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| <h4 class="subsection">13.10.3 Getting into a Consistent State</h4> |
| |
| <p>When dealing with asynchronous operations it is sometimes necessary to |
| get into a consistent state. This would mean for AIO that one wants to |
| know whether a certain request or a group of request were processed. |
| This could be done by waiting for the notification sent by the system |
| after the operation terminated, but this sometimes would mean wasting |
| resources (mainly computation time). Instead POSIX.1b defines two |
| functions which will help with most kinds of consistency. |
| |
| <p>The <code>aio_fsync</code> and <code>aio_fsync64</code> functions are only available |
| if the symbol <code>_POSIX_SYNCHRONIZED_IO</code> is defined in <samp><span class="file">unistd.h</span></samp>. |
| |
| <p><a name="index-synchronizing-1302"></a><!-- aio.h --> |
| <!-- POSIX.1b --> |
| |
| <div class="defun"> |
| — Function: int <b>aio_fsync</b> (<var>int op, struct aiocb *aiocbp</var>)<var><a name="index-aio_005ffsync-1303"></a></var><br> |
| <blockquote><p>Calling this function forces all I/O operations operating queued at the |
| time of the function call operating on the file descriptor |
| <code>aiocbp->aio_fildes</code> into the synchronized I/O completion state |
| (see <a href="Synchronizing-I_002fO.html#Synchronizing-I_002fO">Synchronizing I/O</a>). The <code>aio_fsync</code> function returns |
| immediately but the notification through the method described in |
| <code>aiocbp->aio_sigevent</code> will happen only after all requests for this |
| file descriptor have terminated and the file is synchronized. This also |
| means that requests for this very same file descriptor which are queued |
| after the synchronization request are not affected. |
| |
| <p>If <var>op</var> is <code>O_DSYNC</code> the synchronization happens as with a call |
| to <code>fdatasync</code>. Otherwise <var>op</var> should be <code>O_SYNC</code> and |
| the synchronization happens as with <code>fsync</code>. |
| |
| <p>As long as the synchronization has not happened, a call to |
| <code>aio_error</code> with the reference to the object pointed to by |
| <var>aiocbp</var> returns <code>EINPROGRESS</code>. Once the synchronization is |
| done <code>aio_error</code> return 0 if the synchronization was not |
| successful. Otherwise the value returned is the value to which the |
| <code>fsync</code> or <code>fdatasync</code> function would have set the |
| <code>errno</code> variable. In this case nothing can be assumed about the |
| consistency for the data written to this file descriptor. |
| |
| <p>The return value of this function is 0 if the request was |
| successfully enqueued. Otherwise the return value is -1 and |
| <code>errno</code> is set to one of the following values: |
| |
| <dl> |
| <dt><code>EAGAIN</code><dd>The request could not be enqueued due to temporary lack of resources. |
| <br><dt><code>EBADF</code><dd>The file descriptor <code>aiocbp->aio_fildes</code> is not valid or not open |
| for writing. |
| <br><dt><code>EINVAL</code><dd>The implementation does not support I/O synchronization or the <var>op</var> |
| parameter is other than <code>O_DSYNC</code> and <code>O_SYNC</code>. |
| <br><dt><code>ENOSYS</code><dd>This function is not implemented. |
| </dl> |
| |
| <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> this |
| function is in fact <code>aio_fsync64</code> since the LFS interface |
| transparently replaces the normal implementation. |
| </p></blockquote></div> |
| |
| <!-- aio.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>aio_fsync64</b> (<var>int op, struct aiocb64 *aiocbp</var>)<var><a name="index-aio_005ffsync64-1304"></a></var><br> |
| <blockquote><p>This function is similar to <code>aio_fsync</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_fsync</code> and so |
| transparently replaces the interface for small files on 32 bit |
| machines. |
| </p></blockquote></div> |
| |
| <p>Another method of synchronization is to wait until one or more requests of a |
| specific set terminated. This could be achieved by the <code>aio_*</code> |
| functions to notify the initiating process about the termination but in |
| some situations this is not the ideal solution. In a program which |
| constantly updates clients somehow connected to the server it is not |
| always the best solution to go round robin since some connections might |
| be slow. On the other hand letting the <code>aio_*</code> function notify the |
| caller might also be not the best solution since whenever the process |
| works on preparing data for on client it makes no sense to be |
| interrupted by a notification since the new client will not be handled |
| before the current client is served. For situations like this |
| <code>aio_suspend</code> should be used. |
| |
| <!-- aio.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Function: int <b>aio_suspend</b> (<var>const struct aiocb *const list</var>[]<var>, int nent, const struct timespec *timeout</var>)<var><a name="index-aio_005fsuspend-1305"></a></var><br> |
| <blockquote><p>When calling this function, the calling thread is suspended until at |
| least one of the requests pointed to by the <var>nent</var> elements of the |
| array <var>list</var> has completed. If any of the requests has already |
| completed at the time <code>aio_suspend</code> is called, the function returns |
| immediately. Whether a request has terminated or not is determined by |
| comparing the error status of the request with <code>EINPROGRESS</code>. If |
| an element of <var>list</var> is <code>NULL</code>, the entry is simply ignored. |
| |
| <p>If no request has finished, the calling process is suspended. If |
| <var>timeout</var> is <code>NULL</code>, the process is not woken until a request |
| has finished. If <var>timeout</var> is not <code>NULL</code>, the process remains |
| suspended at least as long as specified in <var>timeout</var>. In this case, |
| <code>aio_suspend</code> returns with an error. |
| |
| <p>The return value of the function is 0 if one or more requests |
| from the <var>list</var> have terminated. Otherwise the function returns |
| -1 and <code>errno</code> is set to one of the following values: |
| |
| <dl> |
| <dt><code>EAGAIN</code><dd>None of the requests from the <var>list</var> completed in the time specified |
| by <var>timeout</var>. |
| <br><dt><code>EINTR</code><dd>A signal interrupted the <code>aio_suspend</code> function. This signal might |
| also be sent by the AIO implementation while signalling the termination |
| of one of the requests. |
| <br><dt><code>ENOSYS</code><dd>The <code>aio_suspend</code> function is not implemented. |
| </dl> |
| |
| <p>When the sources are compiled with <code>_FILE_OFFSET_BITS == 64</code> this |
| function is in fact <code>aio_suspend64</code> since the LFS interface |
| transparently replaces the normal implementation. |
| </p></blockquote></div> |
| |
| <!-- aio.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>aio_suspend64</b> (<var>const struct aiocb64 *const list</var>[]<var>, int nent, const struct timespec *timeout</var>)<var><a name="index-aio_005fsuspend64-1306"></a></var><br> |
| <blockquote><p>This function is similar to <code>aio_suspend</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_suspend</code> and so |
| transparently replaces the interface for small files on 32 bit |
| machines. |
| </p></blockquote></div> |
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