183 lines
7.8 KiB
HTML
183 lines
7.8 KiB
HTML
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<title>fsync</title>
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<basefont size="3"> <a name="fsync"></a> <a name="tag_03_195"></a><!-- fsync -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_195_01"></a>NAME</h4>
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<blockquote>fsync - synchronize changes to a file</blockquote>
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<h4><a name="tag_03_195_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('FSC')">FSC</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/unistd.h.html">unistd.h</a>><br>
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<br>
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int fsync(int</tt> <i>fildes</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_195_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>fsync</i>() function shall request that all data for the open file descriptor named by <i>fildes</i> is to be transferred
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to the storage device associated with the file described by <i>fildes</i> in an implementation-defined manner. The <i>fsync</i>()
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function shall not return until the system has completed that action or until an error is detected.</p>
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<p><sup>[<a href="javascript:open_code('SIO')">SIO</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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If _POSIX_SYNCHRONIZED_IO is defined, the <i>fsync</i>() function shall force all currently queued I/O operations associated with
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the file indicated by file descriptor <i>fildes</i> to the synchronized I/O completion state. All I/O operations shall be completed
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as defined for synchronized I/O file integrity completion. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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</blockquote>
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<h4><a name="tag_03_195_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>fsync</i>() shall return 0. Otherwise, -1 shall be returned and <i>errno</i> set to indicate the
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error. If the <i>fsync</i>() function fails, outstanding I/O operations are not guaranteed to have been completed.</p>
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</blockquote>
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<h4><a name="tag_03_195_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>fsync</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EBADF]</dt>
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<dd>The <i>fildes</i> argument is not a valid descriptor.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>fsync</i>() function was interrupted by a signal.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>fildes</i> argument does not refer to a file on which this operation is possible.</dd>
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<dt>[EIO]</dt>
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<dd>An I/O error occurred while reading from or writing to the file system.</dd>
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</dl>
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<p>In the event that any of the queued I/O operations fail, <i>fsync</i>() shall return the error conditions defined for <a href=
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"../functions/read.html"><i>read</i>()</a> and <a href="../functions/write.html"><i>write</i>()</a>.</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_195_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_195_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <i>fsync</i>() function should be used by programs which require modifications to a file to be completed before continuing;
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for example, a program which contains a simple transaction facility might use it to ensure that all modifications to a file or
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files caused by a transaction are recorded.</p>
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</blockquote>
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<h4><a name="tag_03_195_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The <i>fsync</i>() function is intended to force a physical write of data from the buffer cache, and to assure that after a
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system crash or other failure that all data up to the time of the <i>fsync</i>() call is recorded on the disk. Since the concepts
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of "buffer cache", "system crash", "physical write", and "non-volatile storage" are not defined here, the wording has to be
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more abstract.</p>
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<p>If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on the conformance document to tell the user what can be
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expected from the system. It is explicitly intended that a null implementation is permitted. This could be valid in the case where
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the system cannot assure non-volatile storage under any circumstances or when the system is highly fault-tolerant and the
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functionality is not required. In the middle ground between these extremes, <i>fsync</i>() might or might not actually cause data
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to be written where it is safe from a power failure. The conformance document should identify at least that one configuration
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exists (and how to obtain that configuration) where this can be assured for at least some files that the user can select to use for
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critical data. It is not intended that an exhaustive list is required, but rather sufficient information is provided so that if
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critical data needs to be saved, the user can determine how the system is to be configured to allow the data to be written to
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non-volatile storage.</p>
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<p>It is reasonable to assert that the key aspects of <i>fsync</i>() are unreasonable to test in a test suite. That does not make
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the function any less valuable, just more difficult to test. A formal conformance test should probably force a system crash (power
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shutdown) during the test for this condition, but it needs to be done in such a way that automated testing does not require this to
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be done except when a formal record of the results is being made. It would also not be unreasonable to omit testing for
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<i>fsync</i>(), allowing it to be treated as a quality-of-implementation issue.</p>
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</blockquote>
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<h4><a name="tag_03_195_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_195_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="sync.html"><i>sync</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/unistd.h.html"><i><unistd.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_195_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 3.</p>
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</blockquote>
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<h4><a name="tag_03_195_12"></a>Issue 5</h4>
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<blockquote>
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<p>Aligned with <i>fsync</i>() in the POSIX Realtime Extension. Specifically, the DESCRIPTION and RETURN VALUE sections are much
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expanded, and the ERRORS section is updated to indicate that <i>fsync</i>() can return the error conditions defined for <a href=
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"../functions/read.html"><i>read</i>()</a> and <a href="../functions/write.html"><i>write</i>()</a>.</p>
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</blockquote>
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<h4><a name="tag_03_195_13"></a>Issue 6</h4>
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<blockquote>
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<p>This function is marked as part of the File Synchronization option.</p>
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<p>The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:</p>
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<ul>
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<li>
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<p>The [EINVAL] and [EIO] mandatory error conditions are added.</p>
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</li>
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</ul>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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POSIX ® is a registered Trademark of The IEEE.<br>
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