MPI Segmentation fault when using MPI_Bcast

Question

I'm trying to calculate the difference between the speedup and efficiency of broadcasting an array of 1 million integers to all processes using MPI_Bcast and the usual MPI_send and MPI_Recv. But I don't understand why I'm getting this segmentation fault, any help would be greatly appreciated. here is my code:

#define num_of_ints 1000000
int *create_random_array(int);
int main(int argc, char *argv[]){
int size=0, my_rank =0, i=0;
int tag =99;
double start, end;
int *array = NULL;
srand(time(NULL));

MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

if(my_rank ==0)
    array = create_random_array(size*num_of_ints);
MPI_Barrier(MPI_COMM_WORLD);
start = MPI_Wtime();
MPI_Bcast(&array, num_of_ints, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Barrier(MPI_COMM_WORLD);
end = MPI_Wtime();
printf("time of broadcast using Bcast is %f seconds", end - start);

MPI_Barrier(MPI_COMM_WORLD);
start = MPI_Wtime();
if (my_rank == 0){
    for (i = 1; i < size; i++)
        MPI_Send(&array, num_of_ints, MPI_INT, i,tag, MPI_COMM_WORLD);
}else{
    MPI_Recv(&array, num_of_ints, MPI_INT, 0,tag,MPI_COMM_WORLD, &status);
}
MPI_Barrier(MPI_COMM_WORLD);
end = MPI_Wtime();

printf("time of broadcast using send/recv is %f seconds", end - start);
MPI_Finalize();
return 0;
}
 int *create_random_array(int size){
int i=0;
int *random_arr = (int*)malloc(size*sizeof(int));
for(i=0;i<size;i++)
    random_arr[i] = (rand()/(int)RAND_MAX);
return random_arr;
 }

Use a debugger, and find out where it segfaults, you can't expect us to do this for you. Indent your code properly. Don't cast the return of malloc and friends. — Jens Gustedt

Hristo Iliev Hristo Iliev · Accepted Answer · 2014-04-16T11:26:02

There is a very common mistake in your code that has nothing to do with MPI itself but with the proper use of pointers in C/C++. All MPI communication calls in C expect a pointer to the data and most examples are shown like MPI_Somecall(&var, ...) but there var is usually a scalar integer or floating-point variable. While &var also works if var is a static array, it doesn't work if var is a pointer since &var then gives the location of the pointer itself and not the address that it points to.

In a nutshell:

MPI_Somecall(&var, ...)

when var is:

scalar global/static or automatic variable, e.g. int var;
a global/static or an automatic instance of a structure, e.g. struct foo var;

Otherwise:

MPI_Somecall(var, ...)

when var is:

a static array, e.g. int var[10];
a pointer to a heap variable or array, e.g. int *var = malloc(...);
a pointer in general, e.g. int *var = &some_int_var;
a function argument of a pointer type, e.g. void func(..., int *var, ...);

Replace all instances of &array in your code with array.

MPI Segmentation fault when using MPI_Bcast

2 Answers