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MPI_Cancel(3)                         MPI                        MPI_Cancel(3)


NAME

       MPI_Cancel -  Cancels a communication request


SYNOPSIS

       int MPI_Cancel(MPI_Request *request)


INPUT PARAMETER

       request
              - communication request (handle)


NOTES

       The primary expected use of MPI_Cancel is in multi-buffering schemes,
       where speculative MPI_Irecvs are made.  When the computation completes,
       some of these receive requests may remain; using MPI_Cancel allows the
       user to cancel these unsatisfied requests.

       Cancelling a send operation is much more difficult, in large part
       because the send will usually be at least partially complete (the
       information on the tag, size, and source are usually sent immediately
       to the destination).  Users are advised that cancelling a send, while a
       local operation (as defined by the MPI standard), is likely to be
       expensive (usually generating one or more internal messages).


THREAD AND INTERRUPT SAFETY

       This routine is thread-safe.  This means that this routine may be
       safely used by multiple threads without the need for any user-provided
       thread locks.  However, the routine is not interrupt safe.  Typically,
       this is due to the use of memory allocation routines such as malloc or
       other non-MPICH runtime routines that are themselves not interrupt-
       safe.


NOTES FOR FORTRAN

       All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have
       an additional argument ierr at the end of the argument list.  ierr is
       an integer and has the same meaning as the return value of the routine
       in C.  In Fortran, MPI routines are subroutines, and are invoked with
       the call statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in
       Fortran.


NULL HANDLES

       The MPI 1.1 specification, in the section on opaque objects, explicitly
       disallows freeing a null communicator.  The text from the standard is:
       A null handle argument is an erroneous IN argument in MPI calls, unless an
       exception is explicitly stated in the text that defines the function. Such
       exception is allowed for handles to request objects in Wait and Test calls
       (sections Communication Completion and Multiple Completions ). Otherwise, a
       null handle can only be passed to a function that allocates a new object and
       returns a reference to it in the handle.


ERRORS

       All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error
       value; C routines as the value of the function and Fortran routines in
       the last argument.  Before the value is returned, the current MPI error
       handler is called.  By default, this error handler aborts the MPI job.
       The error handler may be changed with MPI_Comm_set_errhandler (for
       communicators), MPI_File_set_errhandler (for files), and
       MPI_Win_set_errhandler (for RMA windows).  The MPI-1 routine
       MPI_Errhandler_set may be used but its use is deprecated.  The
       predefined error handler MPI_ERRORS_RETURN may be used to cause error
       values to be returned.  Note that MPI does not guarentee that an MPI
       program can continue past an error; however, MPI implementations will
       attempt to continue whenever possible.

       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_REQUEST
              - Invalid MPI_Request .  Either null or, in the case of a
              MPI_Start or MPI_Startall , not a persistent request.
       MPI_ERR_ARG
              - Invalid argument.  Some argument is invalid and is not
              identified by a specific error class (e.g., MPI_ERR_RANK ).


LOCATION

       src/mpi/pt2pt/cancel.c

                                   10/3/2012                     MPI_Cancel(3)

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