Provided by: openmpi-doc_2.1.1-8_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
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