plucky (3) MPI_Cancel.openmpi.3.gz
SYNTAX
C Syntax
#include <mpi.h>
int MPI_Cancel(MPI_Request *request)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_CANCEL(REQUEST, IERROR)
INTEGER REQUEST, IERROR
Fortran 2008 Syntax
USE mpi_f08
MPI_Cancel(request, ierror)
TYPE(MPI_Request), INTENT(IN) :: request
INTEGER, OPTIONAL, INTENT(OUT) :: ierror
INPUT PARAMETER
• request : Communication request (handle).
OUTPUT PARAMETER
• ierror : Fortran only: Error status (integer).
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.
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.
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
COPYRIGHT
2003-2025, The Open MPI Community
Feb 17, 2025 MPI_CANCEL(3)