MPI: Broadcasting a long long int - c

Question

This program estimates Pi by throwing random "darts" (sampling points) to a circle or radius=1 inscribed inside a square board of length=2. Using the relationship
Area of circle / Area of Square = Pi/4
we can estimate Pi using the same relationship expressed as
Darts Inside Circle / Darts Outside Circle = Pi/4
The program works fine when I specify NDARTS in a #define, but when trying to broadcast it as a long long int, read from scanf, I get the following execution error:
mpirun -np 4 ./pi_montecarlo.x
-----------------------------------------------------------------------------
One of the processes started by mpirun has exited with a nonzero exit
code. This typically indicates that the process finished in error.
If your process did not finish in error, be sure to include a "return
0" or "exit(0)" in your C code before exiting the application.
PID 10591 failed on node n0 (127.0.0.1) due to signal 11.
Why?
Is there anything wrong with my MPI_Bcast declaration?
long long int *NDARTS=0;
scanf("%Ld",NDARTS);
MPI_Bcast(NDARTS, 1, MPI_LONG_LONG_INT, 0, MPI_COMM_WORLD);
Full code:
/*
mpicc -g -Wall -lm pi_montecarlo3.c -o pi_montecarlo.x
mpirun -np 4 ./pi_montecarlo.x
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include <mpi.h>
#define MASTER 0
#define PI 3.1415926535
d ouble pseudo_random (double a, double b) {
double r;
r = ((b-a) * ((double) rand() / (double) RAND_MAX)) +a;
return r;
}
int main(int argc, char*argv[]){
long long int *NDARTS=0;
int proc_id,
n_procs,
llimit,
ulimit,
n_circle,
i;
double pi_current,
pi_sum,
x,
y,
z,
error,
start_time,
end_time;
struct timeval stime;
llimit = -1;
ulimit = 1;
n_circle =0;
MPI_Init(&argc, &argv);
MPI_Comm_rank (MPI_COMM_WORLD, &proc_id);
MPI_Comm_size (MPI_COMM_WORLD, &n_procs);
if (proc_id == MASTER){
printf("\nMonte Carlo Method to estimate Pi \n\n");
printf("Introduce Number of Darts \n");
scanf("%Ld",NDARTS);
printf(" Number of processes: %d \n", n_procs);
printf(" Number of darts: %Ld \n", *NDARTS);
MPI_Bcast(NDARTS, 1, MPI_LONG_LONG_INT, 0, MPI_COMM_WORLD);
start_time = MPI_Wtime();
}
gettimeofday(&stime, NULL);
srand(stime.tv_usec * stime.tv_usec * stime.tv_usec * stime.tv_usec);
for (i=1; i<=*NDARTS;i++){
x = pseudo_random(llimit, ulimit);
y = pseudo_random(llimit, ulimit);
z = pow(x,2) + pow(y,2);
if (z<=1.0){
n_circle++;
}
}
pi_current = 4.0 * (double)n_circle / (double) *NDARTS;
MPI_Reduce (&pi_current, &pi_sum, 1, MPI_DOUBLE, MPI_SUM, MASTER, MPI_COMM_WORLD);
if (proc_id == MASTER) {
pi_sum = pi_sum / n_procs;
error = fabs ((pi_sum -PI) / PI) *100;
end_time = MPI_Wtime();
printf("Known value of PI : %11.10f \n", PI);
printf("Estimated Value of PI : %11.10f\n", pi_sum);
printf("Error Percentage : %10.8f\n", error);
printf("Time : %10.8f\n\n", end_time - start_time);
}
MPI_Finalize();
return 0;
}

Answer 1

You're not using scanf() correctly. It should be like this instead:
long long int NDARTS;
scanf("%lld",&NDARTS);
MPI_Bcast(&NDARTS, 1, MPI_LONG_LONG_INT, 0, MPI_COMM_WORLD);
In your current code, long long int *NDARTS=0; effectively initializes NDARTS as a NULL pointer. So scanf() will obviously seg-fault when it tries to write to it.