Segmentation Fault: Struct Serialisation and MPI Data Transfer in C++

Question

I am trying to send a serialised struct from one rank to another. It results in

Segmentation fault: 11

and I have no clue from where it comes.

I have tried to locate the problem by printing out some values and the code always breaks between MPI_Send and MPI_Recv, but since it is segmentation fault one cannot be sure that it is the origin of the problem. Please do enlighten me.

int N = 11;
struct tests{
    int number;
    double *fx;
};

void locateMemoryTests(struct tests *t){
    t->fx = (double*) malloc(N*sizeof(double));
}

void he(struct tests *t, int N){
    int NRank, MyRank;
    MPI_Comm_rank( MPI_COMM_WORLD, &MyRank );
    MPI_Comm_size( MPI_COMM_WORLD, &NRank );
    int st = MyRank * int(N/2);
    int en = (MyRank+1) * int(N/2) + (N%2)*MyRank;

    for (int i=st; i<en; i++){
        t->fx[i] = i*i + 5*(i + t->number);
    }
    const int nitems        = 2;
    int blocklengths[2]     = {1, N};
    MPI_Datatype types[2]   = {MPI_INT, MPI_DOUBLE};
    MPI_Datatype mpi_tests_type;
    MPI_Aint offsets[2];

    offsets[0] = offsetof(tests, number);
    offsets[1] = offsetof(tests, fx);

    MPI_Type_create_struct(nitems, blocklengths, offsets, types, &mpi_tests_type);
    MPI_Type_commit(&mpi_tests_type);
    MPI_Comm_rank(MPI_COMM_WORLD, &MyRank);

    if (MyRank == 0){
        struct tests send;
        send.number = t->number;
        locateMemoryTests(&send);

        for (int i=0; i<N; i++){
            send.fx[i] = t->fx[i];
        }   

        MPI_Send(&send, 2, mpi_tests_type, 1, 111, MPI_COMM_WORLD);
        }
    else if (MyRank == 1){
        MPI_Status status;
        struct tests recv;
        locateMemoryTests(&recv);
        MPI_Recv(&recv, 2, mpi_tests_type, 0, 111, MPI_COMM_WORLD, &status);

    MPI_Type_free(&mpi_tests_type);
}


int main( int argc, char *argv[] ){
    int NRank, MyRank;
    MPI_Init( &argc, &argv );
    MPI_Comm_rank( MPI_COMM_WORLD, &MyRank );
    MPI_Comm_size( MPI_COMM_WORLD, &NRank );

    struct tests tt;
    tt.number = 5;
    locateMemoryTests(&tt);

    he(&tt,N);

    MPI_Finalize();
    return 0;
}

MPI_Recv receives two elements, but recv variable can only hold only one? — Radosław Cybulski
– Radosław Cybulski, Commented Jun 8, 2019 at 15:51
Isn't it the purpose of serialisation, to send/receive a struct with multiple elements? Or do you mean I can only send one type of element? I learnt that "int count" in this case should be the number of the kinds of data my structure has from stackoverflow.com/questions/18165277/… — Sai
– Sai, Commented Jun 8, 2019 at 16:15
I mean you want two elements, but prepared storage only for one. — Radosław Cybulski
– Radosław Cybulski, Commented Jun 8, 2019 at 16:17
Could you please elaborate a bit? I am new to C++ and I am not quite sure why I have prepared storage only for one instead of two. — Sai
– Sai, Commented Jun 8, 2019 at 16:38
Try struct tests recv[2]; and struct tests send[2]; and see if it makes any difference. — Radosław Cybulski
– Radosław Cybulski, Commented Jun 8, 2019 at 16:56

Gilles Gouaillardet · Accepted Answer · 2019-06-10 00:09:24Z

1

Your MPI derived datatype describes the following C struct

struct tests{
    int number;
    double fx[N];
};

but you are using a different one

struct tests{
    int number;
    double *fx;
};

if N is not a constant, you can either declare

struct tests{
    int number;
    double fx[];
};

and it is up to you to correctly allocate such a struct.

An other option is to keep the same struct definition, and manually MPI_Pack() and MPI_Unpack() the data to/from a temporary buffer.

answered Jun 10, 2019 at 0:09

Gilles Gouaillardet

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