Cannot use file i\o when running mpi - c++

Question

I have a problem in MPI, when I run my MPI program from wmpiexec I cannot make any file io. But when I run it in visual studio it works fine.
I tried the usual see fopen() as well as the mpi APIs nothing works.
(the file already exists)
/* 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;
char x='x';
if (rank == 0) {
MPI_File_open(MPI_COMM_SELF, "test.txt", MPI_MODE_RDONLY, 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_read(fh,&x,sizeof(char), MPI_CHAR, &status);
printf("%c",x);
}
getchar();
// fclose(f);
MPI_File_close(&fh);
}
else {
// do nothing
}
MPI_Finalize();
return 0;
}

Answer 1

The error code returned by the function MPI_File_open() can be tested to know why the opening of the file failed. Indeed, as underlined in the documentation of this function:
For MPI I/O function errors, the default error handler is set to MPI_ERRORS_RETURN. The error handler may be changed with MPI_File_set_errhandler; the predefined error handler MPI_ERRORS_ARE_FATAL may be used to make I/O errors fatal. Note that MPI does not guarantee that an MPI program can continue past an error.
As a result the default behavior of MPI_File_open() is not to report errors and try to keep going, even if the file does not exist. Therefore, retreiving the value of the error code and tunning the error handler seems a promissing way to investigate the issue. Here is a sample code code based on yours doing so. It can be compiled by mpicc main.c -o main -std=c99 -Wall and run by mpirun -np 2 main
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// A custom error handler
void file_errhandler_fn(MPI_File * file, int * errorcode, ... );
void file_errhandler_fn(MPI_File * file, int * errorcode, ... ){
if(*errorcode!=MPI_SUCCESS){
//your error handler here...
char error[MPI_MAX_ERROR_STRING+1];
int errorlen;
MPI_Error_string(*errorcode, error, &errorlen);
fprintf(stderr,"error handler: %s\n",error);
fflush(stderr);
//visible painful death, easily spotted
exit(1);
}
}
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;
//creating an error handler for the file, calling your custom function:
MPI_Errhandler errhandler;
MPI_File_create_errhandler(file_errhandler_fn, &errhandler);
char x='x';
if (rank == 0) {
int errorcode=MPI_File_open(MPI_COMM_SELF, "test.txt", MPI_MODE_RDONLY, MPI_INFO_NULL, &fh);
//in case the opening of the file went wrong, handle the error:
if(errorcode!=MPI_SUCCESS){
//your error handler here...
char error[MPI_MAX_ERROR_STRING+1];
int errorlen;
MPI_Error_string(errorcode, error, &errorlen);
fprintf(stderr,"opening file: %s\n",error);
fflush(stderr);
//visible painful death, easily spotted
exit(1);
}
//setting custom error handlerr for further operations
errorcode=MPI_File_set_errhandler(fh,errhandler);
//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);
errorcode=MPI_File_read(fh,&x,sizeof(char), MPI_CHAR, &status);
printf("index %d got %c",i,x);
// the following line should compile fine, but triggers MPI_ERR_COUNT, captured by the custom error handler.
//errorcode=MPI_File_write(fh, &i, -1, MPI_INT, MPI_STATUS_IGNORE);
}
getchar();
// fclose(f);
MPI_File_close(&fh);
}
else {
// do nothing
}
MPI_Errhandler_free(&errhandler);
MPI_Finalize();
return 0;
}
Doing so may not fully solve your issue, but it provides a way to investigate the issue. What is the value of the error code? If it does not help you, let me know so I can suppress my answer an leave your question unnanswered!