mpi_gather structure

Question

I am trying to do sort key-value pairs with qsort. Every proc reads in files with filenames as the proc ids. MPI_Gather sends all the read values to proc 0, which sorts the keys and stores the key-val pairs in a file called "Output". The gather however, does not seem to work. Any help is appreciated. Thanks!

I run the code as

  mpirun -np 3 ./a.out

and my input files are: File "0":

21 bbbb  
2119 iiii
120 hhhh

File "1":

40 dddd 
10 aaaa
100 gggg

File "2":

32 cccc
44 eeee
99 ffff

And the code is:

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

#define BUFSIZE 3
#define BUFLEN 255

struct keyval{
int key;
char val[BUFLEN];
};

typedef struct keyval keyval_s;

typedef int (*compareptr)(const void*, const void*);

int compare (keyval_s * a, keyval_s * b)
{
  return ( a->key - b->key );
}

int main (int argc, char *argv[])
{    
  int values[BUFSIZE];
  keyval_s kv[BUFSIZE], *recv;
  int n, i=0, temp;
  FILE *in, *out;

  int rank, size;
  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);
  char filename[20];
  char data[20];

  if(rank ==0){
recv = (keyval_s *) malloc (size*BUFSIZE*sizeof(keyval_s));
   }

  sprintf(filename, "%d", rank);
  in=fopen(filename,"r");
  while(fscanf(in,"%d %s",&kv[i].key, kv[i].val) != EOF){
printf("Read key %d, data %s from file\n", kv[i].key, kv[i].val);   
i++;
  }
  MPI_Gather(kv,BUFSIZE,MPI_BYTE,recv,BUFSIZE,MPI_BYTE,0,MPI_COMM_WORLD);
  if(rank==0){
 qsort ((void*)&kv, BUFSIZE, sizeof(keyval_s),(compareptr) compare);
 out=fopen("Output","w");
 for (n=0; n<BUFSIZE*size; n++)
    fprintf (out,"%d %s\n",recv[n].key, recv[n].val);
free(recv);
fclose(out);
   }
fclose(in);
return 0;   
}

hcarver · Accepted Answer · 2012-08-02 16:43:53Z

1

The size of the data in MPI_Gather is incorrect. It should be

MPI_Gather(kv, 
           sizeof(keyval_s)*BUFSIZE, 
           MPI_BYTE,
           recv, 
           sizeof(keyval_s)*BUFSIZE, 
           MPI_BYTE, 
           0, 
           MPI_COMM_WORLD);

Note that the recvcount parameter in MPI_Gather is for a message from a single rank, not the total size of the gathered data.

edited Aug 2, 2012 at 16:43

hcarver

7,3445 gold badges44 silver badges67 bronze badges

answered Dec 19, 2011 at 10:59

Greg Inozemtsev

4,67925 silver badges27 bronze badges

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