three pipes

three_pipes.cpp

extern "C" {
#include <sys/wait.h>
#include <unistd.h>
}
#include <iostream>
#include <string>
#include <vector>

int main() {
  int pipes[2][2];

  ::std::vector<const char*> cmd1 = {"ls", "-la", nullptr};
  ::std::vector<const char*> cmd2 = {"grep", "out", nullptr};
  ::std::vector<const char*> cmd3 = {"rev", nullptr};

  for (int i = 0; i < 3; i++) {
    pid_t pid = -1;
    int status = 0;

    if (i == 0) {
      pipe(pipes[0]);
    } else if (i == 1) {
      pipe(pipes[1]);
    }

    switch ((pid = fork())) {
      case -1:
        ::std::perror("fork failed");
        ::std::exit(EXIT_FAILURE);
        break;

      case 0:  // child
        if (i == 0) {
          dup2(pipes[0][1], fileno(stdout));
          close(pipes[0][0]);
          close(pipes[0][1]);

          execvp(cmd1[0], const_cast<char**>(cmd1.data()));
          ::std::perror("exec fail");

        } else if (i == 1) {
          dup2(pipes[0][0], fileno(stdin));
          close(pipes[0][0]);
          close(pipes[0][1]);

          dup2(pipes[1][1], fileno(stdout));
          close(pipes[1][0]);
          close(pipes[1][1]);

          execvp(cmd2[0], const_cast<char**>(cmd2.data()));
          ::std::perror("exec fail");

        } else if (i == 2) {
          dup2(pipes[1][0], fileno(stdin));
          close(pipes[1][0]);
          close(pipes[1][1]);

          execvp(cmd3[0], const_cast<char**>(cmd3.data()));
          ::std::perror("exec fail");
        }
        break;

      default:  // parent
        if (i == 1) {
          close(pipes[0][0]);
          close(pipes[0][1]);
        } else if (i == 2) {
          close(pipes[1][0]);
          close(pipes[1][1]);
        }
        wait(&status);
        break;
    }
  }
}