Abnormal Behaviour string MPI_Send() and MPI_Recv()

Question

My program is compiling and showing abnormal behaviour.

Used following commands for compiling

mpicc one.c -o one

mpiexec -n 2 ./one

I tried to debug it but show no compilation error. I can't understand why my program is behaving abnormally

#include<mpi.h>
#include<stdio.h>
#include<string.h>
int main(int argc,char * argv[])
{

    char str[434];
    int n;
    int rank,size; MPI_Status status;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Comm_size(MPI_COMM_WORLD,&size);
    if(rank == 0)
    {//Sender process
        scanf("%s",&str);

        MPI_Ssend(&n,1,MPI_INT,1,0,MPI_COMM_WORLD);
        MPI_Ssend(&str,n,MPI_CHAR,1,0,MPI_COMM_WORLD);
        printf("Sending word %s in process 0\n",str);
    }
    else
    {//Receiver process
        MPI_Recv(&n,1,MPI_INT,0,0,MPI_COMM_WORLD,&status);
        MPI_Recv(&str,n,MPI_CHAR,0,0,MPI_COMM_WORLD,&status);
        printf("Receiving word %s in process 1\n",str);
    }
    MPI_Finalize();


    return 0;
}

Input:

haha

Actual result

Sending word haha in process 0

Receiving word haha in process 1

Exchange result

Sending word haha in process 0

Receiving word �� in process 1

Enable compiler warnings. You are passing an address of a pointer to an array in e.g. scanf where you should simply pass pointer to the array str. The same problems occurs later in your code. — pbn
You never initialize n. And you should not use &str which is a pointer to the array (and of type char (*)[434]). Use either &str[0] or plain str (which decays to &str[0]). This use of &str is wrong everywhere you use it. — Some programmer dude

Duck Dodgers Duck Dodgers · Accepted Answer · 2019-01-14T14:39:53

As people have kindly pointed out in the comments to your post, you can't use str the way you were, because scanf() expects a char *, while you are passing it char (*)[434], which are not the same.

You would see this error (or warning), if you compiled with the "-Werror -Wformat" flags.

After modifying your code to use an pointer and not a char array, it looks as below:

#include<mpi/mpi.h>
#include<stdio.h>
#include<string.h>
#include <malloc.h>

int main(int argc, char * argv[])
{

    const int size = 434;
    char* str_send = (char*) malloc(size * sizeof(char)); // = "haha";
    char* str_recv = (char*) malloc(size * sizeof(char));
    int n = size;
    int rank;
    MPI_Status status;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Comm_size(MPI_COMM_WORLD,&size);
    if(rank == 0)
    {//Sender process
        scanf("%s", str_send);

        MPI_Ssend(&n,1,MPI_INT,1,0,MPI_COMM_WORLD);
        MPI_Ssend(str_send,n,MPI_CHAR,1,0,MPI_COMM_WORLD);
        printf("Sending word %s in process 0\n",str_send);
    }
    else
    {//Receiver process
        MPI_Recv(&n,1,MPI_INT,0,0,MPI_COMM_WORLD,&status);
        MPI_Recv(str_recv,n,MPI_CHAR,0,0,MPI_COMM_WORLD,&status);
        printf("Receiving word %s in process 1\n",str_recv);
    }
    MPI_Finalize();
    return 0;
}

Which gives the output:

junglefox@ubuntu:~/test_programs$ mpicc main.c

junglefox@ubuntu:~/test_programs$ mpiexec -n 2 ./a.out

haha

Sending word haha in process 0

Receiving word haha in process 1

Since, I feel you want to test the MPI library and not get bogged down into other intricacies, I would suggest using a fixed input and looking for the fixed output, instead of using scanf. As an example,

const char* str_send = "haha";

And then remove or comment out the scanf line.

// scanf("%s", str_send);

Abnormal Behaviour string MPI_Send() and MPI_Recv()

1 Answers