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| <h3 class="section">13.10 Perform I/O Operations in Parallel</h3> |
| |
| <p>The POSIX.1b standard defines a new set of I/O operations which can |
| significantly reduce the time an application spends waiting at I/O. The |
| new functions allow a program to initiate one or more I/O operations and |
| then immediately resume normal work while the I/O operations are |
| executed in parallel. This functionality is available if the |
| <samp><span class="file">unistd.h</span></samp> file defines the symbol <code>_POSIX_ASYNCHRONOUS_IO</code>. |
| |
| <p>These functions are part of the library with realtime functions named |
| <samp><span class="file">librt</span></samp>. They are not actually part of the <samp><span class="file">libc</span></samp> binary. |
| The implementation of these functions can be done using support in the |
| kernel (if available) or using an implementation based on threads at |
| userlevel. In the latter case it might be necessary to link applications |
| with the thread library <samp><span class="file">libpthread</span></samp> in addition to <samp><span class="file">librt</span></samp>. |
| |
| <p>All AIO operations operate on files which were opened previously. There |
| might be arbitrarily many operations running for one file. The |
| asynchronous I/O operations are controlled using a data structure named |
| <code>struct aiocb</code> (<dfn>AIO control block</dfn>). It is defined in |
| <samp><span class="file">aio.h</span></samp> as follows. |
| |
| <!-- aio.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Data Type: <b>struct aiocb</b><var><a name="index-struct-aiocb-1287"></a></var><br> |
| <blockquote><p>The POSIX.1b standard mandates that the <code>struct aiocb</code> structure |
| contains at least the members described in the following table. There |
| might be more elements which are used by the implementation, but |
| depending upon these elements is not portable and is highly deprecated. |
| |
| <dl> |
| <dt><code>int aio_fildes</code><dd>This element specifies the file descriptor to be used for the |
| operation. It must be a legal descriptor, otherwise the operation will |
| fail. |
| |
| <p>The device on which the file is opened must allow the seek operation. |
| I.e., it is not possible to use any of the AIO operations on devices |
| like terminals where an <code>lseek</code> call would lead to an error. |
| |
| <br><dt><code>off_t aio_offset</code><dd>This element specifies the offset in the file at which the operation (input |
| or output) is performed. Since the operations are carried out in arbitrary |
| order and more than one operation for one file descriptor can be |
| started, one cannot expect a current read/write position of the file |
| descriptor. |
| |
| <br><dt><code>volatile void *aio_buf</code><dd>This is a pointer to the buffer with the data to be written or the place |
| where the read data is stored. |
| |
| <br><dt><code>size_t aio_nbytes</code><dd>This element specifies the length of the buffer pointed to by <code>aio_buf</code>. |
| |
| <br><dt><code>int aio_reqprio</code><dd>If the platform has defined <code>_POSIX_PRIORITIZED_IO</code> and |
| <code>_POSIX_PRIORITY_SCHEDULING</code>, the AIO requests are |
| processed based on the current scheduling priority. The |
| <code>aio_reqprio</code> element can then be used to lower the priority of the |
| AIO operation. |
| |
| <br><dt><code>struct sigevent aio_sigevent</code><dd>This element specifies how the calling process is notified once the |
| operation terminates. If the <code>sigev_notify</code> element is |
| <code>SIGEV_NONE</code>, no notification is sent. If it is <code>SIGEV_SIGNAL</code>, |
| the signal determined by <code>sigev_signo</code> is sent. Otherwise, |
| <code>sigev_notify</code> must be <code>SIGEV_THREAD</code>. In this case, a thread |
| is created which starts executing the function pointed to by |
| <code>sigev_notify_function</code>. |
| |
| <br><dt><code>int aio_lio_opcode</code><dd>This element is only used by the <code>lio_listio</code> and |
| <code>lio_listio64</code> functions. Since these functions allow an |
| arbitrary number of operations to start at once, and each operation can be |
| input or output (or nothing), the information must be stored in the |
| control block. The possible values are: |
| |
| <dl> |
| <dt><code>LIO_READ</code><a name="index-LIO_005fREAD-1288"></a><dd>Start a read operation. Read from the file at position |
| <code>aio_offset</code> and store the next <code>aio_nbytes</code> bytes in the |
| buffer pointed to by <code>aio_buf</code>. |
| |
| <br><dt><code>LIO_WRITE</code><a name="index-LIO_005fWRITE-1289"></a><dd>Start a write operation. Write <code>aio_nbytes</code> bytes starting at |
| <code>aio_buf</code> into the file starting at position <code>aio_offset</code>. |
| |
| <br><dt><code>LIO_NOP</code><a name="index-LIO_005fNOP-1290"></a><dd>Do nothing for this control block. This value is useful sometimes when |
| an array of <code>struct aiocb</code> values contains holes, i.e., some of the |
| values must not be handled although the whole array is presented to the |
| <code>lio_listio</code> function. |
| </dl> |
| </dl> |
| |
| <p>When the sources are compiled using <code>_FILE_OFFSET_BITS == 64</code> on a |
| 32 bit machine, this type is in fact <code>struct aiocb64</code>, since the LFS |
| interface transparently replaces the <code>struct aiocb</code> definition. |
| </p></blockquote></div> |
| |
| <p>For use with the AIO functions defined in the LFS, there is a similar type |
| defined which replaces the types of the appropriate members with larger |
| types but otherwise is equivalent to <code>struct aiocb</code>. Particularly, |
| all member names are the same. |
| |
| <!-- aio.h --> |
| <!-- POSIX.1b --> |
| <div class="defun"> |
| — Data Type: <b>struct aiocb64</b><var><a name="index-struct-aiocb64-1291"></a></var><br> |
| <blockquote><dl> |
| <dt><code>int aio_fildes</code><dd>This element specifies the file descriptor which is used for the |
| operation. It must be a legal descriptor since otherwise the operation |
| fails for obvious reasons. |
| |
| <p>The device on which the file is opened must allow the seek operation. |
| I.e., it is not possible to use any of the AIO operations on devices |
| like terminals where an <code>lseek</code> call would lead to an error. |
| |
| <br><dt><code>off64_t aio_offset</code><dd>This element specifies at which offset in the file the operation (input |
| or output) is performed. Since the operation are carried in arbitrary |
| order and more than one operation for one file descriptor can be |
| started, one cannot expect a current read/write position of the file |
| descriptor. |
| |
| <br><dt><code>volatile void *aio_buf</code><dd>This is a pointer to the buffer with the data to be written or the place |
| where the read data is stored. |
| |
| <br><dt><code>size_t aio_nbytes</code><dd>This element specifies the length of the buffer pointed to by <code>aio_buf</code>. |
| |
| <br><dt><code>int aio_reqprio</code><dd>If for the platform <code>_POSIX_PRIORITIZED_IO</code> and |
| <code>_POSIX_PRIORITY_SCHEDULING</code> are defined the AIO requests are |
| processed based on the current scheduling priority. The |
| <code>aio_reqprio</code> element can then be used to lower the priority of the |
| AIO operation. |
| |
| <br><dt><code>struct sigevent aio_sigevent</code><dd>This element specifies how the calling process is notified once the |
| operation terminates. If the <code>sigev_notify</code>, element is |
| <code>SIGEV_NONE</code> no notification is sent. If it is <code>SIGEV_SIGNAL</code>, |
| the signal determined by <code>sigev_signo</code> is sent. Otherwise, |
| <code>sigev_notify</code> must be <code>SIGEV_THREAD</code> in which case a thread |
| which starts executing the function pointed to by |
| <code>sigev_notify_function</code>. |
| |
| <br><dt><code>int aio_lio_opcode</code><dd>This element is only used by the <code>lio_listio</code> and |
| <code>[lio_listio64</code> functions. Since these functions allow an |
| arbitrary number of operations to start at once, and since each operation can be |
| input or output (or nothing), the information must be stored in the |
| control block. See the description of <code>struct aiocb</code> for a description |
| of the possible values. |
| </dl> |
| |
| <p>When the sources are compiled using <code>_FILE_OFFSET_BITS == 64</code> on a |
| 32 bit machine, this type is available under the name <code>struct |
| aiocb64</code>, since the LFS transparently replaces the old interface. |
| </p></blockquote></div> |
| |
| <ul class="menu"> |
| <li><a accesskey="1" href="Asynchronous-Reads_002fWrites.html#Asynchronous-Reads_002fWrites">Asynchronous Reads/Writes</a>: Asynchronous Read and Write Operations. |
| <li><a accesskey="2" href="Status-of-AIO-Operations.html#Status-of-AIO-Operations">Status of AIO Operations</a>: Getting the Status of AIO Operations. |
| <li><a accesskey="3" href="Synchronizing-AIO-Operations.html#Synchronizing-AIO-Operations">Synchronizing AIO Operations</a>: Getting into a consistent state. |
| <li><a accesskey="4" href="Cancel-AIO-Operations.html#Cancel-AIO-Operations">Cancel AIO Operations</a>: Cancellation of AIO Operations. |
| <li><a accesskey="5" href="Configuration-of-AIO.html#Configuration-of-AIO">Configuration of AIO</a>: How to optimize the AIO implementation. |
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