MPI_scatter: Invalid buffer pointer - c++

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;
}