How to handle MPI sendcount of zero - c

Question

What is the correct way to handle a sendcount = 0 when using MPI_Gatherv (or any other function that requires a sendcount) when setting up the displs argument?
I have data that needs to be received by all processors, but all processors might not have any data to send themselves. As an MWE, I tried (on just two processors):
#include <stdlib.h>
#include <stdio.h>
#include <mpi.h>
int main(void)
{
int ntasks;
int thistask;
int n = 0;
int i;
int totcounts = 0;
int *data;
int *rbuf;
int *rcnts;
int *displs;
int *master_data;
int *master_displs;
// Set up mpi
MPI_Init(NULL, NULL);
MPI_Comm_size(MPI_COMM_WORLD, &ntasks);
MPI_Comm_rank(MPI_COMM_WORLD, &thistask);
// Allocate memory for arrays needed by allgatherv
rbuf = calloc(ntasks, sizeof(int));
rcnts = calloc(ntasks, sizeof(int));
displs = calloc(ntasks, sizeof(int));
master_displs = calloc(ntasks, sizeof(int));
// Initialize the counts and displacement arrays
for(i = 0; i < ntasks; i++)
{
rcnts[i] = 1;
displs[i] = i;
}
// Allocate data on just one task, but not others
if(thistask == 1)
{
n = 3;
data = calloc(n, sizeof(int));
for(i = 0; i < n; i++)
{
data[i] = i;
}
}
// Get n so each other processor knows about what others are sending
MPI_Allgatherv(&n, 1, MPI_INT, rbuf, rcnts, displs, MPI_INT, MPI_COMM_WORLD);
// Now that we know how much data each processor is sending, we allocate the array
// to hold it all
for(i = 0; i < ntasks; i++)
{
totcounts += rbuf[i];
}
master_data = calloc(totcounts, sizeof(int));
// Get displs for master data
master_displs[0] = 0;
for(i = 1; i < ntasks; i++)
{
master_displs[i] = master_displs[i - 1] + rbuf[i - 1];
}
// Send each processor's data to all others
MPI_Allgatherv(&data, n, MPI_INT, master_data, rbuf, master_displs, MPI_INT, MPI_COMM_WORLD);
// Print it out to see if it worked
if(thistask == 0)
{
for(i = 0; i < totcounts; i++)
{
printf("master_data[%d] = %d\n", i, master_data[i]);
}
}
// Free
if(thistask == 1)
{
free(data);
}
free(rbuf);
free(rcnts);
free(displs);
free(master_displs);
free(master_data);
MPI_Finalize();
return 0;
}
The way that I've set up master_displs works when every processor has a non-zero n (that is, they have data to send). In this case, both entries will be zero. However, the results of this program are garbage. How would I set up the master_displs array to ensure that master_data holds the correct information (in this case, just master_data[i] = i, as received from task 1)?