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## nng_aio_defer
Defer asynchronous I/O operation.
### Synopsis
```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 the operation associated with _aio_ as deferred for asynchronous completion, registering a cancellation function _fn_ and associated argument _arg_.
This permits the operation to be canceled.
If the _aio_ is 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 for cancellation.
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.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.adoc[`nng_aio_finish`] *exactly once* when they are finished with the operation.
IMPORTANT: Care must be taken to ensure that cancellation and completion of the routine are thread-safe.
This will usually involve the use of locks or other synchronization primitives.
TIP: 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.
TIP: When completing an operation synchronously, calling `nng_aio_defer` is unnecessary.
### See Also
xref:../aio/nng_aio_cancel.adoc[nng_aio_cancel],
xref:nng_aio_finish.adoc[nng_aio_finish],
xref:../aio/nng_aio_result.adoc[nng_aio_result]
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