NAME

       MPI_Waitany - Waits for any specified send or receive to complete.

SYNTAX

C Syntax

       #include <mpi.h>
       int MPI_Waitany(int count, MPI_Request array_of_requests[],
            int *index, MPI_Status *status)

Fortran Syntax

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_WAITANY(COUNT, ARRAY_OF_REQUESTS, INDEX, STATUS, IERROR)
            INTEGER   COUNT, ARRAY_OF_REQUESTS(*), INDEX
            INTEGER   STATUS(MPI_STATUS_SIZE), IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Waitany(count, array_of_requests, index, status, ierror)
            INTEGER, INTENT(IN) :: count
            TYPE(MPI_Request), INTENT(INOUT) :: array_of_requests(count)
            INTEGER, INTENT(OUT) :: index
            TYPE(MPI_Status) :: status
            INTEGER, OPTIONAL, INTENT(OUT) :: ierror

C++ Syntax

       #include <mpi.h>
       static int Request::Waitany(int count, Request
            array_of_requests[], Status& status)

       static int Request::Waitany(int count, Request array_of_requests[])

INPUT PARAMETERS

       count     List length (integer).

       array_of_requests
                 Array of requests (array of handles).

OUTPUT PARAMETERS

       index     Index of handle for operation that completed (integer). In the range 0 to count-1.  In Fortran,
                 the range is 1 to count.

       status    Status object (status).

       IERROR    Fortran only: Error status (integer).

DESCRIPTION

       A call to MPI_Waitany can be used to wait for the completion of one out of several requests.

       The array_of_requests list may contain null or inactive handles. If the list contains no  active  handles
       (list  has length zero or all entries are null or inactive), then the call returns immediately with index
       = MPI_UNDEFINED, and an empty status.

       The execution of MPI_Waitany(count,  array_of_requests,  index,  status)  has  the  same  effect  as  the
       execution  of MPI_Wait(&array_of_requests[i], status), where i is the value returned by index (unless the
       value of index is MPI_UNDEFINED). MPI_Waitany with an array containing one active entry is equivalent  to
       MPI_Wait.

       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.

       Example: Client-server code (starvation can occur).

           CALL MPI_COMM_SIZE(comm, size, ierr)
           CALL MPI_COMM_RANK(comm, rank, ierr)
           IF(rank .GT 0) THEN         ! client code
               DO WHILE(.TRUE.)
                  CALL MPI_ISEND(a, n, MPI_REAL, 0, tag, comm, request, ierr)
                  CALL MPI_WAIT(request, status, ierr)
               END DO
           ELSE         ! rank=0 -- server code
                  DO i=1, size-1
                     CALL MPI_IRECV(a(1,i), n, MPI_REAL, i tag,
                              comm, request_list(i), ierr)
                  END DO
                  DO WHILE(.TRUE.)
                     CALL MPI_WAITANY(size-1, request_list, index, status, ierr)
                     CALL DO_SERVICE(a(1,index))  ! handle one message
                     CALL MPI_IRECV(a(1, index), n, MPI_REAL, index, tag,
                               comm, request_list(index), ierr)
                  END DO
           END IF

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, MPI_File_set_errhandler, or MPI_Win_set_errhandler (depending on the type of MPI
       handle that generated the request); 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.

       Note that per MPI-1 section 3.2.5, MPI exceptions on requests  passed  to  MPI_WAITANY  do  not  set  the
       status.MPI_ERROR  field  in  the returned status.  The error code is passed to the back-end error handler
       and may be passed back to the caller through the return  value  of  MPI_WAITANY  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_File_set_errhandler
       MPI_Test
       MPI_Testall
       MPI_Testany
       MPI_Testsome
       MPI_Wait
       MPI_Waitall
       MPI_Waitsome
       MPI_Win_set_errhandler