int MPI_Cancel(
  MPI_Request *request
);

Parameters

request

[in] communication request (handle)

Remarks

The MPI_CANCEL operation allows pending communications to be canceled. This is required for cleanup. Posting a send or a receive ties up user resources (send or receive buffers), and a cancel may be needed to free these resources gracefully.

A call to MPI_CANCEL marks for cancellation a pending, nonblocking communication operation (send or receive). The cancel call is local. It returns immediately, possibly before the communication is actually canceled. It is still necessary to complete a communication that has been marked for cancellation, using a call to MPI_REQUEST_FREE, MPI_WAIT or MPI_TEST (or any of the derived operations).

If a communication is marked for cancellation, then a MPI_WAIT call for that communication is guaranteed to return, irrespective of the activities of other processes (i.e., MPI_WAIT behaves as a local function); similarly if MPI_TEST is repeatedly called in a busy wait loop for a canceled communication, then MPI_TEST will eventually be successful.

MPI_CANCEL can be used to cancel a communication that uses a persistent request, in the same way it is used for nonpersistent requests. A successful cancellation cancels the active communication, but not the request itself. After the call to MPI_CANCEL and the subsequent call to MPI_WAIT or MPI_TEST, the request becomes inactive and can be activated for a new communication.

The successful cancellation of a buffered send frees the buffer space occupied by the pending message.

Either the cancellation succeeds, or the communication succeeds, but not both. If a send is marked for cancellation, then it must be the case that either the send completes normally, in which case the message sent was received at the destination process, or that the send is successfully canceled, in which case no part of the message was received at the destination. Then, any matching receive has to be satisfied by another send. If a receive is marked for cancellation, then it must be the case that either the receive completes normally, or that the receive is successfully canceled, in which case no part of the receive buffer is altered. Then, any matching send has to be satisfied by another receive.

If the operation has been canceled, then information to that effect will be returned in the status argument of the operation that completes the communication.

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

Null Handles

 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).

Example Code

The following sample code illustrates MPI_Cancel.