MPI_Send double pointer

Question

I have following code:

double * myX;
double * myY;
double * myZ;
int amount;
int count;      // number of process

void SomeClass::someMethod(double *x, double *y, double *z, int amount) {

    if (myId == 0) {
        myX = x;
        myY = y;
        myZ = z;
        amount = amount;
        for(int i = 1; i < count; ++i) {
            MPI_Send(&amount, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
            MPI_Send(myX, amount, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
            MPI_Send(myY, amount, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
            MPI_Send(myX, amount, MPI_DOUBLE, i, 0, MPI_COMM_WORLD);
        }
    }                                         
}

void SomeClass::anotherMethod(void) {

    if(myId != 0) {
        MPI_Recv(&amount, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        MPI_Recv(myX, amount, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        MPI_Recv(myY, amount, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        MPI_Recv(myZ, amount, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);         
    }

    // rest of code
    MPI_Reduce(args);
}

But I have problem because I get Null buffer pointer when I run this code or Segmentation fault when I change something, for example set & before var name and then run.

MPI_init and other required function are called in other class, where I also create this class objects.

Can someone help me?

Your question would be clearer if you explained what part of your code originated the error: Which of your many MPI calls results in Null buffer pointer? What line is the Segmentation fault thrown from? — Edward
– Edward, Commented Jan 20, 2016 at 21:53
@Edward right, it was when I try get data from double array. — Matt
– Matt, Commented Jan 20, 2016 at 23:24

Damyan · Accepted Answer · 2016-01-20 21:09:12Z

3

MPI_Recv will copy the data it receives to the buffer specified by the first parameter (myX in the case below):

MPI_Recv(myX, amount, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

The problem is that you haven't created a buffer to store this.

You could do:

myX = new double[amount];

For example to create the buffer, not forgetting to free the memory again afterwards with:

delete[] myX;

answered Jan 20, 2016 at 21:09

Damyan

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