MPI_Wait(3OpenMPI) MPI_Wait(3OpenMPI)
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
INCLUDE 'mpif.h'
MPI_WAIT(REQUEST, STATUS, IERROR)
INTEGER REQUEST, STATUS(MPI_STATUS_SIZE), 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.
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 sta-tus.MPI_ERROR status.MPI_ERROR
tus.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 han-dler handler
dler 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
Open MPI 1.2 March 2007 MPI_Wait(3OpenMPI)
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