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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>drand48</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="drand48"></a> <a name="tag_03_113"></a><!-- drand48 --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_113_01"></a>NAME</h4><blockquote>drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48, seed48, srand48 - generate uniformly distributedpseudo-random numbers</blockquote><h4><a name="tag_03_113_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><div class="box"><code><tt><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> #include <<a href="../basedefs/stdlib.h.html">stdlib.h</a>><br><br> double drand48(void);<br> double erand48(unsigned short</tt> <i>xsubi</i><tt>[3]);<br> long jrand48(unsigned short</tt> <i>xsubi</i><tt>[3]);<br> void lcong48(unsigned short</tt> <i>param</i><tt>[7]);<br> long lrand48(void);<br> long mrand48(void);<br> long nrand48(unsigned short</tt> <i>xsubi</i><tt>[3]);<br> unsigned short *seed48(unsigned short</tt> <i>seed16v</i><tt>[3]);<br> void srand48(long</tt> <i>seedval</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div><tt><br></tt></blockquote><h4><a name="tag_03_113_03"></a>DESCRIPTION</h4><blockquote><p>This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integerarithmetic.</p><p>The <i>drand48</i>() and <i>erand48</i>() functions shall return non-negative, double-precision, floating-point values,uniformly distributed over the interval [0.0,1.0).</p><p>The <i>lrand48</i>() and <i>nrand48</i>() functions shall return non-negative, long integers, uniformly distributed over theinterval [0,2<sup><small>31</small></sup>).</p><p>The <i>mrand48</i>() and <i>jrand48</i>() functions shall return signed long integers uniformly distributed over the interval[-2<sup><small>31</small></sup>,2<sup><small>31</small></sup>).</p><p>The <i>srand48</i>(), <i>seed48</i>(), and <i>lcong48</i>() functions are initialization entry points, one of which should beinvoked before either <i>drand48</i>(), <i>lrand48</i>(), or <i>mrand48</i>() is called. (Although it is not recommended practice,constant default initializer values shall be supplied automatically if <i>drand48</i>(), <i>lrand48</i>(), or <i>mrand48</i>() iscalled without a prior call to an initialization entry point.) The <i>erand48</i>(), <i>nrand48</i>(), and <i>jrand48</i>()functions do not require an initialization entry point to be called first.</p><p>All the routines work by generating a sequence of 48-bit integer values, X<small><sub>i</sub></small> , according to the linear congruentialformula:</p><blockquote>X<small><sub>n+1</sub></small> = (aX<small><sub>n</sub></small> + c)<small><sub>mod m</sub></small> n>= 0</blockquote><p>The parameter <i>m</i> = 2<small><sup>48</sup></small>; hence 48-bit integer arithmetic is performed. Unless <i>lcong48</i>() is invoked, the multipliervalue <i>a</i> and the addend value <i>c</i> are given by:</p><blockquote><i>a</i> = 5DEECE66D<small><sub>16</sub></small> = 273673163155<small><sub>8</sub></small><p><i>c</i> = B<small><sub>16</sub></small> = 13<small><sub>8</sub></small></blockquote><p>The value returned by any of the <i>drand48</i>(), <i>erand48</i>(), <i>jrand48</i>(), <i>lrand48</i>(), <i>mrand48</i>(), or<i>nrand48</i>() functions is computed by first generating the next 48-bit X<small><sub>i</sub></small> in the sequence. Then the appropriate numberof bits, according to the type of data item to be returned, are copied from the high-order (leftmost) bits of X<small><sub>i</sub></small> andtransformed into the returned value.</p><p>The <i>drand48</i>(), <i>lrand48</i>(), and <i>mrand48</i>() functions store the last 48-bit X<small><sub>i</sub></small> generated in an internalbuffer; that is why the application shall ensure that these are initialized prior to being invoked. The <i>erand48</i>(),<i>nrand48</i>(), and <i>jrand48</i>() functions require the calling program to provide storage for the successive X<small><sub>i</sub></small> valuesin the array specified as an argument when the functions are invoked. That is why these routines do not have to be initialized; thecalling program merely has to place the desired initial value of X<small><sub>i</sub></small> into the array and pass it as an argument. By usingdifferent arguments, <i>erand48</i>(), <i>nrand48</i>(), and <i>jrand48</i>() allow separate modules of a large program to generateseveral <i>independent</i> streams of pseudo-random numbers; that is, the sequence of numbers in each stream shall <i>not</i>depend upon how many times the routines are called to generate numbers for the other streams.</p><p>The initializer function <i>srand48</i>() sets the high-order 32 bits of X<small><sub>i</sub></small> to the low-order 32 bits contained in itsargument. The low-order 16 bits of X<small><sub>i</sub></small> are set to the arbitrary value 330E<small><sub>16</sub></small>.</p><p>The initializer function <i>seed48</i>() sets the value of X<small><sub>i</sub></small> to the 48-bit value specified in the argument array. Thelow-order 16 bits of X<small><sub>i</sub></small> are set to the low-order 16 bits of <i>seed16v</i>[<i>0</i>]. The mid-order 16 bits of X<small><sub>i</sub></small> areset to the low-order 16 bits of <i>seed16v</i>[<i>1</i>]. The high-order 16 bits of X<small><sub>i</sub></small> are set to the low-order 16 bits of<i>seed16v</i>[<i>2</i>]. In addition, the previous value of X<small><sub>i</sub></small> is copied into a 48-bit internal buffer, used only by<i>seed48</i>(), and a pointer to this buffer is the value returned by <i>seed48</i>(). This returned pointer, which can just beignored if not needed, is useful if a program is to be restarted from a given point at some future time-use the pointer to get atand store the last X<small><sub>i</sub></small> value, and then use this value to reinitialize via <i>seed48</i>() when the program is restarted.</p><p>The initializer function <i>lcong48</i>() allows the user to specify the initial X<small><sub>i</sub></small> , the multiplier value <i>a</i>, and theaddend value <i>c</i>. Argument array elements <i>param</i>[<i>0-2</i>] specify X<small><sub>i</sub></small> , <i>param</i>[<i>3-5</i>] specify themultiplier <i>a</i>, and <i>param</i>[<i>6</i>] specifies the 16-bit addend <i>c</i>. After <i>lcong48</i>() is called, a subsequent call toeither <i>srand48</i>() or <i>seed48</i>() shall restore the standard multiplier and addend values, <i>a</i> and <i>c,</i>specified above.</p><p>The <i>drand48</i>(), <i>lrand48</i>(), and <i>mrand48</i>() functions need not be reentrant. A function that is not required tobe reentrant is not required to be thread-safe.</p></blockquote><h4><a name="tag_03_113_04"></a>RETURN VALUE</h4><blockquote><p>As described in the DESCRIPTION above.</p></blockquote><h4><a name="tag_03_113_05"></a>ERRORS</h4><blockquote><p>No errors are defined.</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_113_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_113_07"></a>APPLICATION USAGE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_113_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_113_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_113_10"></a>SEE ALSO</h4><blockquote><p><a href="rand.html"><i>rand</i>()</a>, the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/stdlib.h.html"><i><stdlib.h></i></a></p></blockquote><h4><a name="tag_03_113_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 1. Derived from Issue 1 of the SVID.</p></blockquote><h4><a name="tag_03_113_12"></a>Issue 5</h4><blockquote><p>A note indicating that these functions need not be reentrant is added to the DESCRIPTION.</p></blockquote><h4><a name="tag_03_113_13"></a>Issue 6</h4><blockquote><p>The DESCRIPTION is updated to avoid use of the term "must" for application requirements.<br></p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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