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| <h4 class="subsection">13.10.4 Cancellation of AIO Operations</h4> |
| |
| <p>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. |
| |
| <!-- aio.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>aio_cancel</b> (<var>int fildes, struct aiocb *aiocbp</var>)<var><a name="index-aio_005fcancel-1307"></a></var><br> |
| <blockquote><p>The <code>aio_cancel</code> function can be used to cancel one or more |
| outstanding requests. If the <var>aiocbp</var> parameter is <code>NULL</code>, the |
| function tries to cancel all of the outstanding requests which would process |
| the file descriptor <var>fildes</var> (i.e., whose <code>aio_fildes</code> member |
| is <var>fildes</var>). If <var>aiocbp</var> is not <code>NULL</code>, <code>aio_cancel</code> |
| attempts to cancel the specific request pointed to by <var>aiocbp</var>. |
| |
| <p>For requests which were successfully canceled, the normal notification |
| about the termination of the request should take place. I.e., depending |
| on the <code>struct sigevent</code> 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. |
| |
| <p>After a request is successfully canceled, a call to <code>aio_error</code> with |
| a reference to this request as the parameter will return |
| <code>ECANCELED</code> and a call to <code>aio_return</code> will return -1. |
| If the request wasn't canceled and is still running the error status is |
| still <code>EINPROGRESS</code>. |
| |
| <p>The return value of the function is <code>AIO_CANCELED</code> 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 <code>AIO_NOTCANCELED</code>. In this case <code>aio_error</code> |
| must be used to find out which of the, perhaps multiple, requests (in |
| <var>aiocbp</var> is <code>NULL</code>) weren't successfully canceled. If all |
| requests already terminated at the time <code>aio_cancel</code> is called the |
| return value is <code>AIO_ALLDONE</code>. |
| |
| <p>If an error occurred during the execution of <code>aio_cancel</code> the |
| function returns -1 and sets <code>errno</code> to one of the following |
| values. |
| |
| <dl> |
| <dt><code>EBADF</code><dd>The file descriptor <var>fildes</var> is not valid. |
| <br><dt><code>ENOSYS</code><dd><code>aio_cancel</code> is not implemented. |
| </dl> |
| |
| <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code>, this |
| function is in fact <code>aio_cancel64</code> since the LFS interface |
| transparently replaces the normal implementation. |
| </p></blockquote></div> |
| |
| <!-- aio.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>aio_cancel64</b> (<var>int fildes, struct aiocb64 *aiocbp</var>)<var><a name="index-aio_005fcancel64-1308"></a></var><br> |
| <blockquote><p>This function is similar to <code>aio_cancel</code> with the only difference |
| that the argument is a reference to a variable of type <code>struct |
| aiocb64</code>. |
| |
| <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code>, this |
| function is available under the name <code>aio_cancel</code> and so |
| transparently replaces the interface for small files on 32 bit |
| machines. |
| </p></blockquote></div> |
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