MPI_Scatter分段错误（信号11）

Question

I'm trying to Parallelize some piece of code that calculates matrices, but I'm getting segmentation fault error. I already debug the code, but I cant find out the problem.

Here is the snippet where I declare the matrice:

double **A, **B, **C, *tmp1,*tmp2,*tmp3;
int N;
int myrank, P, from, to;

N = 100;

tmp1 = (double *) malloc (sizeof(double ) * N * N);
tmp2 = (double *) malloc (sizeof(double ) * N * N);
tmp3 = (double *) malloc (sizeof(double ) * N * N);
A = (double **) malloc (sizeof(double *) * N);
B = (double **) malloc (sizeof(double *) * N);
C = (double **) malloc (sizeof(double *) * N);

from = myrank * N/P;
to = (myrank+1) * N/P;

Then here is where I inicialize the MPI

MPI_Init (&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
MPI_Comm_size(MPI_COMM_WORLD, &P);

But somehow, I can't scatter...

MPI_Bcast(B, N*N, MPI_INT, 0, MPI_COMM_WORLD);

printf("Rank: %d\n", myrank);

MPI_Scatter(A, N*N/P, MPI_INT, &A[from], N*N/P, MPI_INT, 0, MPI_COMM_WORLD);

I can't wrote all the code here because I'm not allowed...But the problem is in Scatter it self. What I'am doing wrong?

Answer 1

You cannot directly send a 2-D matrix ( double** ). Instead, you have to pass the address of the actual data: &A[0][0] . For receiving, you also have to pass an address to where the data is actually stored. In your case this would be: &A[from][0] instead of &A[from] .

Try this as the scatter:

MPI_Scatter(&A[0][0], N*N/P, MPI_INT, &A[from][0], N*N/P, MPI_INT, 0, MPI_COMM_WORLD);

On another note, on the receiving process all values in A other than those received will remain invalid. You might want to consider creating a separate receive buffer of the size required:

double* local_A = (double*)malloc(sizeof(double)*N*N/P);
MPI_Scatter(&A[0][0], N*N/P, MPI_INT, local_A, N*N/P, MPI_INT, 0, MPI_COMM_WORLD);