what I understand studying MPI specification is that an MPI send primitive refer to a memory location (or a send buffer) pointed by the data to be sent and take the data in that location which then passed as a message to the another Process.

Though it is true that virtual address of a give process will be meaningless in another process memory address; It is ok to send data pointed by pointer such as void pointer as MPI will any way pass the data itself as a message

For example the following works correctly:

    // Sender Side.
    int x = 100;
    void* snd;
    MPI_Send(snd,4,MPI_BYTE,1,0,MPI_COMM_WORLD);   

    // Receiver Side.
    void* rcv;
    MPI_Recv(rcv, 4,MPI_BYTE,0,0,MPI_COMM_WORLD); 

but when I add void* snd in a struct and try to send the struct this will no succeed.

I don't understand why the previous example work correctly but not the following.

Here, I have defined a typedef struct and then create an MPI_DataType from it. With the same explanation of the above the following should also have succeed, unfortunately it is not working.

here is the code:

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

    int main(int args, char *argv[])
    {
        int rank, source =0, tag=1, dest=1;
        int bloackCount[2];

        MPI_Init(&args, &argv);

        typedef struct {
            void* data;
            int tag; 
        } data;

        data myData;    

        MPI_Datatype structType, oldType[2];
        MPI_Status stat;

        /* MPI_Aint type used to idetify byte displacement of each block (array)*/      
        MPI_Aint offsets[2], extent;
        MPI_Comm_rank(MPI_COMM_WORLD, &rank);


        offsets[0] = 0;
        oldType[0] = MPI_BYTE;
            bloackCount[0] = 1;

        MPI_Type_extent(MPI_INT, &extent);

        offsets[1] = 4 * extent;  /*let say the MPI_BYTE will contain ineteger :         size of int * extent */
        oldType[1] = MPI_INT;
        bloackCount[1] = 1;

        MPI_Type_create_struct(2, bloackCount,offsets,oldType, &structType);
        MPI_Type_commit(&structType);


        if(rank == 0){
    int x = 100;
    myData.data = &x;
    myData.tag = 99;
    MPI_Send(&myData,1,structType, dest, tag, MPI_COMM_WORLD);
}
if(rank == 1 ){ 
    MPI_Recv(&myData, 1, structType, source, tag, MPI_COMM_WORLD, &stat);
          // with out this the following printf() will properly print the value 99 for 
          // myData.tag
    int x = *(int *) myData.data;
    printf(" \n Process %d, Received : %d , %d \n\n", rank , myData.tag, x); 
    }   
       MPI_Type_free(&structType);             
       MPI_Finalize();
    }

Error message running the code: [Looks like I am trying to access an invalid memory address space in the second process]

    [ubuntu:04123] *** Process received signal ***
    [ubuntu:04123] Signal: Segmentation fault (11)
    [ubuntu:04123] Signal code: Address not mapped (1)
    [ubuntu:04123] Failing at address: 0xbfe008bc
    [ubuntu:04123] [ 0] [0xb778240c]
    [ubuntu:04123] [ 1] GenericstructType(main+0x161) [0x8048935]
    [ubuntu:04123] [ 2] /lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)         [0xb750f4d3]
    [ubuntu:04123] [ 3] GenericstructType() [0x8048741]
    [ubuntu:04123] *** End of error message ***

Can some please explain to me why it is not working. any advice will also be appreciated

thanks,