MPI_Cancel(3)

NAME
MPI_Cancel - Cancels a communication request

SYNOPSIS
int MPI_Cancel(MPI_Request * request)

INPUT PARAMETERS
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 ).