pipes/pipe_ls_wc.c

#include <sys/wait.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int pfd[2];                                     /* Pipe file descriptors */

    if (pipe(pfd) == -1)                            /* Create pipe */
        errExit("pipe");

    switch (fork()) {
    case -1:
        errExit("fork");

    case 0:             /* First child: exec 'ls' to write to pipe */
        if (close(pfd[0]) == -1)                    /* Read end is unused */
            errExit("close 1");

        /* Duplicate stdout on write end of pipe; close duplicated descriptor */

        if (pfd[1] != STDOUT_FILENO) {              /* Defensive check */
            if (dup2(pfd[1], STDOUT_FILENO) == -1)
                errExit("dup2 1");
            if (close(pfd[1]) == -1)
                errExit("close 2");
        }

        execlp("ls", "ls", (char *) NULL);          /* Writes to pipe */
        errExit("execlp ls");

    default:            /* Parent falls through to create next child */
        break;
    }

    switch (fork()) {
    case -1:
        errExit("fork");

    case 0:             /* Second child: exec 'wc' to read from pipe */
        if (close(pfd[1]) == -1)                    /* Write end is unused */
            errExit("close 3");

        /* Duplicate stdin on read end of pipe; close duplicated descriptor */

        if (pfd[0] != STDIN_FILENO) {               /* Defensive check */
            if (dup2(pfd[0], STDIN_FILENO) == -1)
                errExit("dup2 2");
            if (close(pfd[0]) == -1)
                errExit("close 4");
        }

        execlp("wc", "wc", "-l", (char *) NULL);
        errExit("execlp wc");

    default: /* Parent falls through */
        break;
    }

    /* Parent closes unused file descriptors for pipe, and waits for children */

    if (close(pfd[0]) == -1)
        errExit("close 5");
    if (close(pfd[1]) == -1)
        errExit("close 6");
    if (wait(NULL) == -1)
        errExit("wait 1");
    if (wait(NULL) == -1)
        errExit("wait 2");

    exit(EXIT_SUCCESS);
}