The aio_cancel function attempts to cancel outstanding asynchronous I/O
requests for the file descriptor fd. If aiocbp is NULL, all such
requests are cancelled. Otherwise, only the request described by the control
block pointed to by aiocbp is cancelled.
Normal asynchronous notification occurs for cancelled requests. The request
return status is set to -1, and the request error status is set to ECANCELED.
The control block of requests that cannot be cancelled is not changed.
If aiocbp is not NULL, and fd differs from the file descriptor
with which the asynchronous operation was initiated, unspecified results
Which operations are cancelable is implementation-defined.
This function returns AIO_CANCELED if all requests were successfully cancelled.
It returns AIO_NOTCANCELED when at least one of the requests specified was not
cancelled because it was in progress. In this case one may check the status of
individual requests using aio_error(3). This function returns
AIO_ALLDONE when all requests had been completed already before this call.
When some error occurs, -1 is returned, and errno is set appropriately.