writing to text file in MPI - c

Question

I'm trying to write to text file with MPI but the file is not created.
I need only to write at the master (rank = 0), but nothing works.
It only working when I running the program in console (and save corrupt element) and not in Mpich2 and
I attached the code.
Thanks for helping.
/* -*- Mode: C; c-basic-offset:4 ; -*- */
/*
* (C) 2001 by Argonne National Laboratory.
* See COPYRIGHT in top-level directory.
*/
/* This is an interactive version of cpi */
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc,char *argv[])
{
int namelen, numprocs, rank;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Get_processor_name(processor_name,&namelen);
MPI_Status status;
FILE* f = fopen("test.txt","wb+");
if (rank == 0) {
for (int i=0; i < 5; i++){
fprintf(f,"%d \n",i);
}
fclose(f);
}
else {
// do nothing
}
MPI_Finalize();
return 0;
}

Answer 1

In the sample code you posted, all processes open the file and only process 0 closes it. Could you try the following modification ?
if (rank == 0) {
FILE* f = fopen("test.txt","wb+");
if(f==NULL){printf("failed to open file: permission issue ?\n");exit(1);}
for (int i=0; i < 5; i++){
fprintf(f,"%d \n",i);
}
fclose(f);
}
Since your code seems to come from the Argonne National Laboratory, I suppose that it is ran on a cluster using a particular file system.
The following code is based on yours. It makes use of MPI_File_open() and MPI_File_write() on a single process, using MPI_COMM_SELF.
/* This is an interactive version of cpi */
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc,char *argv[])
{
int namelen, numprocs, rank;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Get_processor_name(processor_name,&namelen);
MPI_Status status;
MPI_File fh;
if (rank == 0) {
MPI_File_open(MPI_COMM_SELF, "test.txt",MPI_MODE_CREATE | MPI_MODE_WRONLY,MPI_INFO_NULL,&fh);
//FILE* f = fopen("test.txt","wb+");
//if(f==NULL){
//printf("failed to open file\n");exit(1);
//}
for (int i=0; i < 5; i++){
char buf[42];
//fprintf(f,"%d \n",i);
snprintf(buf,42,"%d \n",i);
MPI_File_write(fh,buf,strlen(buf), MPI_CHAR,&status);
}
// fclose(f);
MPI_File_close(&fh);
}
else {
// do nothing
}
MPI_Finalize();
return 0;
}
Please make sure that you hae the permission to write in the considered folder. Make sure that all nodes can access this folder ! Try some folders like your folder in /tmp or /scratch... Your cluster may have some sort of documentation somewhere telling you where you can write files !