Implementing `ls -la | sort`.

pipe.c

// Heavily modified from https://brandonwamboldt.ca/how-linux-pipes-work-under-the-hood-1518/.
#include <stdio.h>
#include <unistd.h> // STDOUT_FILENO
#include <sys/types.h> // wait()
#include <sys/wait.h> // wait()

#define READ_END 0
#define WRITE_END 1

#define FORK_CHILD 0
#define FORK_PARENT 1

// ls -la | sort
int main(int argc, char **argv) {
  // Create a pipe in parent
  int pipefd[2];
  if (pipe(pipefd) == -1) {
    printf("parent: failed to create pipe\n");
    return 1;
  }

  // We need to fork two processes, one for ls, and another one for sort
  int pid_ls = fork();
  if (pid_ls == -1) {
    printf("parent: could not fork process to run ls\n");
    return 1;
  }

  if (pid_ls == FORK_CHILD) { // Child
    printf("child: running ls\n");

    // http://man7.org/linux/man-pages/man3/stdin.3.html
    // Merge stdout of ls to write end of pipe.
    if (dup2(pipefd[WRITE_END], STDOUT_FILENO) == -1) {
      printf("child: failed to call dup2 for ls\n");
      return 1;
    }
    close(pipefd[READ_END]);
    close(pipefd[WRITE_END]);

    // Replace the current process with ls.
    // execve() does not return on success, and the text, data, bss, and stack
    // of the calling process are overwritten by that of the program loaded.
    // Ref: https://linux.die.net/man/2/execve
    char *ls_argv[] = {"/bin/ls", "-la", NULL};
    execve("/bin/ls", ls_argv, NULL);

    // We will only return here if execve failed.
    printf("child: ls failed\n");
    return -1;
  }

  // Parent (FORK_PARENT)
  // Continue forking
  int pid_sort = fork();
  if (pid_sort == -1) {
    printf("parent: could not fork process to run sort\n");
    return 1;
  }

  if (pid_sort == FORK_CHILD) { // Child
    printf("child: running sort\n");

    // http://man7.org/linux/man-pages/man3/stdin.3.html
    // Merge stdin of sort to read end of pipe.
    if (dup2(pipefd[READ_END], STDIN_FILENO) == -1) {
      printf("child: failed to call dup2 for sort\n");
      return 1;
    }
    close(pipefd[READ_END]);
    close(pipefd[WRITE_END]);

    // Replace the current process with sort.
    // execve() does not return on success, and the text, data, bss, and stack
    // of the calling process are overwritten by that of the program loaded.
    // Ref: https://linux.die.net/man/2/execve
    char *sort_argv[] = {"/usr/bin/sort", NULL};
    execve("/usr/bin/sort", sort_argv, NULL);

    // We will only return here if execve failed.
    printf("child: sort failed\n");
    return -1;
  }

  close(pipefd[READ_END]);
  close(pipefd[WRITE_END]);

  printf("parent: wait for children\n");
  while (wait(NULL) > 0);
  printf("parent: done!\n");

  return 0;
}