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 canceled. Otherwise, only the request described by the control block pointed to by aiocbp is canceled. (See aio(7) for a description of the aiocb structure.)
Normal asynchronous notification occurs for canceled requests (see aio(7) and sigevent(7)). The request return status (aio_return(3)) is set to -1, and the request error status (aio_error(3)) is set to ECANCELED. The control block of requests that cannot be canceled is not changed.
If the request could not be canceled, then it will terminate in the usual way after performing the I/O operation. (In this case, aio_error(3) will return the status EINPROGRESSS.)
If aiocbp is not NULL, and fd differs from the file descriptor with which the asynchronous operation was initiated, unspecified results occur.
Which operations are cancelable is implementation-defined.