cache and retrieve a process rank with a communicator

/* cache_test.c -- cache and retrieve a process rank with a communicator
*
* Input: none
* Output: Message from the process whose rank was cached
*
* See Chap 8, pp. 139 & ff in PPMPI
*/
#include
#include "mpi.h"
#include

main(int argc, char* argv[]) {
int p;
int my_rank;
MPI_Comm io_comm; /* Communicator for I/O */
int IO_KEY; /* I/O process attribute key */
int* io_rank_ptr; /* Attributes are pointers */
void* extra_arg; /* Unused */
int flag;
int* io_rank_attr;

MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &p);
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

/* Get a separate communicator for I/O functions by */
/* duplicating MPI_COMM_WORLD */
MPI_Comm_dup(MPI_COMM_WORLD, &io_comm);

/* Create the attribute key */
MPI_Keyval_create(MPI_DUP_FN, MPI_NULL_DELETE_FN,
&IO_KEY, extra_arg);

/* Allocate storage for the attribute content */
io_rank_ptr = (int*) malloc(sizeof(int));

/* Set the attribute content */
*io_rank_ptr = 0;

/* Cache the attribute with io_comm */
MPI_Attr_put(io_comm, IO_KEY, io_rank_ptr);

/* Retrieve the I/O process rank */
MPI_Attr_get(io_comm, IO_KEY, &io_rank_attr, &flag);

/* If flag == 0, something went wrong: */
/* there's no attribute cached. */
if ((flag != 0) && (my_rank == *io_rank_attr)) {
printf("Greetings from the I/O Process!\n");
printf("My rank is %d\n", *io_rank_attr);
}

MPI_Finalize();
} /* main */