Using the pipe() system call

Question

I've been trying to use the pipe() system call to create a shell that supports piping (with an arbitrary number of commands).

Unfortunately, I haven't had much luck using pipe(). After spending a few days looking at various online resources, I decided to put together an oversimplified program that has the same effect as executing ls | sort to see if I could even get a pipe to work for two sibling, child processes. Here's the code:

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

void run(char *cmd) {
   char *args[2];
   args[0] = cmd;
   args[1] = NULL;

   execvp(cmd, args);
}

int main(void) {
    int filedes[2];
    pipe(filedes);

    pid_t pid_a, pid_b;

    if (!(pid_a = fork())) {
        dup2(filedes[1], 1);
        run("ls");
    }

    if (!(pid_b = fork())) {
        dup2(filedes[0], 0);
        run("sort");
    }

    waitpid(pid_a, NULL, 0);
    waitpid(pid_b, NULL, 0);

    return 0;
}

The pipe is created in the parent and I know that after the execvp() call, each child process inherits the file descriptors that pipe() creates in the parent. For the ls process, I'm using dup2() to redirect its standard out (1) to the write-end of the pipe and for the sort process, standard in (0) is being redirected to the read-end of the pipe.

Finally, I wait for both processes to finish before exiting.

My intuition tells me this should work, but it doesn't!

Any suggestions?

Answer 1

You have to close the pipes you're not using. at least sort will read from its stdin until stdin is closed.

In this case, it's stdin is never closed, as you still have 2 open filedescriptors for it.

• filedes[0] in the ls child (this likely gets closed when ls finishes)
• filedes[0] in the parent program (this never gets closed as you waitpid() for sort to end, but it never will because the parent keeps its stdin open)

Change your program to

if (!(pid_a = fork())) {
    dup2(filedes[1], 1);
    closepipes(filedes);
    run("ls");
}

if (!(pid_b = fork())) {
    dup2(filedes[0], 0);
    closepipes(filedes);
    run("sort");
}
closepipe(filedes);
waitpid(pid_a, NULL, 0);
waitpid(pid_b, NULL, 0);

where closepipes is something like

void closepipes(int *fds)
{ 
 close(fds[0]);
 close(fds[1]);
}