:( Sorry, Another MPI Problem

This is a discussion on :( Sorry, Another MPI Problem within the C++ Programming forums, part of the General Programming Boards category; Hi, I can't seem to find a help forum dedicated to MPI, this was the closest I could find. Anyways, ...

  1. #1
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    :( Sorry, Another MPI Problem

    Hi, I can't seem to find a help forum dedicated to MPI, this was the closest I could find. Anyways, I am having problem with the following code:

    Code:
    #include <iostream>
    #include <math.h>
    #include <bitset>
    #include <fstream>
    #include <mpi.h>
    
    using namespace std;
    
    MPI_Request req;
    MPI_Status stat;
    
    MPI_Datatype type[2]={MPI_INT,MPI_DOUBLE};
    MPI_Datatype MSG;
    int blocklen[2]={1,1};
    MPI_Aint disp[2]={0,sizeof(int)};
    
    struct message{
        int position;
        double data;   // *** HERE ***
    };
    message snd,rcv;
    
    int main(int argc,char *argv[]){
    
        MPI_Init(&argc,&argv);
        MPI_Type_struct(2,blocklen,disp,type,&MSG);
        MPI_Type_commit(&MSG);
        double tt=0.0;
        int myRank,nProcs;
        MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
        MPI_Comm_size(MPI_COMM_WORLD, &nProcs);
        if(myRank==0){
            tt=3.0;
            snd.data=tt;
            snd.position=1111;
            rcv.position=snd.position;
            for(int i=1;i<nProcs;i++){
                MPI_Isend(&snd,1,MSG,i,0,MPI_COMM_WORLD,&req);
                MPI_Wait(&req,&stat);
            }
        }
        else{
            MPI_Recv(&rcv,1,MSG,0,0,MPI_COMM_WORLD,&stat);
            tt=rcv.data;
        }
        cout << "Server " << myRank << " has " << tt << " and " << rcv.position << endl;
    
    }
    Basically all the code does is have the root process send the double 3.0 to the other process. When I run this code all servers but server 0 receive the message as 0 not 3. However, if I just change the line marks //*** HERE ** in struct to "float data" and use data as a float instead it works. Does anybody have any idea why this might be happening? I've tried fiddling with the MPI_Aint disp[2]={0,sizeof(int)}; line but that doesn't seem to do anything. Any help would be greatly appreciated.

    P.S. please don't suggest I use bcast or some such , I need the code to have a coupled Isend and Rcv
    Last edited by maverick_starst; 07-18-2009 at 10:16 PM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well most (if not all) of the MPI functions return a status result.
    How about some RTM and adding some error checking.

    Perhaps it will tell you why it isn't working, rather than you just "*shrug* it doesn't work"
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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