Behavior of MPI_Send and MPI_Recv

Question

Why these lines of code:

if(my_rank != 0) {
    sprintf(msg, "Hello from %d of %d...", my_rank, comm_sz);
    if(my_rank == 2) {
        sleep(2);
        sprintf(msg, "Hello from %d of %d, I have slept 2 seconds...", my_rank, comm_sz);
    }
    MPI_Send(msg, strlen(msg), MPI_CHAR, 0, 0, MPI_COMM_WORLD);
}
else {
    printf("Hello from the chosen Master %d\n", my_rank);
    for(i = 1; i < comm_sz; i++) {
        MPI_Recv(msg, MAX_STRING, MPI_CHAR, i, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        printf("%s\n", msg);
    }
}

give this result?

Hello from the chosen Master 0  
Hello from 1 of 5...  
Hello from 2 of 5, I have slept 2 seconds...  
Hello from 3 of 5... have slept 2 seconds...  
Hello from 4 of 5... have slept 2 seconds...

Doesn't each process have its copy of 'msg' ?

What do you expect? Note that MPI does not work on threads, but processes. — Zulan
– Zulan, Commented May 24, 2017 at 20:00
@Zulan I expect that only the second process would write I have slept 2 seconds... — glc78
– glc78, Commented May 24, 2017 at 20:02

Zulan · Accepted Answer · 2017-05-24 20:23:46Z

3

strlen() does not include the null terminator, hence it will not be sent to the master. Receiving the message from rank 3 will not overwrite the later part of the string, so it is still displayed. You should use strlen(msg) + 1 as send count.

answered May 24, 2017 at 20:23

Zulan

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