Read input integer from external file for MPI - mpi

Question

How could I read external input file for mpi? I need to read one integer from external file (zadanie4_vstup.txt), to compute simple factorial. I have tried to substitute the second argument in MPI_Init() with address of int variable (n), but it looks it is nonsense.
Thank you.
#include <stdio.h>
#include <mpi.h>
int main(int argc, char ** argv)
{
FILE *fr, *fw;
fr = fopen("zadanie4_vstup.txt", "r");
fw = fopen("zadanie4_vystup.txt", "w");
int nproc, me;
int fakt=1, i, buff, n;
MPI_Status stat;
fscanf(fr, "%d", &n);
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &nproc);
MPI_Comm_rank(MPI_COMM_WORLD, &me);
#pragma omp parallel for private(i) reduction(*:fakt)
for(i=me*n/nproc+1; i<=(me+1)*n/nproc; i++) {
fakt *= i;
}
if(nproc > 1) {
if(me == 0) {
for(i=1; i<nproc; i++) {
MPI_Recv(&buff, 1, MPI_INT, i, 0, MPI_COMM_WORLD, &stat);
fakt*=buff;
}
} else {
MPI_Send(&fakt, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
}
if(me == 0) {
fprintf(fw, "%d! = %d\n", n, fakt);
}
fclose(fr);
fclose(fw);
MPI_Finalize();
}

Answer 1

here is a version of your program that reads n on the command line.
note i simplified the communications by using MPI_Reduce()
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
int main(int argc, char *argv[]) {
int nproc, me;
int fakt=1, res, i, buff, n;
MPI_Status stat;
MPI_Init(&argc, &argv);
n = atoi(argv[1]);
MPI_Comm_size(MPI_COMM_WORLD, &nproc);
MPI_Comm_rank(MPI_COMM_WORLD, &me);
#pragma omp parallel for private(i) reduction(*:fakt)
for(i=me*n/nproc+1; i<=(me+1)*n/nproc; i++) {
fakt *= i;
}
MPI_Reduce(&fakt, &res, 1, MPI_INT, MPI_PROD, 0, MPI_COMM_WORLD);
if(me == 0) {
printf("%d! = %d\n", n, res);
}
MPI_Finalize();
return 0;
}
for example
$ mpirun -np 4 ./fakt 6
6! = 720