#### 13.10.4 Cancellation of AIO Operations

When one or more requests are asynchronously processed, it might be useful in some situations to cancel a selected operation, e.g., if it becomes obvious that the written data is no longer accurate and would have to be overwritten soon. As an example, assume an application, which writes data in files in a situation where new incoming data would have to be written in a file which will be updated by an enqueued request. The POSIX AIO implementation provides such a function, but this function is not capable of forcing the cancellation of the request. It is up to the implementation to decide whether it is possible to cancel the operation or not. Therefore using this function is merely a hint.

— Function: int aio_cancel (int fildes, struct aiocb *aiocbp)

> The `aio_cancel` function can be used to cancel one or more outstanding requests. If the aiocbp parameter is `NULL`, the function tries to cancel all of the outstanding requests which would process the file descriptor fildes (i.e., whose `aio_fildes` member is fildes). If aiocbp is not `NULL`, `aio_cancel` attempts to cancel the specific request pointed to by aiocbp.

> For requests which were successfully canceled, the normal notification about the termination of the request should take place. I.e., depending on the `struct sigevent` object which controls this, nothing happens, a signal is sent or a thread is started. If the request cannot be canceled, it terminates the usual way after performing the operation.

> After a request is successfully canceled, a call to `aio_error` with a reference to this request as the parameter will return `ECANCELED` and a call to `aio_return` will return -1. If the request wasn't canceled and is still running the error status is still `EINPROGRESS`.

> The return value of the function is `AIO_CANCELED` if there were requests which haven't terminated and which were successfully canceled. If there is one or more requests left which couldn't be canceled, the return value is `AIO_NOTCANCELED`. In this case `aio_error` must be used to find out which of the, perhaps multiple, requests (in aiocbp is `NULL`) weren't successfully canceled. If all requests already terminated at the time `aio_cancel` is called the return value is `AIO_ALLDONE`.

> If an error occurred during the execution of `aio_cancel` the function returns -1 and sets `errno` to one of the following values.

> `EBADF`
>   The file descriptor fildes is not valid.
> `ENOSYS`
>   `aio_cancel` is not implemented.

> When the sources are compiled with `_FILE_OFFSET_BITS == 64`, this function is in fact `aio_cancel64` since the LFS interface transparently replaces the normal implementation.

— Function: int aio_cancel64 (int fildes, struct aiocb64 *aiocbp)

> This function is similar to `aio_cancel` with the only difference that the argument is a reference to a variable of type `struct aiocb64`.

> When the sources are compiled with `_FILE_OFFSET_BITS == 64`, this function is available under the name `aio_cancel` and so transparently replaces the interface for small files on 32 bit machines.