NAME

       MPI_Wait - Waits for an MPI send or receive to complete.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Wait(MPI_Request *request, MPI_Status *status)

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_WAIT(REQUEST, STATUS, IERROR)
            INTEGER   REQUEST, STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Wait(request, status, ierror)
            TYPE(MPI_Request), INTENT(INOUT) :: request
            TYPE(MPI_Status) :: status
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

C++ Syntax

       #include <mpi.h>
       void Request::Wait(Status& status)

       void Request::Wait()

INPUT PARAMETER

       request   Request (handle).

OUTPUT PARAMETERS

       status    Status object (status).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       A  call  to  MPI_Wait returns when the operation identified by request is complete. If the
       communication object associated with this request was created by  a  nonblocking  send  or
       receive  call,  then  the  object  is  deallocated by the call to MPI_Wait and the request
       handle is set to MPI_REQUEST_NULL.

       The call returns, in status, information on the completed operation. The  content  of  the
       status object for a receive operation can be accessed as described in Section 3.2.5 of the
       MPI-1 Standard, "Return Status." The status object for a send operation may be queried  by
       a call to MPI_Test_cancelled (see Section 3.8 of the MPI-1 Standard, "Probe and Cancel").

       If  your  application does not need to examine the status field, you can save resources by
       using the predefined  constant  MPI_STATUS_IGNORE  as  a  special  value  for  the  status
       argument.

       One is allowed to call MPI_Wait with a null or inactive request argument. In this case the
       operation returns immediately with empty status.

NOTES

       Successful return of MPI_Wait after an MPI_Ibsend implies that the user send buffer can be
       reused    i.e.,   data   has  been  sent  out  or  copied  into  a  buffer  attached  with
       MPI_Buffer_attach. Note that, at this point, we can no longer cancel the  send  (for  more
       information,  see  Section  3.8  of the MPI-1 Standard, "Probe and Cancel"). If a matching
       receive is never posted, then the buffer cannot be freed. This runs  somewhat  counter  to
       the  stated goal of MPI_Cancel (always being able to free program space that was committed
       to the communication subsystem).

       Example: Simple usage of nonblocking operations and  MPI_Wait.

           CALL MPI_COMM_RANK(comm, rank, ierr)
           IF(rank.EQ.0) THEN
               CALL MPI_ISEND(a(1), 10, MPI_REAL, 1, tag, comm, request, ierr)
               **** do some computation ****
               CALL MPI_WAIT(request, status, ierr)
           ELSE
               CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr)
               **** do some computation ****
               CALL MPI_WAIT(request, status, ierr)
           END IF

ERRORS

       Almost all MPI routines return an error value; C routines as the value of the function and
       Fortran  routines in the last argument. C++ functions do not return errors. If the default
       error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then  on  error  the  C++  exception
       mechanism will be used to throw an MPI::Exception object.

       Before  the  error value is returned, the current MPI error handler is called. By default,
       this error handler aborts the MPI job, except for I/O function errors. The  error  handler
       may    be    changed    with    MPI_Comm_set_errhandler,    MPI_File_set_errhandler,    or
       MPI_Win_set_errhandler (depending on the type of MPI handle that generated  the  request);
       the  predefined  error  handler  MPI_ERRORS_RETURN may be used to cause error values to be
       returned. Note that MPI does not guarantee that an MPI program can continue past an error.

       Note that per MPI-1 section 3.2.5, MPI exceptions on requests passed to  MPI_WAIT  do  not
       set  the  status.MPI_ERROR  field in the returned status.  The error code is passed to the
       back-end error handler and may be passed back to the caller through the  return  value  of
       MPI_WAIT  if  the  back-end  error  handler returns it.  The pre-defined MPI error handler
       MPI_ERRORS_RETURN exhibits this behavior, for example.

SEE ALSO

       MPI_Comm_set_errhandler
       MPI_File_set_errhandler
       MPI_Test
       MPI_Testall
       MPI_Testany
       MPI_Testsome
       MPI_Waitall
       MPI_Waitany
       MPI_Waitsome
       MPI_Win_set_errhandler