MPI_WAIT(3)					     Open MPI					       MPI_WAIT(3)

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

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Wait(MPI_Request *request, MPI_Status *status)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_WAIT(REQUEST, STATUS, IERROR)
	       INTEGER REQUEST, STATUS(MPI_STATUS_SIZE), IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Wait(request, status, ierror)
	       TYPE(MPI_Request), INTENT(INOUT) :: request
	       TYPE(MPI_Status) :: status
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETER
       • request: Request (handle).

OUTPUT PARAMETERS
       • status: Status object (status).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       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 the “Return Status” subsection of the “Point-to-Point
       Communication” chapter in the MPI Standard <https://www.mpi-forum.org/docs/>.  The status object for a send
       operation may be queried by a call to MPI_Test_cancelled <#mpi-test-cancelled> (see the “Probe and  Cancel”
       section in the MPI Standard <https://www.mpi-forum.org/docs/>).

       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.

NOTES
       Successful  return  of  MPI_Wait after an MPI_Ibsend <#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	<#
       mpi-buffer-attach>.  Note  that, at this point, we can no longer cancel the send (for more information, see
       the “Probe and Cancel” section in the  MPI  Standard  <https://www.mpi-forum.org/docs/>).   If  a  matching
       receive	is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of
       MPI_Cancel <#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 == 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 IF (rank == 1) THEN
	      CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr)
	      **** do some computation ****
	      CALL MPI_WAIT(request, status, ierr)
	  END IF

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.

       Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI Standard
       <https://www.mpi-forum.org/docs/>,  MPI	errors	on  requests  passed  to   MPI_Wait   do   not	 set   the
       status.MPI_ERROR	 field	in  the	 returned  status.   The error code is always 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:

	  • MPI_Comm_set_errhandler <#mpi-comm-set-errhandler>

	  • MPI_File_set_errhandler <#mpi-file-set-errhandler>

	  • MPI_Test <#mpi-test>

	  • MPI_Testall <#mpi-testall>

	  • MPI_Testany <#mpi-testany>

	  • MPI_Testsome <#mpi-testsome>

	  • MPI_Waitall <#mpi-waitall>

	  • MPI_Waitany <#mpi-waitany>

	  • MPI_Waitsome <#mpi-waitsome>

	  • MPI_Win_set_errhandler <#mpi-win-set-errhandler>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					       MPI_WAIT(3)
