Fatal error in MPI_gather (matrix multiplication using MPI)

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C

parallel

Hello, I've just recently been messing around with my parallel programming in MPI (OpenMP after this!) and I ran a code to multiply matrices on a virtual machine (Ubuntu) with 4 cores. Unfortunately, I got an error:

Fatal error in PMPI_Gather: Invalid buffer pointer, error stack:
PMPI_Gather(856): MPI_Gather(sbuf=0x601d20, scount=100, MPI_FLOAT, rbuf=0x601d20, rcount=100, MPI_FLOAT, root=0, MPI_COMM_WORLD) failed
PMPI_Gather(797): Buffers must not be aliased

This is the code obtained from SpeedGoComputing:

#include <mpi.h>

const int size = 20;

float a[20][20];
float b[20][20];
float c[20][20];

void multiply(int istart, int iend)
{
	int i,j,k;
    for (i = istart; i <= iend; ++i) {
        for (j = 0; j < size; ++j) {
            for (k = 0; k < size; ++k) {
                c[i][j] += a[i][k] * b[k][j];
            }
        }
    }
}

int main(int argc, char* argv[])
{
    int rank, nproc;
    int istart, iend;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &nproc);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0) {
        // Initialize buffers.
		int i;
		int j;
        for (i = 0; i < size; ++i) {
            for (j = 0; j < size; ++j) {
                a[i][j] = (float)i + j;
                b[i][j] = (float)i - j;
                c[i][j] = 0.0f;
            }
        }
    }

    // Broadcast matrices to all workers.
    MPI_Bcast(a, size*size, MPI_FLOAT, 0,MPI_COMM_WORLD);
    MPI_Bcast(b, size*size, MPI_FLOAT, 0,MPI_COMM_WORLD);
    MPI_Bcast(c, size*size, MPI_FLOAT, 0,MPI_COMM_WORLD);

    // Partition work by i-for-loop.
    istart = (size / nproc) * rank;
    iend = (size / nproc) * (rank + 1) - 1;

    // Compute matrix multiplication in [istart,iend]
    // of i-for-loop.
    // C <- C + A x B
    multiply(istart, iend);

    // Gather computed results.
    MPI_Gather(c + (size/nproc*rank),
               size*size/nproc,
               MPI_FLOAT,
               c + (size/nproc*rank),
               size*size/nproc,
               MPI_FLOAT,
               0,
               MPI_COMM_WORLD);
			   



    if (rank == 0) {
        // Compute remaining multiplications
        // when size % nproc > 0.
        if (size % nproc > 0) {
            multiply((size/nproc)*nproc, size-1);
        }
    }

    MPI_Finalize();
    return 0;
}

What I have tried:

I suspect there's something wrong with the sendbuf argument of MPI_Gather, so I changed it to MPI_IN_PLACE but even that didn't work. I tried MPI_gatherV too and that didn't work.

I've tried debugging this in Visual Studio and it says there was a breakpoint reached. At the "if (rank == 0)": line. Perhaps it has something to do with pointers and addresses, I'm not very sure. I will be reading up on it.
If anyone could help, please and thank you! I'm not very adept at C.