Feeding stdout to a child process which will execv() sort

Question

I am trying to find out how I can send output of one process into a child process. I have gone down a journey learning of file descriptors and pipes. I think I am almost there but am missing a key component.

This is what I have so far:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    int fd[2];
    pid_t sort_pid;


    /* Create the pipe */
    if(pipe(fd) == -1) {
        fprintf(stderr, "Pipe failed\n");
        exit(EXIT_FAILURE);
    }
    /* create child process that will sort */
    sort_pid = fork();
    if(sort_pid < 0) { // failed to fork
        fprintf(stderr, "Child Fork failed\n");
        exit(EXIT_FAILURE);
    }
    else if(sort_pid == 0) { // child process
        close(0);   // close stdin
        dup2(fd[0], 0); // make stdin same as fd[0]
        close(fd[1]); // don't need this end of the pipe
        execlp("D:/Cygwin/bin/sort", "sort", NULL);
    }
    else { // parent process
        close(1); // close stdout
        dup2(fd[1], 1); // make stdout same as fd[1]
        close(fd[0]); // don't need this end of the pipe
        printf("Hello\n");
        printf("Bye\n");
        printf("Hi\n");
        printf("G'day\n");
        printf("It Works!\n");
        wait(NULL);
    }

    return EXIT_SUCCESS;
}

This doesn't work, as it seems to go into an endless loop or something. I tried combinations of the wait() but that doesnt help either.

I am doing this to learn how to apply this idea in my actual program. In my actual program I read files, parse them line by line and save the processed data to a static array of structs. I want to be able to then generate output based on these results and use the fork() and execv() syscalls to sort the output.

This is ultimately for a project in uni.

These are similar examples which I dissected to get to the stage I am at so far:

• pipe() and fork() in c
• How to call UNIX sort command on data in pipe
• Using dup,pipe,fifo to communicate with the child process

Furthermore I read the manual pages on the relevant syscalls to try and understand them. I will admit my knowledge of pipes and using them is still basically nothing, as this is my first every try with them.

Any help is appreciated, even further sources of information I could look into myself. I seem to have exhausted most of the useful stuff a google search give me.

Answer 1

sort will read until it encounters end-of-file. You therefore have to close the write-end of the pipe if you want it to complete. Because of the dup2, you have two copies of the open file description, so you need

1. close(fd[1]); anytime after the call to dup2
2. close(1); after you're done writing to (the new) stdout

Make sure to fflush(stdout) before the second of these to ensure that all your data actually made it into the pipe.

(This is a simple example of a deadlock: sort is waiting on the pipe to close, which will happen when the parent exits. But the parent won't exit until it finishes waiting on the child to exit…)