.TH MPI_Cancel 3 "11/12/2019" " " "MPI"

.SH NAME

MPI_Cancel \-  Cancels a communication request 

.SH SYNOPSIS

.nf

int MPI_Cancel(MPI_Request * request)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B request 

- communication request (handle)

.PD 1

.SH NOTES

The primary expected use of 

.I MPI_Cancel

is in multi-buffering

schemes, where speculative 

.I MPI_Irecvs

are made.  When the computation

completes, some of these receive requests may remain; using 

.I 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).

.SH 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 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 

.I MPI_WTIME

and 

.I MPI_WTICK

) have

an additional argument 

.I ierr

at the end of the argument list.  

.I 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

.I call

statement.

All MPI objects (e.g., 

.I MPI_Datatype

, 

.I MPI_Comm

) are of type 

.I INTEGER

in Fortran.

.SH 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:

.nf

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.

.fi

.SH ERRORS

All MPI routines (except 

.I MPI_Wtime

and 

.I 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 

.I MPI_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  The predefined error handler

.I MPI_ERRORS_RETURN

may be used to cause error values to be returned.

Note that MPI does 

.B not

guarentee that an MPI program can continue past

an error; however, MPI implementations will attempt to continue whenever

possible.

.PD 0

.TP

.B MPI_SUCCESS 

- No error; MPI routine completed successfully.

.PD 1

.PD 0

.TP

.B MPI_ERR_REQUEST 

- Invalid 

.I MPI_Request

\&.

Either null or, in the case of a

.I MPI_Start

or 

.I MPI_Startall

, not a persistent request.

.PD 1

.PD 0

.TP

.B MPI_ERR_ARG 

- Invalid argument.  Some argument is invalid and is not

identified by a specific error class (e.g., 

.I MPI_ERR_RANK

).

.PD 1