---
name: "nng_aio_defer"
section: "3"
platform: "linux"
package: "libnng-dev"
version: "1.10.1-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/nng_aio_defer"
---

# nng\_aio\_defer(3)

defer asynchronous I/O operation

## NAME

nng\_aio\_defer - defer asynchronous I/O operation

## SYNOPSIS

```
#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 `nng_aio_finish()` is called, with the error code specified by *err*.

**Important**\

It is mandatory that I/O providers call `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

[nng\_aio\_alloc(3)](/nng_aio_alloc/3.md), [nng\_aio\_cancel(3)](/nng_aio_cancel/3.md), [nng\_aio\_finish(3)](/nng_aio_finish/3.md), [nng\_aio\_result(3)](/nng_aio_result/3.md), [nng\_aio(5)](/nng_aio/5.md), [nng(7)](/nng/7.md)