C Syntax

       2003-2025, The Open MPI Community

                                                  Jun 07, 2025                                       MPI_WAIT(3)

       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 MPIStandard.  The status object for a send operation may be queried  by  a
       call to MPI_Test_cancelled (see the “Probe and Cancel” section in the MPIStandard).

       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.

       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_thread,  after
       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 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_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 was called on that communicator. If called on a
         window or file, acts as if 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 then MPI_Comm_set_errhandler

       • MPI_File_create_errhandler then MPI_File_set_errhandler

       • MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

       • MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

       See the MPImanpage for a full list of MPIerrorcodes.

       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 MPIStandard, 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.

       SEEALSO:

          • MPI_Comm_set_errhandler

          • MPI_File_set_errhandler

          • MPI_Test

          • MPI_Testall

          • MPI_Testany

          • MPI_Testsome

          • MPI_Waitall

          • MPI_Waitany

          • MPI_Waitsome

          • MPI_Win_set_errhandler

       • request: Request (handle).

       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 the “Probe and Cancel”  section  in  the  MPIStandard).   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).

       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

       • status: Status object (status).

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

CSyntax
          #include <mpi.h>

          int MPI_Wait(MPI_Request *request, MPI_Status *status)

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

   Fortran2008Syntax
          USE mpi_f08
          MPI_Wait(request, status, ierror)
               TYPE(MPI_Request), INTENT(INOUT) :: request
               TYPE(MPI_Status) :: status
               INTEGER, OPTIONAL, INTENT(OUT) :: ierror