[SOLVED] Error on sending argv

awiles · 04-21-2010, 08:45 AM

Hi all,
I write a simple MPI program to send a text message to another process. The code is below.
(test.c)

Code:

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

int main(int argc, char* argv[]) {
    int dest, noProcesses, processId;
    MPI_Status status;
    
    char* buffer;
    
    char* text = "ABCDEF";
    
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &noProcesses);
    MPI_Comm_rank(MPI_COMM_WORLD, &processId);
    
    buffer = (char*) malloc(256 * sizeof(char));
    
    if (processId == 0) {
      fprintf(stdout, "Master: sending %s to %d\n", text, 1);
      MPI_Send((void *)&text, strlen(text) + 1, MPI_CHAR, 1, 0, MPI_COMM_WORLD);
    } else {
      MPI_Recv(&buffer, 128, MPI_CHAR, MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
      fprintf(stdout, "Slave: received %s from %d\n", buffer, status.MPI_SOURCE);
    }
    MPI_Finalize();
    return 0;
}

After compiling and executing it I get the following output:

Code:

[root@cluster Desktop]# mpicc -o test test.c
[root@cluster Desktop]# mpirun -np 2 test
Master: sending ABCDEF to 1
Slave: received ABCDEF from 0

In the source code above, I replace

Code:

char* text = "ABCDEF";

by

Code:

char* text = argv[1];

then compile and execute it again with the following commands:

Code:

[root@cluster Desktop]# mpicc -o test test.c
[root@cluster Desktop]# mpirun -np 2 test ABCDEF

Then I get the following output:

Code:

Master: sending ABCDEF to 1
[cluster:03917] *** Process received signal ***
[cluster:03917] Signal: Segmentation fault (11)
[cluster:03917] Signal code: Address not mapped (1)
[cluster:03917] Failing at address: 0xbfa445a2
[cluster:03917] [ 0] [0x959440]
[cluster:03917] [ 1] /lib/libc.so.6(_IO_fprintf+0x22) [0x76be02]
[cluster:03917] [ 2] test(main+0x143) [0x80488b7]
[cluster:03917] [ 3] /lib/libc.so.6(__libc_start_main+0xdc) [0x73be8c]
[cluster:03917] [ 4] test [0x80486c1]
[cluster:03917] *** End of error message ***
--------------------------------------------------------------------------
mpirun noticed that process rank 1 with PID 3917 on node cluster.hpc.org exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

I’m very confused because the only difference between the two source codes is the difference between

Code:

char* text = "ABCDEF";

and

Code:

char* text = argv[1];

Can any one help me why the results are so different? How can I send argv[i] to another process?
Thank you very much!

nowonmai · 04-21-2010, 08:53 AM

If you go back over the compiler output, you'll probably have a better idea...
Also, brush up on indirection & string copying.

Sergei Steshenko · 04-21-2010, 09:00 AM

Quote:

Originally Posted by awiles

Hi all,
I write a simple MPI program to send a text message to another process. The code is below.
(test.c)

Code:

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

int main(int argc, char* argv[]) {
    int dest, noProcesses, processId;
    MPI_Status status;
    
    char* buffer;
    
    char* text = "ABCDEF";
    
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &noProcesses);
    MPI_Comm_rank(MPI_COMM_WORLD, &processId);
    
    buffer = (char*) malloc(256 * sizeof(char));
    
    if (processId == 0) {
      fprintf(stdout, "Master: sending %s to %d\n", text, 1);
      MPI_Send((void *)&text, strlen(text) + 1, MPI_CHAR, 1, 0, MPI_COMM_WORLD);
    } else {
      MPI_Recv(&buffer, 128, MPI_CHAR, MPI_ANY_SOURCE, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
      fprintf(stdout, "Slave: received %s from %d\n", buffer, status.MPI_SOURCE);
    }
    MPI_Finalize();
    return 0;
}

After compiling and executing it I get the following output:

Code:

[root@cluster Desktop]# mpicc -o test test.c
[root@cluster Desktop]# mpirun -np 2 test
Master: sending ABCDEF to 1
Slave: received ABCDEF from 0

In the source code above, I replace

Code:

char* text = "ABCDEF";

by

Code:

char* text = argv[1];

then compile and execute it again with the following commands:

Code:

[root@cluster Desktop]# mpicc -o test test.c
[root@cluster Desktop]# mpirun -np 2 test ABCDEF

Then I get the following output:

Code:

Master: sending ABCDEF to 1
[cluster:03917] *** Process received signal ***
[cluster:03917] Signal: Segmentation fault (11)
[cluster:03917] Signal code: Address not mapped (1)
[cluster:03917] Failing at address: 0xbfa445a2
[cluster:03917] [ 0] [0x959440]
[cluster:03917] [ 1] /lib/libc.so.6(_IO_fprintf+0x22) [0x76be02]
[cluster:03917] [ 2] test(main+0x143) [0x80488b7]
[cluster:03917] [ 3] /lib/libc.so.6(__libc_start_main+0xdc) [0x73be8c]
[cluster:03917] [ 4] test [0x80486c1]
[cluster:03917] *** End of error message ***
--------------------------------------------------------------------------
mpirun noticed that process rank 1 with PID 3917 on node cluster.hpc.org exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

I’m very confused because the only difference between the two source codes is the difference between

Code:

char* text = "ABCDEF";

and

Code:

char* text = argv[1];

Can any one help me why the results are so different? How can I send argv[i] to another process?
Thank you very much!

Do you really need the item in red ? I.e. do you really need address of pointer rather than the pointer itself ?

And regardless of your problem you must check what 'malloc' returns.

Anyway, put diagnostic

Code:

fprintf(stderr, ...);

messages in order to understand what is the last before segfault executed statement.

awiles · 04-21-2010, 09:50 AM

Quote:

Originally Posted by Sergei Steshenko

Do you really need the item in red ? I.e. do you really need address of pointer rather than the pointer itself ?

And regardless of your problem you must check what 'malloc' returns.

Anyway, put diagnostic

Code:

fprintf(stderr, ...);

messages in order to understand what is the last before segfault executed statement.

Thanks for your reply. After removing the two ampersands, the application runs without any error.
Thanks again