/* For those who would like to continue to learn more about UNIX programming,
 *  see the FAQ:      http://www.erlenstar.demon.co.uk/unix/faq_toc.html
 *  or the YoLinux tutorial (e.g. Fork/Exec):
 *                    http://www.yolinux.com/TUTORIALS/ForkExecProcesses.html
 *  for the many details of UNIX programming.
 *
 * This version includes error checking.  See man pages for details.
 */

#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>    /* 'man fprintf' says we need this. */
#include <sys/wait.h> /* 'man waitpid' says we need this. */


/* Just in case these aren't already defined. */
#define STDIN 0
#define STDOUT 1

int main() {
    int pipe_fd[2];       /* 'man pipe' says its arg is this type */
    int fd;               /* 'man dup' says its arg is this type */
    pid_t child1, child2; /* 'man fork' says it returns type 'pid_t' */
    char * argvChild[2];

    printf("Executing \"ls | wc\";\n");
    printf("Number of files in curr dir is (first # is answer): ");
    fflush(stdout);  /* Force printing to complete, before continuing. */

    if ( -1 == pipe(pipe_fd) ) perror("pipe");
    child1 = fork();
    /* child1 > 0 implies that we're still the parent. */
    if (child1 > 0) child2 = fork();
    if (child1 == 0) { /* if we are child1, do:  "ls | ..." */
        if ( -1 == close(STDOUT) ) perror("close");  /* close  */
        fd = dup(pipe_fd[1]); /* set up empty STDOUT to be pipe_fd[1] */
        if ( -1 == fd ) perror("dup");
        if ( fd != STDOUT ) fprintf(stderr, "Pipe output not at STDOUT.\n");
        close(pipe_fd[0]); /* never used by child1 */
        close(pipe_fd[1]); /* not needed any more */
        argvChild[0] = "ls"; argvChild[1] = NULL;
        if ( -1 == execvp(argvChild[0], argvChild) ) perror("execvp");
    } else if (child2 == 0) { /* if we are child2, do:  "... | wc" */
        if ( -1 == close(STDIN) ) perror("close");  /* close  */
        fd = dup(pipe_fd[0]); /* set up empty STDIN to be pipe_fd[0] */
        if ( -1 == fd ) perror("dup");
        if ( fd != STDIN ) fprintf(stderr, "Pipe input not at STDIN.\n");
        close(pipe_fd[0]); /* not needed any more */
        close(pipe_fd[1]); /* never used by child2 */
        argvChild[0] = "wc"; argvChild[1] = NULL;
        if ( -1 == execvp(argvChild[0], argvChild) ) perror("execvp");
    } else { /* else we're parent */
        int status;
        /* Close parent copy of pipes;
         * In particular, if pipe_fd[1] not closed, child2 will hang
         *   forever waiting since parent could also write to pipe_fd[1]
         */
        close(pipe_fd[0]);
        close(pipe_fd[1]);
        if ( -1 == waitpid(child1, &status, 0) ) perror("waitpid");
        /* Optionally, check return status.  This is what main() returns. */
        if (WIFEXITED(status) == 0)
            printf("child1 returned w/ error code %d\n", WEXITSTATUS(status));
        if ( -1 == waitpid(child2, &status, 0) ) perror("waitpid");
        /* Optionally, check return status.  This is what main() returns. */
        if (WIFEXITED(status) == 0)
            printf("child2 returned w/ error code %d\n", WEXITSTATUS(status));
    }
    return 0;  /* returning 0 from main() means success. */
}