send_recv_ord_sem.cpp

/*
 * Copyright 2021 Jeisson Hidalgo-Cespedes - Universidad de Costa Rica
 * Creates a secondary thread that greets in the standard output
 */

#include <mpi.h>
#include <cstdint>
#include <iostream>

int main(int argc, char* argv[]) {
  if (MPI_Init(&argc, &argv) == MPI_SUCCESS) {
    int my_rank = -1;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    int process_count = -1;
    MPI_Comm_size(MPI_COMM_WORLD, &process_count);

    char hostname[MPI_MAX_PROCESSOR_NAME];
    int hostname_len = -1;
    MPI_Get_processor_name(hostname, &hostname_len);

    const int32_t previous = (process_count + my_rank - 1) % process_count;
    const int32_t next = (my_rank + 1) % process_count;
    int32_t can_print = 1;

    if (my_rank != 0) {
      MPI_Recv(&can_print, /*capacity*/ 1, MPI_INT32_T, /*source*/previous
        , /*tag*/ 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    }

    std::cout << "Hello from main thread of process " << my_rank
      << " of " << process_count << " on " << hostname << std::endl;

    if (my_rank < process_count - 1) {
      MPI_Send(&can_print, /*count*/ 1, MPI_INT32_T, /*dest*/ next, /*tag*/ 0
        , MPI_COMM_WORLD);
    }

    MPI_Finalize();
  } else {
    std::cout << "Error: could not init MPI environment" << std::endl;
  }
  return 0;
}