NAME

       MPI_Cancel -  Cancels a communication request

SYNOPSIS

       #include <mpi.h>
       int MPI_Cancel(MPI_Request *preq)

INPUT PARAMETER

       preq   - communication request (handle)

NOTES

       Cancel  has  only  been  implemented  for receive requests; it is a no-op for send requests.  The primary
       expected use of MPI_Cancel is in multi-buffering schemes, where speculative  MPI_Irecv  calls  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).  As such, LAM/MPI has not implemented the canceling of send operations.

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."

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.

ERRORS

       If an error occurs in an MPI function, the current MPI error handler is called to handle it.  By default,
       this  error  handler  aborts the MPI job.  The error handler may be changed with MPI_Errhandler_set ; the
       predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be  returned  (in  C  and
       Fortran;  this  error  handler is less useful in with the C++ MPI bindings.  The predefined error handler
       MPI::ERRORS_THROW_EXCEPTIONS should be used in C++ if the error value needs to be recovered).  Note  that
       MPI does not guarantee that an MPI program can continue past an error.

       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.  The C++ bindings for MPI do not return error values;
       instead, error values are communicated  by  throwing  exceptions  of  type  MPI::Exception  (but  not  by
       default).  Exceptions are only thrown if the error value is not MPI::SUCCESS .

       Note that if the MPI::ERRORS_RETURN handler is set in C++, while MPI functions will return upon an error,
       there will be no way to recover what the actual error value was.
       MPI_SUCCESS
              - No error; MPI routine completed successfully.
       MPI_ERR_REQUEST
              -  Invalid  MPI_Request  ;  either  null  or,  MPI_REQUEST_NULL  ,  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.
              This is typically a NULL pointer or other such error.

MORE INFORMATION

       For  more  information,  please  see the official MPI Forum web site, which contains the text of both the
       MPI-1 and MPI-2 standards.  These documents contain detailed information about each MPI function (most of
       which is not duplicated in these man pages).

       http://www.mpi-forum.org/

ACKNOWLEDGEMENTS

       The LAM Team would like the thank the MPICH Team for the handy program to generate man  pages  ("doctext"
       from  ftp://ftp.mcs.anl.gov/pub/sowing/sowing.tar.gz ), the initial formatting, and some initial text for
       most of the MPI-1 man pages.

LOCATION

       cancel.c

LAM/MPI 7.1.4                                       6/24/2006                                      MPI_Cancel(3)