= nng_aio_defer(3)
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//
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// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
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// Copyright 2018 Capitar IT Group BV <info@capitar.com>
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//
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// This document is supplied under the terms of the MIT License, a
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// copy of which should be located in the distribution where this
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// file was obtained (LICENSE.txt). A copy of the license may also be
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// found online at https://opensource.org/licenses/MIT.
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//
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== NAME
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nng_aio_defer - defer asynchronous I/O operation
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== SYNOPSIS
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[source, c]
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----
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#include <nng/nng.h>
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typedef void (*nng_aio_cancelfn)(nng_aio *aio, void *arg, int err);
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void nng_aio_defer(nng_aio *aio, nng_aio_cancelfn fn, void *arg);
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----
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== DESCRIPTION
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The `nng_aio_defer()` function marks operation associated with _aio_ as
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being deferred for asynchronous completion, and also registers a cancellation
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function _fn_ and associated argument _arg_, thereby
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permitting the operation to be canceled.
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If the _aio_ is being canceled, the cancellation routine _fn_ will be called
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with the _aio_, the _arg_ specified by `nng_aio_defer()`, and an error
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value in _err_, which is the reason that the operation is being canceled.
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The operation may not be cancelable; for example it may have already been
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completed, or be in a state where it is no longer possible to unschedule it.
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In this case, the _cancelfn_ should just return without making any changes.
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If the cancellation routine successfully canceled the operation, it should
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ensure that xref:nng_aio_finish.3.adoc[`nng_aio_finish()`] is called, with the
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error code specified by _err_.
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IMPORTANT: It is mandatory that I/O providers call
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xref:nng_aio_finish.3.adoc[`nng_aio_finish()`]
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*EXACTLY ONCE* when they are finished with the operation.
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NOTE: This function is only for I/O providers (those actually performing
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the operation such as HTTP handler functions or transport providers); ordinary
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users of the _aio_ should not call this function.
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NOTE: Care must be taken to ensure that cancellation and completion of
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the routine are multi-thread safe; this will usually involve the use
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of locks or other synchronization primitives.
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TIP: For operations that complete synchronously, without any need to be
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deferred, the provider should not bother to call `nng_aio_defer()`,
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although it is harmless if it does.
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== RETURN VALUES
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None.
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== ERRORS
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None.
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== SEE ALSO
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[.text-left]
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xref:nng_aio_alloc.3.adoc[nng_aio_alloc(3)],
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xref:nng_aio_cancel.3.adoc[nng_aio_cancel(3)],
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xref:nng_aio_finish.3.adoc[nng_aio_finish(3)],
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xref:nng_aio_result.3.adoc[nng_aio_result(3)],
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xref:nng_aio.5.adoc[nng_aio(5)],
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xref:nng.7.adoc[nng(7)]
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