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