Should MPI_IRecv/MPI_ISend have the same `count`? - parallel-processing

Question

Should a pair of MPI_IRecv/MPI_ISend get the same count?
int MPI_Irecv(void *buf, int count, MPI_Datatype datatype, int source,
int tag, MPI_Comm comm, MPI_Request *request)
...
count
number of elements in receive buffer (integer)
The documentation seems to suggest it should not but I am confused by the wording and it is formulated a bit differently from MPI_Recv. I am attaching an example which works as I am expecting if I pass different count.
isend.c
#include <stdio.h>
#include <mpi.h>
#define send_cnt 1
#define recv_cnt 10
#define SEND 0 /* who sends and who receives? */
#define RECV 1
#define TAG 0
#define COMM MPI_COMM_WORLD
MPI_Status status;
MPI_Request request;
void send() {
int dest = RECV;
int buf[] = {42};
MPI_Isend(buf, send_cnt, MPI_INT, dest, TAG, COMM, &request);
MPI_Wait(&request, &status);
}
void recv() {
int dest = SEND;
int buf[123];
MPI_Irecv(buf, recv_cnt, MPI_INT, dest, TAG, COMM, &request);
MPI_Wait(&request, &status);
printf("recv: %d\n", buf[0]);
}
int main(int argc, char *argv[]) {
int rank;
MPI_Init(&argc, &argv);
MPI_Comm_rank(COMM, &rank);
if (rank == SEND) send();
else recv();
MPI_Finalize();
return 0;
}

Answer 1

Both MPI_Recv and MPI_Irecv take as an argument the maximum amount of data elements that are allowed to be written into the buffer. This number does not necessarily have to be equal to the count passed to MPI_Send - it could be larger or smaller. When there is not enough space in the receive buffer to accommodate the message, MPI will signal a truncation error. When there is more space than the message size, only a part of the buffer will be written over. In the latter case one could use MPI_Get_count to examine the MPI status object returned by MPI_Recv / MPI_Test* / MPI_Wait*.
There is a legitimate case of the count passed to MPI_(I)Recv being smaller than the count passed to MPI_Send - different datatypes. For example, one can send 10 MPI_INTs and receive a single element of a contiguous datatype that consists of 10 MPI_INTs. In your case both the send and the receive operations use the same datatype, therefore the receive count must be at least as big as the send count.
By the way, your code is missing a call to wait/test the request created by MPI_Isend, which is erroneous.