An error occured in MPI_Recv while sending an array - c

Question

#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main(int argc, char **argv)
{
int N;
scanf("%d", &N);
double *a = (double *)malloc(N * sizeof(double));
int i, rank, size, tag = 99, tag1 = 100;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (rank == 0)
{
for(int j=0;j<N;++j)
{
a[j] = j+0.1;
}
for (i = 1; i < size; i++)
{
MPI_Send(&N, 1, MPI_INT, i, tag1, MPI_COMM_WORLD);
MPI_Send(a, N, MPI_DOUBLE, i, tag, MPI_COMM_WORLD);
}
}
else
{
MPI_Recv(&N, 1, MPI_INT, 0, tag1, MPI_COMM_WORLD, &status);
MPI_Recv(a, N, MPI_DOUBLE, 0, tag, MPI_COMM_WORLD, &status);
// for(int j=0;j<N*2;++j)
// printf("%d %f\n", rank, a[j]);
}
MPI_Barrier(MPI_COMM_WORLD);
printf("Message from process %d : %f\n", rank, a[rank]);
MPI_Finalize();
return 0;
}
I'm creating the array 'a' in 0th process and sending it to remaining proccesses. But I'm getting the following error upon doing this.
[nikhil:8599] *** An error occurred in MPI_Recv
[nikhil:8599] *** reported by process [4228579329,1]
[nikhil:8599] *** on communicator MPI_COMM_WORLD
[nikhil:8599] *** MPI_ERR_BUFFER: invalid buffer pointer
[nikhil:8599] *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort,
[nikhil:8599] *** and potentially your MPI job)
[nikhil:08593] 2 more processes have sent help message help-mpi-errors.txt / mpi_errors_are_fatal
[nikhil:08593] Set MCA parameter "orte_base_help_aggregate" to 0 to see all help / error messages
Can anybody explain why I'm getting this error?
As you can see in the code there's a for loop containing a print statement which is commented. The weird thing is... upon uncommenting that loop. It's working fine.

Answer 1

Thoughts:
MPI_Init should be the first thing in your program.
Only one rank should scanf.
N is not communicated across ranks, so you are allocating memory of undefined size.
Define variables as close to their point of usage as possible. Putting int i at the top of your function is a disaster waiting to happen.
The barrier at the end is unnecessary.
All the ranks need to allocate their own memory.
That gets us to this code:
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
int main(int argc, char **argv){
MPI_Init(&argc, &argv);
const int tag = 99;
const int tag1 = 100;
int rank, size;
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
double *a; //Pointer to the memory we will allocate
int N;
if (rank == 0){
scanf("%d", &N);
a = (double *)malloc(N * sizeof(double));
for(int j=0;j<N;++j){
a[j] = j+0.1;
}
for (int i = 1; i < size; i++){
MPI_Send(&N, 1, MPI_INT, i, tag1, MPI_COMM_WORLD);
MPI_Send(a, N, MPI_DOUBLE, i, tag, MPI_COMM_WORLD);
}
} else {
MPI_Status status;
MPI_Recv(&N, 1, MPI_INT, 0, tag1, MPI_COMM_WORLD, &status);
//Have to allocate memory on all ranks
a = (double *)malloc(N * sizeof(double));
MPI_Recv(a, N, MPI_DOUBLE, 0, tag, MPI_COMM_WORLD, &status);
// for(int j=0;j<N*2;++j)
// printf("%d %f\n", rank, a[j]);
}
printf("Message from process %d : %f\n", rank, a[rank]);
MPI_Finalize();
return 0;
}
Doing it better
The broadcast command is your friend here:
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
#define MPI_Error_Check(x) {const int err=x; if(x!=MPI_SUCCESS) { fprintf(stderr, "MPI ERROR %d at %d.", err, __LINE__);}}
int main(int argc, char **argv){
MPI_Init(&argc, &argv);
int rank, size;
MPI_Error_Check(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
MPI_Error_Check(MPI_Comm_size(MPI_COMM_WORLD, &size));
int N;
if (rank==0){
scanf("%d", &N);
}
MPI_Error_Check(MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD));
double *a = (double *)malloc(N * sizeof(double));
if(rank==0){
for(int j=0;j<N;++j){
a[j] = j+0.1;
}
}
printf("Message from process %d : N=%d\n", rank, N);
MPI_Error_Check(MPI_Bcast(a, N, MPI_DOUBLE, 0, MPI_COMM_WORLD));
fprintf(stderr, "Message from process %d : %f\n", rank, a[rank]);
free(a);
MPI_Finalize();
return 0;
}
Doing It Even Better
The fastest form of communication is no communication at all. In your case, once the value N is known each rank can recreate the data on its own:
#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"
#define MPI_Error_Check(x) {const int err=x; if(x!=MPI_SUCCESS) { fprintf(stderr, "MPI ERROR %d at %d.", err, __LINE__);}}
int main(int argc, char **argv){
MPI_Init(&argc, &argv);
int rank, size;
MPI_Error_Check(MPI_Comm_rank(MPI_COMM_WORLD, &rank));
MPI_Error_Check(MPI_Comm_size(MPI_COMM_WORLD, &size));
int N;
if (rank==0){
scanf("%d", &N);
}
MPI_Error_Check(MPI_Bcast(&N, 1, MPI_INT, 0, MPI_COMM_WORLD));
double *a = (double *)malloc(N * sizeof(double));
for(int j=0;j<N;++j){
a[j] = j+0.1;
}
printf("Message from process %d : N=%d\n", rank, N);
fprintf(stderr, "Message from process %d : %f\n", rank, a[rank]);
free(a);
MPI_Finalize();
return 0;
}