MPI_Send_init

int MPI_Send_init(
  void *buf,
  int count,
  MPI_Datatype datatype,
  int dest,
  int tag,
  MPI_Comm comm,
  MPI_Request *request
);

Parameters

buf

[in] initial address of send buffer (choice)

count

[in] number of elements sent (integer)

datatype

[in] type of each element (handle)

dest

[in] rank of destination (integer)

tag

[in] message tag (integer)

comm

[in] communicator (handle)

request

[out] communication request (handle)

Remarks

Creates a persistent communication request for a standard mode send operation, and binds to it all the arguments of a send operation.

Often a communication with the same argument list is repeatedly executed within the inner loop of a parallel computation. In such a situation, it may be possible to optimize the communication by binding the list of communication arguments to a persistent communication request once and, then, repeatedly using the request to initiate and complete messages. The persistent request thus created can be thought of as a communication port or a ``half-channel.'' It does not provide the full functionality of a conventional channel, since there is no binding of the send port to the receive port. This construct allows reduction of the overhead for communication between the process and communication controller, but not of the overhead for communication between one communication controller and another. It is not necessary that messages sent with a persistent request be received by a receive operation using a persistent request, or vice versa.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS

No error; MPI routine completed successfully.

MPI_ERR_COUNT

Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_TYPE

Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).

MPI_ERR_RANK

Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_SOURCE.

MPI_ERR_TAG

Invalid tag argument. Tags must be non-negative; tags in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_TAG. The largest tag value is available through the the attribute MPI_TAG_UB.

MPI_ERR_COMM

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

MPI_ERR_INTERN

This error is returned when some part of the MPICH implementation is unable to acquire memory.

See Also

Example Code

The following sample code illustrates MPI_Send_init.

#include "mpi.h"
#include <stdlib.h>
 
int main(int argc, char *argv[])
{
    MPI_Request r;
    MPI_Status s;
    int flag;
    int buf[10];
    int rbuf[10];
    int tag = 27;
    int dest = 0;
    int rank, size, i;
 
    MPI_Init( &argc, &argv );
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
 
    /* Create a persistent send request */
    MPI_Send_init( buf, 10, MPI_INT, dest, tag, MPI_COMM_WORLD, &r );
 
    /* Use that request */
    if (rank == 0) {
        int i;
        MPI_Request *rr = (MPI_Request *)malloc(size * sizeof(MPI_Request));
        for (i=0; i<size; i++) {
            MPI_Irecv( rbuf, 10, MPI_INT, i, tag, MPI_COMM_WORLD, &rr[i] );
        }
        MPI_Start( &r );
        MPI_Wait( &r, &s );
        MPI_Waitall( size, rr, MPI_STATUSES_IGNORE );
        free(rr);
    }
    else {
        MPI_Start( &r );
        MPI_Wait( &r, &s );
    }
 
    MPI_Request_free( &r );
 
 
    if (rank == 0)
    {
        MPI_Request sr;
        /* Create a persistent receive request */
        MPI_Recv_init( rbuf, 10, MPI_INT, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &r );
        MPI_Isend( buf, 10, MPI_INT, 0, tag, MPI_COMM_WORLD, &sr );
        for (i=0; i<size; i++) {
            MPI_Start( &r );
            MPI_Wait( &r, &s );
        }
        MPI_Wait( &sr, &s );
        MPI_Request_free( &r );
    }
    else {
        MPI_Send( buf, 10, MPI_INT, 0, tag, MPI_COMM_WORLD );
    }
 
    MPI_Finalize();
    return 0;
}