MPI_scatter: Invalid buffer pointer

Question

Can anybody tell what am I doing wrong due to which I am getting this error. Code:

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

void transpose(int ** p, int row, int col)
{
    int ** tempVar;
    tempVar = (int *)malloc(sizeof(int *)* row);
    int i = 0;
    for (; i < row; i++)
    {
        tempVar[i] = (int *)malloc(sizeof (int *)* col);
        int j = 0;
        while (j < col)
        {
            tempVar[i][j] = p[j][i];
            j++;
        }
    }
    p = tempVar;
}
void main(int argc, char * argv[])
{
    int rank, size;
    MPI_Init(argc, argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    int d[] = { 1000, 1000, 1000, 1000, 1000, 1000 };
    int vt[6] = { 1000, 1000, 1000, 1000, 1000, 1000 };
    int ** p;
    p = (int *)malloc(sizeof(int *)* 6);
    int i = 0;
    int row = 6;
    int col = 6;
    while (i < 6)
    {
        p[i] = (int *)malloc(sizeof(int *)* 6);
        /*int j = 0;
        if (rank == 0)
        {
            while (j < 6)
            {
                scanf("%d", p[i][j]);
                j++;
            }
        }*/
        i++;    
    }
    p[0][0] = 0; p[0][1] =2 ; p[0][2] =3 ; p[0][3] =1 ; p[0][4] =1000 ; p[0][5] =1000 ;
    p[1][0] = 2; p[1][1] = 0; p[1][2] = 1000; p[1][3] = 1000; p[1][4] = 5; p[1][5] = 1000;
    p[2][0] = 3; p[2][1] = 1000; p[2][2] = 0; p[2][3] = 1000; p[2][4] = 1000; p[2][5] = 1;

    p[3][0] = 1; p[3][1] = 1000; p[3][2] = 1000; p[3][3] = 0; p[3][4] = 4; p[3][5] = 3;

    p[4][0] = 1000; p[4][1] = 5; p[4][2] = 1000; p[4][3] = 4; p[4][4] = 0; p[4][5] = 2;

    p[5][0] = 1000; p[5][1] = 1000; p[5][2] = 1; p[5][3] = 3; p[5][4] = 2; p[5][5] = 0;
    int smallest;   
    if (rank == 0)
    {
        //transpose(&p , row , col);
        smallest = 0;
        vt[smallest] = smallest;
        //MPI_Bcast();
    }
    int  vt1, d1;
    vt1 = d1 = 0;

    int roww[6];
    MPI_Scatter(vt, 6, MPI_INT, vt1, 1, MPI_INT, 0, MPI_COMM_WORLD);
    MPI_Scatter(d, 6, MPI_INT, d1, 1, MPI_INT, 0, MPI_COMM_WORLD);
    MPI_Scatter(p, row *row, MPI_INT,roww, 6, MPI_INT, 0, MPI_COMM_WORLD);
    i = 0;
    while (i < (row*row)/size)
    {
        MPI_Bcast(smallest, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (vt1 != rank)
        {
            if (roww[smallest] != 1000)
            {   
                if (d1 > roww[smallest])
                    d1 = roww[smallest];
            }
        }

        MPI_Gather(d1, 1, MPI_INT, d, row, MPI_INT, 0, MPI_COMM_WORLD);
        if (rank == 0)
        {
            smallest = d[0];
            int k = 1;
            int index = 0;
            while (k < 6)
            {
                if (d[k] < smallest)
                {
                    smallest = d[k];
                    index = k;
                }
                k++;
            }
            vt[k] = index;

        }
        MPI_Scatter(vt, 6, MPI_INT, vt1, (row) / size, MPI_INT, 0, MPI_COMM_WORLD);
        MPI_Scatter(d, 6, MPI_INT, d1, (row) / size, MPI_INT, 0, MPI_COMM_WORLD);
i++;

    }
    MPI_Finalize();
}

The error that I am getting is

Fatal Error: fatal error in MPI_Scatter: Invalid buffer pointer, error stack:
MPI_Scatter(760): MPI_Scatter(sbuf=0x0085f7ac , scount , MPI_INT , rbuf =0x0000000 , rcount =1, MPI_INT , root= 0 , MPI_COMM_WORLD) failed

Answer 1

The code you provided compiles with lots of warnings that are not to be ignored such as :

 passing argument 2 of ‘MPI_Init’ from incompatible pointer type

Look carefully at the prototype of functions : int* fun(int* b); is likely to fail if you call something like int d;fun(d);. If the function needs a pointer to the data, fun(&d) may work better. This problem occurs many times, as MPI functions are called.

More : the function transpose(int ** p) tries to modify p by doing p= tempVar. As signaled by @WhozCraig, by doing int **p;...;transpose(p,...), a copy of p in the scope of the function transpose() is modified, but not p. Hence, the right prototype of this function is transpose(int ***p,...) and the right way to call it is int** p;...;transpose(&p,...);

Regarding memory allocation : you found a way to allocate 2D array ! But the data is not contiguous in memory since rows are allocated one at a time. If you plan to use MPI functions such as MPI_Scatter(), allocating a contiguous 2D array is the right way to go (more).

Additional advice : call free() at the right time to free the memory and avoid memory leaks. Do not cast the return of malloc()

Here is a piece of code that should compile well with mpicc main.c -o main -Wall. The option -Wall enables all warnings. It seems to run fine, though i did not check if the result is correct.

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

void transpose(int *** p, int row, int col)
{
    int ** tempVar;
    tempVar = malloc(sizeof(int *)* row);
    if (tempVar==NULL){printf("malloc failed\n"); exit (1);}
    tempVar[0] = malloc(sizeof (int )* col*row);
    if (tempVar[0]==NULL){printf("malloc failed\n"); exit (1);}
    int i = 0;
    for (i=0; i < row; i++)
    {
        tempVar[i] = &tempVar[0][col*i];
        int j = 0;
        while (j < col)
        {
            tempVar[i][j] = (*p)[j][i];
            j++;
        }
    }
    free((*p)[0]);
    free(*p);
    *p = tempVar;
}
int main(int argc, char * argv[])
{
    int rank, size;
    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    int d[] = { 1000, 1000, 1000, 1000, 1000, 1000 };
    int vt[6] = { 1000, 1000, 1000, 1000, 1000, 1000 };
    int ** p;

    int i = 0;
    int row = 6;
    int col = 6;
    p = malloc(sizeof(int *)* row);
    if (p==NULL){printf("malloc failed\n"); exit (1);}
    p[0] = malloc(sizeof(int )* row*col);
    if (p[0]==NULL) {printf("malloc failed\n"); exit (1);}
    while (i < row)
    {
        p[i] = &p[0][i*col];
        /*int j = 0;
        if (rank == 0)
        {
            while (j < 6)
            {
                scanf("%d", p[i][j]);
                j++;
            }
        }*/
        i++;    
    }
    p[0][0] = 0; p[0][1] =2 ; p[0][2] =3 ; p[0][3] =1 ; p[0][4] =1000 ; p[0][5] =1000 ;
    p[1][0] = 2; p[1][1] = 0; p[1][2] = 1000; p[1][3] = 1000; p[1][4] = 5; p[1][5] = 1000;
    p[2][0] = 3; p[2][1] = 1000; p[2][2] = 0; p[2][3] = 1000; p[2][4] = 1000; p[2][5] = 1;

    p[3][0] = 1; p[3][1] = 1000; p[3][2] = 1000; p[3][3] = 0; p[3][4] = 4; p[3][5] = 3;

    p[4][0] = 1000; p[4][1] = 5; p[4][2] = 1000; p[4][3] = 4; p[4][4] = 0; p[4][5] = 2;

    p[5][0] = 1000; p[5][1] = 1000; p[5][2] = 1; p[5][3] = 3; p[5][4] = 2; p[5][5] = 0;
    int smallest;   
    if (rank == 0)
    {
        //transpose(&p , row , col);
        smallest = 0;
        vt[smallest] = smallest;
        //MPI_Bcast();
    }
    int  vt1, d1;
    vt1 = d1 = 0;

    int roww[col];
    MPI_Scatter(vt, 1, MPI_INT, &vt1, 1, MPI_INT, 0, MPI_COMM_WORLD);
    MPI_Scatter(d, 1, MPI_INT, &d1, 1, MPI_INT, 0, MPI_COMM_WORLD);
    MPI_Scatter(&p[0][0], col, MPI_INT,roww, col, MPI_INT, 0, MPI_COMM_WORLD);
    i = 0;
    while (i < (row*row)/size)
    {
        MPI_Bcast(&smallest, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (vt1 != rank)
        {
            if (roww[smallest] != 1000)
            {   
                if (d1 > roww[smallest])
                    d1 = roww[smallest];
            }
        }

        MPI_Gather(&d1, 1, MPI_INT, d, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (rank == 0)
        {
            smallest = d[0];
            int k = 1;
            int index = 0;
            while (k < 6)
            {
                if (d[k] < smallest)
                {
                    smallest = d[k];
                    index = k;
                }
                k++;
            }
            vt[k] = index;

        }
        MPI_Scatter(vt, 1, MPI_INT, &vt1, 1, MPI_INT, 0, MPI_COMM_WORLD);
        MPI_Scatter(d, 1, MPI_INT, &d1, 1, MPI_INT, 0, MPI_COMM_WORLD);
        i++;

    }

    free(p[0]);
    free(p);
    MPI_Finalize();
    return 0;
}