## 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]