17.2.422. MPI_Waitany

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

17.2.422.1. SYNTAX

17.2.422.1.1. C Syntax

#include <mpi.h>

int MPI_Waitany(int count, MPI_Request array_of_requests[],
     int *index, MPI_Status *status)

17.2.422.1.2. 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

17.2.422.1.3. 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

17.2.422.2. INPUT PARAMETERS

  • count: List length (integer).

  • array_of_requests: Array of requests (array of handles).

17.2.422.3. 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).

17.2.422.4. 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

17.2.422.5. ERRORS

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

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 errors 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.