.\"Copyright 2007, Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.TH MPI_Wait 3OpenMPI "March 2007" "Open MPI 1.2" " "
.SH NAME
\fBMPI_Wait\fP \- Waits for an MPI send or receive to complete.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Wait(MPI_Request *\fIrequest\fP, MPI_Status\fI *status\fP)

.SH Fortran Syntax
.nf
INCLUDE 'mpif.h'
MPI_WAIT(\fIREQUEST, STATUS, IERROR\fP)
	INTEGER	\fIREQUEST, STATUS(MPI_STATUS_SIZE), IERROR\fP 

.SH C++ Syntax
.nf
#include <mpi.h>
void Request::Wait(Status& \fIstatus\fP)

void Request::Wait() 

.SH INPUT PARAMETER
.ft R
.TP 1i
request      
Request (handle).
.sp
.SH OUTPUT PARAMETERS
.ft R
.TP 1i
status      
Status object (status).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer). 

.SH DESCRIPTION
.ft R
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. 
.sp
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").
.sp
If your application does not need to examine the \fIstatus\fP field, you can save resources by using the predefined constant MPI_STATUS_IGNORE as a special value for the \fIstatus\fP argument. 
.sp
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). 
.sp
Example: Simple usage of nonblocking operations and  MPI_Wait. 
.sp
.nf
    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 
.ni

.SH 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.
.sp
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; 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.  

.SH SEE ALSO
.ft R
.sp
MPI_Test
.br
MPI_Testall
.br
MPI_Testany
.br
MPI_Testsome
.br
MPI_Waitall
.br
MPI_Waitany
.br
MPI_Waitsome
.br

' @(#)MPI_Wait.3 1.20 06/03/09