Question

I am very new to MPI programming, I am trying to run a program that should sort e.g. 8 or 16 or more numbers in parallel, I am getting segmentation error when I compiled and ran the program. Below is the code that I wan to compile. Any suggestion will be appreciated.

#include <stdio.h>

#include <stdlib.h> 

#include <mpi.h> 


/* The IncOrder function that is called by qsort is defined as follows */ 

int IncOrder(const void *e1, const void *e2) 

{ 
  return (*((int *)e1) - *((int *)e2)); 
} 

int main(int argc, char *argv[]){ 
     int n;         /* The total number of elements to be sorted */ 
     int npes;      /* The total number of processes */ 
     int myrank;    /* The rank of the calling process */ 
         int nlocal;    /* The local num of elements, and d array that stores */ 
     int *elmnts;   /* The array that stores the local elements */ 
         int *relmnts;  /* The array that stores the received elements */ 
     int oddrank;   /* The rank of d process during odd-phase comm */ 
     int evenrank;  /* The rank of the process during even-phase com */ 
     int *wspace;   /* Working space during the compare-split operation */ 
     int i; 
     MPI_Status status; 

     /* Initialize MPI and get system information */ 
     MPI_Init(&argc, &argv); 
     MPI_Comm_size(MPI_COMM_WORLD, &npes); 
     MPI_Comm_rank(MPI_COMM_WORLD, &myrank); 

     n = atoi(argv[1]); 
     nlocal = n/npes; /* Compute the number of elements to be stored locally. */ 

     /* Allocate memory for the various arrays */ 
     elmnts  = (int *)malloc(nlocal*sizeof(int)); 
     relmnts = (int *)malloc(nlocal*sizeof(int)); 
     wspace  = (int *)malloc(nlocal*sizeof(int)); 

     /* Fill-in the elmnts array with random elements */ 
     srandom(myrank); 
     for (i=0; i<nlocal; i++) 
          elmnts[i] = random(); 

      /* Sort the local elements using the built-in quicksort routine */ 
          qsort(elmnts, nlocal, sizeof(int), IncOrder); 
              /* Determine the rank of the processors that myrank needs to com during 
      * ics/ccc.gifthe */ 
      /* odd and even phases of the algorithm */ 
      if (myrank%2 == 0) { 
          oddrank  = myrank-1; 
          evenrank = myrank+1; 
      } else { 
          oddrank  = myrank+1; 
          evenrank = myrank-1; 
                                  } 

      /* Set the ranks of the processors at the end of the linear */ 
      if (oddrank == -1 || oddrank == npes) 
          oddrank = MPI_PROC_NULL; 
      if (evenrank == -1 || evenrank == npes) 
          evenrank = MPI_PROC_NULL; 

      /* Get into the main loop of the odd-even sorting algorithm */ 
      for (i=0; i<npes-1; i++) { 
          if (i%2 == 1) /* Odd phase */ 
              MPI_Sendrecv(elmnts, nlocal, MPI_INT, oddrank, 1, relmnts, 
              nlocal, MPI_INT, oddrank, 1, MPI_COMM_WORLD, &status); 
          else /* Even phase */ 
              MPI_Sendrecv(elmnts, nlocal, MPI_INT, evenrank, 1, relmnts, 
              nlocal, MPI_INT, evenrank, 1, MPI_COMM_WORLD, &status); 

              CompareSplit(nlocal, elmnts, relmnts, wspace, myrank < status.MPI_SOURCE); 
       } 
MPI_Gather(elmnts,nlocal,MPI_INT,relmnts,n,MPI_INT,0,MPI_COMM_WORLD);


/* The master host display the sorted array */
int len = sizeof(elmnts)/sizeof(int);
if(myrank == 0) {

    printf("\nSorted array :\n");
    for (i=0;i<len;i++) {
        printf("%d; ",relmnts[i]); 
    }

    printf("\n");

}


          free(elmnts); free(relmnts); free(wspace); 
      MPI_Finalize(); 
} 

/* This is the CompareSplit function */ 
CompareSplit(int nlocal, int *elmnts, int *relmnts, int *wspace,  int keepsmall){ 
    int i, j, k; 

        for (i=0; i<nlocal; i++) 
            wspace[i] = elmnts[i]; /* Copy the elmnts array into the wspace array */ 

        if (keepsmall) { /* Keep the nlocal smaller elements */ 
         for (i=j=k=0; k<nlocal; k++) { 
            if (j == nlocal || (i < nlocal && wspace[i] < relmnts[j])) 
                elmnts[k] = wspace[i++]; 
            else 
                elmnts[k] = relmnts[j++]; 
         } 
     } else { /* Keep the nlocal larger elements */ 
            for (i=k=nlocal-1, j=nlocal-1; k>=0; k--) { 
              if (j == 0 || (i >= 0 && wspace[i] >= relmnts[j])) 
                  elmnts[k] = wspace[i--]; 
          else 
              elmnts[k] = relmnts[j--]; 
        } 
     } 
} 

Answer 1

Your segmentation fault comes from this line

    n = atoi(argv[1]);

Since argv[1] contains the string (null), n gets a zero value. Change n to a custom integer like: n=5 and everything will work fine.