, 199);
            buff[count] = '\0';
            if (count == 0)
                break;
            strcat (data, buff);
        } while(1);
        pipe (p3);
        close(p1[0]);
        write (p3[1], data, sizeof (data));
        fsync(p3[1]);
        if (fork() == 0) {
            int c;
            close(0);
            dup(p3[0]);
            close(p3[1]);
            execlp ("grep", "grep", "-a", "^d", NULL);
            exit (0);
        }
        close(p3[1]);
        wait(NULL);
        pipe (p4);
        write (p4[1], data, sizeof (data));
        if (fork() == 0) {
            close(0);
            dup(p4[0]);
            close(p4[1]);
            execlp ("grep", "grep", "-a", "^-", NULL);
            exit (0);
        }
        close(p4[1]);
        wait(NULL);
        pipe (p5);
        write (p5[1], data, sizeof (data));
        if (fork() == 0) {
            close(0);
            dup(p5[0]);
            close(p5[1]);
            execlp ("grep", "grep", "-a", "^c", NULL);
            exit (0);
        }
        close(p5[1]);
        wait(NULL);
    }
    }