Receiving
C | Fortran-2008 | Fortran-90
The MPI_IN_PLACE is an option used in MPI routines that involve both a send and a receive phase, such as MPI_Gather and its variants. By passing MPI_IN_PLACE as the send buffer, the MPI routine knows that the send and receive buffer are one same buffer.
Note: the in-place variant of MPI_Sendrecv does not rely on MPI_IN_PLACE but is a distinct routine MPI_Sendrecv_replace.
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
/**
* @brief Illustrates how to use an in-place gather.
* @details This application is meant to be run with 4 MPI processes. Every MPI
* process begins with a value, then MPI process 0 is picked to gather all these
* values and print them. It can be visualised as follows:
*
* +-----------+ +-----------+ +-----------+ +-----------+
* | Process 0 | | Process 1 | | Process 2 | | Process 3 |
* +-+-------+-+ +-+-------+-+ +-+-------+-+ +-+-------+-+
* | Value | | Value | | Value | | Value |
* | 0 | | 100 | | 200 | | 300 |
* +-------+ +-------+ +-------+ +-------+
* \ | | /
* \ | | /
* \ | | /
* \ | | /
* \ | | /
* \ | | /
* +-----+-----+-----+-----+
* | 0 | 100 | 200 | 300 |
* +-----+-----+-----+-----+
* | Process 0 |
* +-----------------------+
**/
int main(int argc, char* argv[])
{
MPI_Init(&argc, &argv);
// Get number of processes and check that 4 processes are used
int size;
MPI_Comm_size(MPI_COMM_WORLD, &size);
if(size != 4)
{
printf("This application is meant to be run with 4 MPI processes.\n");
MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
}
// Determine root's rank
int root_rank = 0;
// Get my rank
int my_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
// Define my value
int my_value = my_rank * 100;
printf("Process %d, my value = %d.\n", my_rank, my_value);
if(my_rank == root_rank)
{
int buffer[4];
buffer[my_rank] = my_value * 100;
MPI_Gather(MPI_IN_PLACE, 1, MPI_INT, buffer, 1, MPI_INT, root_rank, MPI_COMM_WORLD);
printf("Values collected on process %d: %d, %d, %d, %d.\n", my_rank, buffer[0], buffer[1], buffer[2], buffer[3]);
}
else
{
MPI_Gather(&my_value, 1, MPI_INT, NULL, 0, MPI_INT, root_rank, MPI_COMM_WORLD);
}
MPI_Finalize();
return EXIT_SUCCESS;
}