pipeexample.c

// how to create a process pipeline in C, or: 
//  how does the shell execute something like this:
//// $ awk '{print $3}' < /proc/mounts | sort | uniq -c

#define _GNU_SOURCE // needed for pipe2
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>

// i like the error check macro
#define CHECK(x) if ((x) < 0) { perror(#x); exit(1); }

// because i get them backwards all the time
#define READ_END(pipe) pipe[0]
#define WRITE_END(pipe) pipe[1]

int main(int argc, char **argv) {
    int procmounts = open("/proc/mounts", O_RDONLY | O_CLOEXEC);
    CHECK(procmounts);

    int awktosort[2];
    // we pass O_CLOEXEC to pipe so that when our child procs
    // dup2 over their stdin/stdout and then exec() their new process
    // that new process will not have the higher-numbered fds open,
    // only the stdio fds.
    CHECK(pipe2(awktosort, O_CLOEXEC));

    int awk = fork();
    CHECK(awk);
    if (awk == 0) {
        // replace our stdin with the procmounts fd
        CHECK(dup2(procmounts, 0));
        // send our stdout to sort
        CHECK(dup2(WRITE_END(awktosort), 1));
        // using execlp to search $PATH is -kiinda- cheating since that's a lot
        // more work from libc than just a syscall wrapper
        execlp("awk", "awk", "{print $3}", NULL);
        exit(1); // should not get here
    }
    close(procmounts);
    close(WRITE_END(awktosort));

    int sorttouniq[2];
    CHECK(pipe2(sorttouniq, O_CLOEXEC));

    int sort = fork();
    CHECK(sort);
    if (sort == 0) {
        // replace our stdin with the pipe connected to awk's stdout
        CHECK(dup2(READ_END(awktosort), 0));
        // replace our stdout with the pipe to uniq
        CHECK(dup2(WRITE_END(sorttouniq), 1));
        execlp("sort", "sort", NULL);
        exit(1);
    }
    close(READ_END(awktosort));
    close(WRITE_END(sorttouniq));

    int uniq = fork();
    CHECK(uniq);
    if (uniq == 0) {
        CHECK(dup2(READ_END(sorttouniq), 0));
        execlp("uniq", "uniq", "-c", NULL);
        exit(1);
    }
    close(READ_END(sorttouniq));

    int wstat;
    // just like an actual shell, we only preserve the exit code
    // of the last process in the pipeline
    waitpid(uniq, &wstat, 0);
    if (WIFEXITED(wstat)) {
        return WEXITSTATUS(wstat);
    }
    return 1;
}