CODEWITHSUNDEEP
cmpi
ZPA
ZPA

Reputation: 97

User input variable value in C

I am a beginner in C (have few experiences in C++) and I have a problem with user input variable value. I wrote MPI_Scatter and MPI_Gather program in C which would calculate the total sum of number of inputted integers on every node.

The problem is: if I define variable input (see code below) as input=5; it will calculate the whole sum for all 4 nodes (210). If I set the input for scanf, a result will be only 15. It seems that the variable change its value. Can you help me please? Code:

#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
#define MASTER 0
int main(int argc, char** argv){
   int id, nproc;
   MPI_Init(&argc, &argv);
   MPI_Comm_size(MPI_COMM_WORLD, &nproc);
   MPI_Comm_rank(MPI_COMM_WOLRD, &id);
   MPI_Status status;
   int input=0; // <- problematic variable
   int nodeValue=0;
   int size;
   int *array;
   if(id == MASTER){
     printf("How many elements per node? ");
     scanf("%d", &input);
     nodeValue = input;
   }
   MPI_Barrier(MPI_COMM_WORLD);
   if(id == MASTER){
     size = input * nproc;
     array = malloc(sizeof(int)*size);
     ...
   }
}

Upvotes: 2

Views: 3285

Answers (1)

Habi Sheriff
Habi Sheriff

Reputation: 179

Your query is similar to below stack overflow issue: Visit In MPI for multiple process scanf taking input only once and assign garbage values to other?

Option given as: please read input from file.

Here is the sample code to read from file:

  #include "mpi.h"
  #include <stdio.h>
  #include <stdlib.h>

  int main(int argc, char *argv[])
  {
    int myid, nprocs;
    int *array;
    int size, i;
    FILE *fp;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &myid);
    MPI_Comm_size(MPI_COMM_WORLD, &nprocs);

    if(myid ==0)
    {
     fp = fopen("test", "r");
     fscanf(fp, "%d", &size);
    }

   /* MPI_Bcast(void *buffer,
           int count,
           MPI_Datatype datatype,
           int root,
           MPI_Comm comm) */
   MPI_Bcast(&size,1, MPI_INT, 0, MPI_COMM_WORLD);
   array = (int *) malloc(size* sizeof(int));

   if(myid ==0)
   {
    for(i = 0; i < size; i++)
    {
     fscanf(fp, "%d", &array[i]);
    }
   }
    MPI_Bcast(array, size, MPI_INT, 0, MPI_COMM_WORLD);

    MPI_Finalize();
  }

Here is the link has some examples: Visit https://docs.loni.org/wiki/c_mpi_examples

If it is really needs an user input: option we have are

1) Read from command line arguments or from file (write a code to input to that file - though it is a lengthy approach)

2) STDIN won't work as expected after MPI_Init. Try to put your scanf statement before MPI_Init.

Modified code: Please try this:

    int id, nproc;
    MPI_Status status;
    int input=0; // <- problematic variable
    int nodeValue=0;
    int size;
    int *array;
    if(id == MASTER){
     printf("How many elements per node? ");
     scanf("%d", &input);
     nodeValue = input;
    }
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &nproc);
    MPI_Comm_rank(MPI_COMM_WOLRD, &id);

Upvotes: 5

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