MPI_REQUEST_FREE(3)				     Open MPI				       MPI_REQUEST_FREE(3)

MPI_Request_free — Frees a communication request object.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Request_free(MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: 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

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 <#mpi-wait> or MPI_Test <#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 == 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 IF (rank == 1) THEN
	      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	<#
       mpi-recv-init> or MPI_Send_init <#mpi-send-init> and friends. However, it can be used  to  free	a  request
       created with MPI_Irecv <#mpi-irecv> or MPI_Isend <#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 <#mpi-wait> ).

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Isend <#mpi-isend>

	  • MPI_Irecv <#mpi-irecv>

	  • MPI_Issend <#mpi-issend>

	  • MPI_Ibsend <#mpi-ibsend>

	  • MPI_Irsend <#mpi-irsend>

	  • MPI_Recv_init <#mpi-recv-init>

	  • MPI_Send_init <#mpi-send-init>

	  • MPI_Ssend_init <#mpi-ssend-init>

	  • MPI_Rsend_init <#mpi-rsend-init>

	  • MPI_Test <#mpi-test>

	  • MPI_Wait <#mpi-wait>

	  • MPI_Waitall <#mpi-waitall>

	  • MPI_Waitany <#mpi-waitany>

	  • MPI_Waitsome <#mpi-waitsome>

	  • MPI_Testall <#mpi-testall>

	  • MPI_Testany <#mpi-testany>

	  • MPI_Testsome <#mpi-testsome>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				       MPI_REQUEST_FREE(3)
