17.2.33. MPI_Cancel — Open MPI 5.0.3 documentation

# 17.2.33. MPI_Cancel

MPI_Cancel — Cancels a communication request.

## 17.2.33.1. SYNTAX

### 17.2.33.1.1. C Syntax

    #include <mpi.h>

    int MPI_Cancel(MPI_Request *request)

### 17.2.33.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'

    MPI_CANCEL(REQUEST, IERROR)
        INTEGER REQUEST, IERROR

### 17.2.33.1.3. Fortran 2008 Syntax

    USE mpi_f08

    MPI_Cancel(request, ierror)
        TYPE(MPI_Request), INTENT(IN) :: request
        INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.33.2. INPUT PARAMETER

- `request` : Communication request (handle).

## 17.2.33.3. OUTPUT PARAMETER

- `ierror` : Fortran only: Error status (integer).

## 17.2.33.4. DESCRIPTION

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 an 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 (see Section 3.9 in the MPI Standard, “Persistent Communication Requests”) 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 is received at the destination process, or that the send is successfully canceled, in which case no part of the message is 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.

## 17.2.33.5. 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 requests may remain; using MPI_Cancel allows the user to cancel these unsatisfied requests.

## 17.2.33.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Probe