= nng_aio_defer(3)
//
// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
// Copyright 2018 Capitar IT Group BV <info@capitar.com>
//
// This document is supplied under the terms of the MIT License, a
// copy of which should be located in the distribution where this
// file was obtained (LICENSE.txt).  A copy of the license may also be
// found online at https://opensource.org/licenses/MIT.
//

== NAME

nng_aio_defer - defer asynchronous I/O operation

== SYNOPSIS

[source, c]
----
#include <nng/nng.h>

typedef void (*nng_aio_cancelfn)(nng_aio *aio, void *arg, int err);

void nng_aio_defer(nng_aio *aio, nng_aio_cancelfn fn, void *arg);
----

== DESCRIPTION

The `nng_aio_defer()` function marks operation associated with _aio_ as
being deferred for asynchronous completion, and also registers a cancellation
function _fn_ and associated argument _arg_, thereby
permitting the operation to be canceled.

If the _aio_ is being canceled, the cancellation routine _fn_ will be called
with the _aio_, the _arg_ specified by `nng_aio_defer()`, and an error
value in _err_, which is the reason that the operation is being canceled.

The operation may not be cancelable; for example it may have already been
completed, or be in a state where it is no longer possible to unschedule it.
In this case, the _cancelfn_ should just return without making any changes.

If the cancellation routine successfully canceled the operation, it should
ensure that xref:nng_aio_finish.3.adoc[`nng_aio_finish()`] is called, with the
error code specified by _err_.

IMPORTANT: It is mandatory that I/O providers call
xref:nng_aio_finish.3.adoc[`nng_aio_finish()`]
*EXACTLY ONCE* when they are finished with the operation.

NOTE: This function is only for I/O providers (those actually performing
the operation such as HTTP handler functions or transport providers); ordinary
users of the _aio_ should not call this function.

NOTE: Care must be taken to ensure that cancellation and completion of
the routine are multi-thread safe; this will usually involve the use
of locks or other synchronization primitives.

TIP: For operations that complete synchronously, without any need to be
deferred, the provider should not bother to call `nng_aio_defer()`,
although it is harmless if it does.

== RETURN VALUES

None.

== ERRORS

None.

== SEE ALSO

[.text-left]
xref:nng_aio_alloc.3.adoc[nng_aio_alloc(3)],
xref:nng_aio_cancel.3.adoc[nng_aio_cancel(3)],
xref:nng_aio_finish.3.adoc[nng_aio_finish(3)],
xref:nng_aio_result.3.adoc[nng_aio_result(3)],
xref:nng_aio.5.adoc[nng_aio(5)],
xref:nng.7.adoc[nng(7)]