Initialize an array using openmpi once - c

Question

I am trying to run some tests using OPENmpi processing data in an array by spliting up the work across nodes (the second part is with matricies). I am running into some problems now because the data array is being initialized every time and I don't know how to prevent this from happening.
How, using ANSI C can I create a variable length array, using OPENmpi once? I tried making it static and global, but nothing.
#define NUM_THREADS 4
#define NUM_DATA 1000
static int *list = NULL;
int main(int argc, char *argv[]) {
int numprocs, rank, namelen;
char processor_name[MPI_MAX_PROCESSOR_NAME];
int n = NUM_DATA*NUM_DATA;
printf("hi\n");
int i;
if(list == NULL)
{
printf("ho\n");
list = malloc(n*sizeof(int));
for(i = 0 ; i < n; i++)
{
list[i] = rand() % 1000;
}
}
int position;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Get_processor_name(processor_name, &namelen);
printf("Process %d on %s out of %d\n", rank,processor_name, numprocs);
clock_t start = clock();
position = n / NUM_THREADS * rank;
search(list,position, n / NUM_THREADS * (rank + 1));
printf("Time elapsed: %f seconds\n", ((double)clock() - (double)start) /(double) CLOCKS_PER_SEC);
free(list);
MPI_Finalize();
return 0;
}

Answer 1

Probably the easiest way is to have the rank 0 process do the initialization while the other processes block. Then once the initialization is done, have them all start their work.
A basic example trying to call your search function (NB: it's dry-coded):
#define NUM_THREADS 4
#define NUM_DATA 1000
int main(int argc, char *argv[]) {
int *list;
int numprocs, rank, namelen, i, n;
int chunksize,offset;
char processor_name[MPI_MAX_PROCESSOR_NAME];
n= NUM_DATA * NUM_DATA;
MPI_Status stat;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Get_processor_name(processor_name, &namelen);
//note you'll need to handle n%NUM_THREADS !=0, but i'm ignoring that for now
chunksize = n / NUM_THREADS;
if (rank == 0) {
//Think of this as a master process
//Do your initialization in this process
list = malloc(n*sizeof(int));
for(i = 0 ; i < n; i++)
{
list[i] = rand() % 1000;
}
// Once you're ready, send each slave process a chunk to work on
offset = chunksize;
for(i = 1; i < numprocs; i++) {
MPI_Send(&list[offset], chunksize, MPI_INT, i, 0, MPI_COMM_WORLD);
offset += chunksize
}
search(list, 0, chunksize);
//If you need some sort of response back from the slaves, do a recv loop here
} else {
// If you're not the master, you're a slave process, so wait to receive data
list = malloc(chunksize*sizeof(int));
MPI_Recv(list, chunksize, MPI_INT, 0, 0, MPI_COMM_WORLD, &stat);
// Now you can do work on your portion
search(list, 0, chunksize);
//If you need to send something back to the master, do it here.
}
MPI_Finalize();
}