I'm running into strange behavior when trying to use MPI_Gatherv, and I must be misunderstanding something basic about the function. To demonstrate, I've simplified my code down to a toy problem that is hopefully easy to understand without much context.
int * gatherv_counts = (int *)malloc(2*sizeof(int));
int * gatherv_displacements = (int *)malloc(2*sizeof(int));
double * receive_buffer = (double *)malloc(10*sizeof(double));
for(int i = 0; i < 2; i++)
{
gatherv_counts[i] = 5;
gatherv_displacements[i] = 0;
}
double * data = (double *)malloc(5*sizeof(double));
for(int i = 0; i < 5; i++)
{
data[i] = (double)(mpirank + 2);
}
int mpiret = MPI_Gatherv( data, 5, MPI_DOUBLE, receive_buffer, gatherv_counts, gatherv_displacements, MPI_DOUBLE, 0, mpicomm);
if (mpirank == 0) {
FILE *file = fopen("output.txt", "a");
for (int i = 0; i < 10; i++) {
fprintf (file, "%16.15e \n", receive_buffer[i]);
}
fflush(file);
fclose(file);
}
I'm running this with 2 MPI processes. The data from each process is just 5 doubles (for a total combined size of 10), with values set to the rank+2. The counts are just hardcoded to 5, and displacements are all 0. It's about as simple a usage of MPI_Gatherv as I can imagine, and I'd expect that receive_buffer to be [2,2,2,2,2,3,3,3,3,3] after completion.
Instead, the receive_buffer is [3,3,3,3,3, 0,0,4.940656458412465e-324, 1.182342568274937e-316, 1.146400746224656e+248 ]. It seems to have completely skipped the data from rank 0 (the root of MPI_Gatherv), taken the data from rank 1, and left the remaining space filled with garbage. Could someone please explain what is going wrong here?
Also, for the record, I've seen a number of similarly titled questions, but these do not appear to be the same problem: MPI_Gatherv is not collecting data correctly
