non-blocking.c

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/**
 * non-blocking.c
 *
 * Demonstrates using the MPI_Iprobe function to check if a message is
 * available. If it is, then the message is received and status printed. If not,
 * then the main process (rank 0) keeps incrementing its counter.
 *
 * NOTE: Ranks other than 0 and 1 will be ignored.
 *
 * Compile: mpicc -g -Wall non-blocking.c -o non-blocking
 * Run: mpirun -n 2 ./non-blocking
 */
#include <mpi.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    MPI_Init(NULL, NULL);

    /* Get the rank of this process */
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0) {
        int random;
        unsigned long counter = 0;
        int flag;

        while (true) {
            MPI_Iprobe(MPI_ANY_SOURCE, 0, MPI_COMM_WORLD,
                    &flag, MPI_STATUS_IGNORE);
            if (flag == 1) {
                MPI_Recv(&random, 1, MPI_INT, 1, 0,
                        MPI_COMM_WORLD, MPI_STATUS_IGNORE);
                printf("Received number: %d; counter: %lu\n", random, counter);
            }
            counter++;
        }
    } else if (rank == 1) {
        /* I will send a random number to rank 3 after sleeping a random number
         * of seconds */
        int random;
        int sleep_time;
        while (true) {
            random = rand();
            sleep_time = rand() % 3;
            sleep(sleep_time);
            MPI_Send(&random, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
        }
    }

    MPI_Finalize();
    return 0;
}