161 lines
7.2 KiB
HTML
161 lines
7.2 KiB
HTML
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<title>flockfile</title>
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<basefont size="3"> <a name="flockfile"></a> <a name="tag_03_168"></a><!-- flockfile -->
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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_168_01"></a>NAME</h4>
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<blockquote>flockfile, ftrylockfile, funlockfile - stdio locking functions</blockquote>
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<h4><a name="tag_03_168_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('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/stdio.h.html">stdio.h</a>><br>
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<br>
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void flockfile(FILE *</tt><i>file</i><tt>);<br>
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int ftrylockfile(FILE *</tt><i>file</i><tt>);<br>
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void funlockfile(FILE *</tt><i>file</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_168_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>These functions shall provide for explicit application-level locking of stdio ( <b>FILE *</b>) objects. These functions can be
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used by a thread to delineate a sequence of I/O statements that are executed as a unit.</p>
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<p>The <i>flockfile</i>() function shall acquire for a thread ownership of a ( <b>FILE *</b>) object.</p>
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<p>The <i>ftrylockfile</i>() function shall acquire for a thread ownership of a ( <b>FILE *</b>) object if the object is available;
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<i>ftrylockfile</i>() is a non-blocking version of <i>flockfile</i>().</p>
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<p>The <i>funlockfile</i>() function shall relinquish the ownership granted to the thread. The behavior is undefined if a thread
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other than the current owner calls the <i>funlockfile</i>() function.</p>
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<p>The functions shall behave as if there is a lock count associated with each ( <b>FILE *</b>) object. This count is implicitly
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initialized to zero when the ( <b>FILE *</b>) object is created. The ( <b>FILE *</b>) object is unlocked when the count is zero.
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When the count is positive, a single thread owns the ( <b>FILE *</b>) object. When the <i>flockfile</i>() function is called, if
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the count is zero or if the count is positive and the caller owns the ( <b>FILE *</b>) object, the count shall be incremented.
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Otherwise, the calling thread shall be suspended, waiting for the count to return to zero. Each call to <i>funlockfile</i>() shall
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decrement the count. This allows matching calls to <i>flockfile</i>() (or successful calls to <i>ftrylockfile</i>()) and
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<i>funlockfile</i>() to be nested.</p>
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<p>All functions that reference ( <b>FILE *</b>) objects shall behave as if they use <i>flockfile</i>() and <i>funlockfile</i>()
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internally to obtain ownership of these ( <b>FILE *</b>) objects.</p>
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</blockquote>
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<h4><a name="tag_03_168_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>None for <i>flockfile</i>() and <i>funlockfile</i>().</p>
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<p>The <i>ftrylockfile</i>() function shall return zero for success and non-zero to indicate that the lock cannot be acquired.</p>
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</blockquote>
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<h4><a name="tag_03_168_05"></a>ERRORS</h4>
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<blockquote>
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<p>No errors are defined.</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_168_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_168_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>Applications using these functions may be subject to priority inversion, as discussed in the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/xbd_chap03.html#tag_03_285">Section 3.285, Priority Inversion</a>.</p>
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</blockquote>
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<h4><a name="tag_03_168_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The <i>flockfile</i>() and <i>funlockfile</i>() functions provide an orthogonal mutual-exclusion lock for each <b>FILE</b>. The
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<i>ftrylockfile</i>() function provides a non-blocking attempt to acquire a file lock, analogous to <a href=
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"../functions/pthread_mutex_trylock.html"><i>pthread_mutex_trylock</i>()</a>.</p>
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<p>These locks behave as if they are the same as those used internally by <i>stdio</i> for thread-safety. This both provides
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thread-safety of these functions without requiring a second level of internal locking and allows functions in <i>stdio</i> to be
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implemented in terms of other <i>stdio</i> functions.</p>
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<p>Application writers and implementors should be aware that there are potential deadlock problems on <b>FILE</b> objects. For
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example, the line-buffered flushing semantics of <i>stdio</i> (requested via {_IOLBF}) require that certain input operations
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sometimes cause the buffered contents of implementation-defined line-buffered output streams to be flushed. If two threads each
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hold the lock on the other's <b>FILE</b>, deadlock ensues. This type of deadlock can be avoided by acquiring <b>FILE</b> locks in a
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consistent order. In particular, the line-buffered output stream deadlock can typically be avoided by acquiring locks on input
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streams before locks on output streams if a thread would be acquiring both.</p>
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<p>In summary, threads sharing <i>stdio</i> streams with other threads can use <i>flockfile</i>() and <i>funlockfile</i>() to cause
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sequences of I/O performed by a single thread to be kept bundled. The only case where the use of <i>flockfile</i>() and
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<i>funlockfile</i>() is required is to provide a scope protecting uses of the
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<i>*_unlocked</i>() functions/macros. This moves the cost/performance tradeoff to the optimal
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point.</p>
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</blockquote>
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<h4><a name="tag_03_168_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_168_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="getc_unlocked.html"><i>getc_unlocked</i>()</a> , <a href="putc_unlocked.html"><i>putc_unlocked</i>()</a> , the Base
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Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/stdio.h.html"><i><stdio.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_168_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p>
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</blockquote>
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<h4><a name="tag_03_168_12"></a>Issue 6</h4>
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<blockquote>
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<p>These functions are marked as part of the Thread-Safe Functions option.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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