using_pipe.cpp

// based on the example from
// https://stackoverflow.com/questions/13041416/redirect-stdout-of-two-processes-to-another-processs-stdin-in-linux-c
#include <vector>

#include <iostream>
#include <signal.h>
#include <unistd.h>

/// Entry point of process A
int procA(void) {
  // Process A writing to C
  for (int i = 0; i < 100; i++) {
    std::cout << "Hi" << std::endl;
    usleep(5000);
  }
  std::cout << "[A] Sleeping" << std::endl;
  sleep(6);
  std::cout << "[A] Exiting" << std::endl;
  return 0;
}

/// Entry point of process B
int procB(void) {
  // Process B writing to C
  while (!std::cin.eof()) {
    // read a line of input until EOL and store in a string
    std::string line;
    std::getline(std::cin, line);
    if (line.size() > 0)
      std::cout << line << std::endl;
  }
  std::cout << "[B] saw EOF" << std::endl;
  return 0;
}

/// Entry point of process C
int procC(void) {
  // Process C reading from both A and B
  while (!std::cin.eof()) {
    // read a line of input until EOL and store in a string
    std::string line;
    std::getline(std::cin, line);
    if (line.size() > 0)
      std::cout << "[C]: " << line << std::endl;
  }
  return 0;
}

int main(void) {
  std::vector<pid_t> kids;
  // create a pipe
  int ABtoC[2];
  pipe(ABtoC);

  pid_t child_pid;
  child_pid = fork();
  if (child_pid == 0) {
    // redirect stdout to the pipe
    dup2(ABtoC[1], STDOUT_FILENO);
    close(ABtoC[0]);
    close(ABtoC[1]); // Close this too!

    return procA();
  } else if (child_pid < 0) {
    std::cerr << "Error: could not fork\n";
    return 1;
  }

  kids.push_back(child_pid);

  child_pid = fork();
  if (child_pid == 0) {
    // redirect stdin from the pipe
    dup2(ABtoC[0], STDIN_FILENO);
    close(ABtoC[1]);
    close(ABtoC[0]);

    return procC();
  } else if (child_pid < 0) {
    std::cerr << "Error: could not fork\n";
    return 1;
  }

  kids.push_back(child_pid);
  child_pid = 0;

  // redirect stdout to the pipe
  dup2(ABtoC[1], STDOUT_FILENO);
  close(ABtoC[0]);
  close(ABtoC[1]);

  // start process B
  int res = procB();

  // send kill signal to all children
  for (pid_t k : kids) {
    int status;
    kill(k, SIGTERM);
    waitpid(k, &status, 0);
  }

  return res;
}