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MPI_Wait(3) man page (version 1.8)

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


C Syntax

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

Fortran Syntax

INCLUDE ’mpif.h’

C++ Syntax

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

Input Parameter

Request (handle).

Output Parameters

Status object (status).
Fortran only: Error status (integer).


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)
        CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr)

        **** do some computation ****
        CALL MPI_WAIT(request, status, ierr)
    END IF


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


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