"ls | sort" with pipes and forks?

[alwut]

I'm practicing with exec and pipes and forks but I can't seem to understand how to have "sort" sort the output of "ls". I'm actually unsure of how to do this. I have my code below.

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

int main(int argc, char** argv) {

    int fd[2];
    int pid;
    char read_buf[READ_SIZE + 1];
    int chars_read;

    /*
     * Check for adequate args.
     */
    if (argc < 1) {
        fprintf(stderr, "usage: grab-stdout prog arg ...");
        exit(-1);
    }

    /*
     * Create a pipe that will have its output written to by the executed
     * program.
     */
    pipe(fd);

    /*
     * Fork off a process to exec the program.
     */
    if ((pid = fork()) < 0) {
        perror("fork");
  exit(-1);
    }

    /*
     * Forked child has its stdout be the write end of the pipe.
     */
    else if (pid == 0) {

        /*
         * Don't need read end of pipe in child.
         */
        close(fd[0]);

        /*
         * This use of dup2 makes the output end of the pipe be stdout.
         */
        dup2(fd[1], STDOUT_FILENO);

        /*
         * Don't need fd[1] after the dup2.
         */
        close(fd[1]);

        /*
         * Exec the program given in the command line, including any args.
         */

        execlp("ls", "ls", 0);
        
        perror("exec1");
        exit(-1);
    }

    /*
     * Parent takes its input from the read end of the pipe.
     */
    else {
        /*
         * Don't need write end of pipe in parent.
         */
        close(fd[1]);

        //I'm unsure of what to do here but this is all I could think of
        execlp("sort", "sort", "-r", fd[0] );

        perror("exec2");
        exit(-1);

    }

    exit(0);
}

Thanks for any input.

[Adak]

* Parent takes its input from the read end of the pipe.
. . .
* Don't need write end of pipe in parent.

OMG! Thanks for the laugh!

[Salem]

> execlp("sort", "sort", "-r", fd[0] );
Well you use the same dup2 (for stdin of course) and then
execlp("sort", "sort", "-r" );

[alwut]

ok i got it, Thanks Salem!