Trying to MPI_Send and MPI_Recv char array getting garbage

Question

I'm trying to create a log file from each processor and then send that to the root as a char array. I first send the length and then I send the data. The length sends fine, but the data is always garbage! Here is my code:

MPI_Barrier (MPI_COMM_WORLD);
string out = "";

MPI_Status status[2];
MPI_Request reqs[num_procs];

string log = "TEST";
int length = log.length();
char* temp = (char *) malloc(length+1);
strcpy(temp, log.c_str());

if (my_id != 0)
{
    MPI_Send (&length, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
    MPI_Send (&temp, length+1, MPI_CHAR, 0, 1, MPI_COMM_WORLD);
}   
else {      
    int length;
    for (int i = 1; i < num_procs; i++)
    {
        MPI_Recv (&length, 2, MPI_INT, i, 1, MPI_COMM_WORLD, &status[0]);
        char* rec_buf;
        rec_buf = (char *) malloc(length+1);
        MPI_Recv (rec_buf, length+1, MPI_CHAR, i, 1, MPI_COMM_WORLD, &status[1]);
        out += rec_buf;
        free(rec_buf);
    }
}

MPI_Barrier (MPI_COMM_WORLD);
free(temp);

You can add an answer. You just have to wait a certain amount of time. If you go back and add your answer now, it won't show up as unanswered and you can mark it as answered by yourself (after another waiting period). — Wesley Bland
– Wesley Bland, Commented Jul 9, 2013 at 16:23
Why are you sending one words with MPI_Send (&length, 1, ...) but receiving two words MPI_Recv (&length, 2, ...)? — Always Asking
– Always Asking, Commented Dec 3, 2013 at 18:25
Also, why all your message tags are 1? Quoting from Message Passing Interface (MPI): "Send and receive operations should match message tags." — Always Asking
– Always Asking, Commented Dec 3, 2013 at 18:27

niklasfi · Accepted Answer · 2015-06-30 12:09:51Z

5

You are passing a char** to MPI_Send instead of a char* this causes memory corruption, or in your case the garbled output you are getting. Everything should be fine if you use

MPI_Send (temp, length+1, MPI_CHAR, 0, 1, MPI_COMM_WORLD);

(note the removed & in front of the first argument, temp.)

edited Jun 30, 2015 at 12:09

answered Apr 28, 2014 at 20:56

niklasfi

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