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

4.1.6                                             Sep 30, 2023                                    MPI_Waitany(3)