240 lines
9.1 KiB
HTML
240 lines
9.1 KiB
HTML
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<title>rand</title>
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<basefont size="3"> <a name="rand"></a> <a name="tag_03_592"></a><!-- rand -->
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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_592_01"></a>NAME</h4>
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<blockquote>rand, rand_r, srand - pseudo-random number generator</blockquote>
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<h4><a name="tag_03_592_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<p><code><tt>#include <<a href="../basedefs/stdlib.h.html">stdlib.h</a>><br>
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<br>
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int rand(void);<br>
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</tt></code></p>
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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"> int rand_r(unsigned *</tt><i>seed</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]"
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border="0"></tt></code></div>
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<tt><br>
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void srand(unsigned</tt> <i>seed</i><tt>);<br>
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</tt></blockquote>
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<h4><a name="tag_03_592_03"></a>DESCRIPTION</h4>
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<blockquote>
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<div class="box"><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]"
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border="0"> The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the
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requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to
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the ISO C standard. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></div>
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<p>The <i>rand</i>() function shall compute a sequence of pseudo-random integers in the range [0, {RAND_MAX}] <sup>[<a href=
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"javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> with a
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period of at least 2<sup><small>32</small></sup>. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> The
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<i>rand</i>() function need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.
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<img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p><sup>[<a href="javascript:open_code('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The <i>rand_r</i>() function shall compute a sequence of pseudo-random integers in the range [0, {RAND_MAX}]. (The value of the
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{RAND_MAX} macro shall be at least 32767.)</p>
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<p>If <i>rand_r</i>() is called with the same initial value for the object pointed to by <i>seed</i> and that object is not
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modified between successive returns and calls to <i>rand_r</i>(), the same sequence shall be generated. <img src=
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"../images/opt-end.gif" alt="[Option End]" border="0"></p>
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<p>The <i>srand</i>() function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent
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calls to <i>rand</i>(). If <i>srand</i>() is then called with the same seed value, the sequence of pseudo-random numbers shall be
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repeated. If <i>rand</i>() is called before any calls to <i>srand</i>() are made, the same sequence shall be generated as when
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<i>srand</i>() is first called with a seed value of 1.</p>
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<p>The implementation shall behave as if no function defined in this volume of IEEE Std 1003.1-2001 calls <i>rand</i>()
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or <i>srand</i>().</p>
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</blockquote>
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<h4><a name="tag_03_592_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>The <i>rand</i>() function shall return the next pseudo-random number in the sequence.</p>
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<p><sup>[<a href="javascript:open_code('TSF')">TSF</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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The <i>rand_r</i>() function shall return a pseudo-random integer. <img src="../images/opt-end.gif" alt="[Option End]" border=
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"0"></p>
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<p>The <i>srand</i>() function shall not return a value.</p>
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</blockquote>
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<h4><a name="tag_03_592_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_592_06"></a>EXAMPLES</h4>
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<blockquote>
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<h5><a name="tag_03_592_06_01"></a>Generating a Pseudo-Random Number Sequence</h5>
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<p>The following example demonstrates how to generate a sequence of pseudo-random numbers.</p>
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<pre>
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<tt>#include <stdio.h>
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#include <stdlib.h>
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...
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long count, i;
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char *keystr;
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int elementlen, len;
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char c;
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...
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/* Initial random number generator. */
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srand(1);
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<br>
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/* Create keys using only lowercase characters */
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len = 0;
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for (i=0; i<count; i++) {
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while (len < elementlen) {
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c = (char) (rand() % 128);
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if (islower(c))
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keystr[len++] = c;
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}
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<br>
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keystr[len] = '\0';
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printf("%s Element%0*ld\n", keystr, elementlen, i);
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len = 0;
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}
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</tt>
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</pre>
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<h5><a name="tag_03_592_06_02"></a>Generating the Same Sequence on Different Machines</h5>
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<p>The following code defines a pair of functions that could be incorporated into applications wishing to ensure that the same
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sequence of numbers is generated across different machines.</p>
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<pre>
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<tt>static unsigned long next = 1;
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int myrand(void) /* RAND_MAX assumed to be 32767. */
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{
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next = next * 1103515245 + 12345;
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return((unsigned)(next/65536) % 32768);
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}
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<br>
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void mysrand(unsigned seed)
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{
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next = seed;
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}
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_592_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <a href="../functions/drand48.html"><i>drand48</i>()</a> function provides a much more elaborate random number
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generator.</p>
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<p>The limitations on the amount of state that can be carried between one function call and another mean the <i>rand_r</i>()
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function can never be implemented in a way which satisfies all of the requirements on a pseudo-random number generator. Therefore
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this function should be avoided whenever non-trivial requirements (including safety) have to be fulfilled.</p>
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</blockquote>
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<h4><a name="tag_03_592_08"></a>RATIONALE</h4>
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<blockquote>
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<p>The ISO C standard <i>rand</i>() and <i>srand</i>() functions allow per-process pseudo-random streams shared by all
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threads. Those two functions need not change, but there has to be mutual-exclusion that prevents interference between two threads
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concurrently accessing the random number generator.</p>
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<p>With regard to <i>rand</i>(), there are two different behaviors that may be wanted in a multi-threaded program:</p>
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<ol>
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<li>
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<p>A single per-process sequence of pseudo-random numbers that is shared by all threads that call <i>rand</i>()</p>
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</li>
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<li>
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<p>A different sequence of pseudo-random numbers for each thread that calls <i>rand</i>()</p>
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</li>
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</ol>
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<p>This is provided by the modified thread-safe function based on whether the seed value is global to the entire process or local
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to each thread.</p>
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<p>This does not address the known deficiencies of the <i>rand</i>() function implementations, which have been approached by
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maintaining more state. In effect, this specifies new thread-safe forms of a deficient function.</p>
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</blockquote>
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<h4><a name="tag_03_592_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_592_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="drand48.html"><i>drand48</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/stdlib.h.html"><i><stdlib.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_592_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 1. Derived from Issue 1 of the SVID.</p>
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</blockquote>
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<h4><a name="tag_03_592_12"></a>Issue 5</h4>
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<blockquote>
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<p>The <i>rand_r</i>() function is included for alignment with the POSIX Threads Extension.</p>
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<p>A note indicating that the <i>rand</i>() function need not be reentrant is added to the DESCRIPTION.</p>
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</blockquote>
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<h4><a name="tag_03_592_13"></a>Issue 6</h4>
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<blockquote>
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<p>Extensions beyond the ISO C standard are marked.</p>
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<p>The <i>rand_r</i>() function is 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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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
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"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
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