Sending typedef struct containing void* by creating MPI drived datatype.
- by hankol
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,
