Provided by: openmpi-doc_4.1.6-7ubuntu2_all 
NAME
MPI_Request_free - Frees a communication request object.
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Request_free(MPI_Request *request)
Fortran Syntax
INCLUDE 'mpif.h'
MPI_REQUEST_FREE(REQUEST, IERROR)
INTEGER REQUEST, IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Request_free(request, ierror)
TYPE(MPI_Request), INTENT(INOUT) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
C++ Syntax
#include <mpi.h>
void Request::Free()
INPUT/OUTPUT PARAMETER
request
Communication request (handle).
DESCRIPTION
This operation allows a request object to be deallocated without waiting for the associated communication
to complete.
MPI_Request_free marks the request object for deallocation and sets request to MPI_REQUEST_NULL. Any
ongoing communication that is associated with the request will be allowed to complete. The request will
be deallocated only after its completion.
NOTES
Once a request is freed by a call to MPI_Request_free, it is not possible to check for the successful
completion of the associated communication with calls to MPI_Wait or MPI_Test. Also, if an error occurs
subsequently during the communication, an error code cannot be returned to the user -- such an error must
be treated as fatal. Questions arise as to how one knows when the operations have completed when using
MPI_Request_free. Depending on the program logic, there may be other ways in which the program knows that
certain operations have completed and this makes usage of MPI_Request_free practical. For example, an
active send request could be freed when the logic of the program is such that the receiver sends a reply
to the message sent -- the arrival of the reply informs the sender that the send has completed and the
send buffer can be reused. An active receive request should never be freed, as the receiver will have no
way to verify that the receive has completed and the receive buffer can be reused.
Example:
CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
IF(rank.EQ.0) THEN
DO i=1, n
CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
CALL MPI_REQUEST_FREE(req, ierr)
CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
CALL MPI_WAIT(req, status, ierr)
END DO
ELSE ! rank.EQ.1
CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
CALL MPI_WAIT(req, status)
DO I=1, n-1
CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
CALL MPI_REQUEST_FREE(req, ierr)
CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
CALL MPI_WAIT(req, status, ierr)
END DO
CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
CALL MPI_WAIT(req, status)
END IF
This routine is normally used to free persistent requests created with either MPI_Recv_init or
MPI_Send_init and friends. However, it can be used to free a request created with MPI_Irecv or MPI_Isend
and friends; in that case the use can not use the test/wait routines on the request.
It is permitted to free an active request. However, once freed, you can not use the request in a wait or
test routine (e.g., MPI_Wait ).
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; 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.
SEE ALSO
MPI_Isend
MPI_Irecv
MPI_Issend
MPI_Ibsend
MPI_Irsend
MPI_Recv_init
MPI_Send_init
MPI_Ssend_init
MPI_Rsend_init
MPI_Test
MPI_Wait
MPI_Waitall
MPI_Waitany
MPI_Waitsome
MPI_Testall
MPI_Testany
MPI_Testsome
4.1.6 Sep 30, 2023 MPI_Request_free(3)