OPENMPI broadcast mpi_bcast issue

Question

I am trying to understand the mpi_bcast. more specifically, in the below code I am expecting the end result would be buf0->1990 and buf1-->1991.

#include <mpi.h>
#include <stdio.h>

int main(int argc, char** argv) {
        int rank;
        int buf0;
        int buf1;
        const int root=0;

        MPI_Init(&argc, &argv);
        MPI_Comm_rank(MPI_COMM_WORLD, &rank);
        if(rank == 0){
           buf0 = 1990;
        } else {
           buf1 = 1991;
        }

        printf("[%d]: Before Bcast, buf0 is %d\n", rank, buf0);
        printf("[%d]: Before Bcast, buf1 is %d\n", rank, buf1);

        /* everyone calls bcast, data is taken from root and ends up in everyone's buf */
        MPI_Bcast(&buf0, 1, MPI_INT, 0, MPI_COMM_WORLD);
        MPI_Bcast(&buf1, 1, MPI_INT, 1, MPI_COMM_WORLD);

        printf("[%d]: After Bcast, buf0 is %d\n", rank, buf0);
        printf("[%d]: After Bcast, buf1 is %d\n", rank, buf1);

        MPI_Finalize();
        return 0;
}

However the output that I get is as follows, I was expecting that both the threads would have the value of buf0 and buf1 syncronized however buf1 for thread 0 is 0. Am I missing something here.

mpirun -np 2 mpi_counter_1
[0]: Before Bcast, buf0 is 1990
[0]: Before Bcast, buf1 is 0
[1]: Before Bcast, buf0 is 0
[1]: Before Bcast, buf1 is 1991
[0]: After Bcast, buf0 is 1990
[0]: After Bcast, buf1 is 0
[1]: After Bcast, buf0 is 1990
[1]: After Bcast, buf1 is 1991

Your code works just fine for me (mpich 3.0.4 and gcc 4.9.3) as it should (AFAICS). The behaviour you experience looks very fishy to me. This can be a mistake in compiling and/or running the code (discrepancy between the MPI library used at compilation time, and the launcher). Or this can be a bug in the MPI library (that seems unlikely since that would be the hell of a bug). — Gilles, Mar 02 '16 at 08:04
I agree with @Gilles, one thing that looks a little bit fishy is the missing `./` before the call to `mpi_counter_1` (see [this answer](http://stackoverflow.com/a/6331085/620382)). Maybe it is using a different binary than you expect? — Zulan, Mar 02 '16 at 08:32
Agree with the previous two comments. The code works fine on OpenMPI 1.10 and gnu 4.8.x compilers. — Harald, Mar 02 '16 at 10:21
Was interesting, I inited the buf0 and buf1 to 0 and now it works fine. Not sure what could cause it. — user2766839, Mar 03 '16 at 00:29

OPENMPI broadcast mpi_bcast issue

0 Answers0