MPI_Probe sample

test.cpp

#include <cstdio>
#include <mpi.h>
#include <string>

void send() {
  std::string str = "Abracadabra";
  MPI_Send(str.data(), str.size(), MPI_CHAR, 1, 0, MPI_COMM_WORLD);
  printf("Sent: %s\n", str.c_str());
}

void receive() {
  MPI_Status status;
  MPI_Probe(0, 0, MPI_COMM_WORLD, &status);
  int length = 0;
  MPI_Get_count(&status, MPI_CHAR, &length);
  std::unique_ptr<char[]> buf(new char[length]);
  MPI_Recv(buf.get(), length, MPI_CHAR, 0, 0, MPI_COMM_WORLD, &status);
  printf("Recieved: %s\n", buf.get());
}

int main(int argc, char **argv) {
  MPI_Init(&argc, &argv);
  int rank = 0;
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  if (rank == 0) {
    send();
  } else {
    receive();
  }
  MPI_Finalize();
}

$ mpic++ test.cpp
$ mpirun -np 2 ./a.out
Sent: Abracadabra
Recieved: Abracadabra