| <html lang="en"> |
| <head> |
| <title>ISO Random - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Pseudo_002dRandom-Numbers.html#Pseudo_002dRandom-Numbers" title="Pseudo-Random Numbers"> |
| <link rel="next" href="BSD-Random.html#BSD-Random" title="BSD Random"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="ISO-Random"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="BSD-Random.html#BSD-Random">BSD Random</a>, |
| Up: <a rel="up" accesskey="u" href="Pseudo_002dRandom-Numbers.html#Pseudo_002dRandom-Numbers">Pseudo-Random Numbers</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">19.8.1 ISO C Random Number Functions</h4> |
| |
| <p>This section describes the random number functions that are part of |
| the ISO C<!-- /@w --> standard. |
| |
| <p>To use these facilities, you should include the header file |
| <samp><span class="file">stdlib.h</span></samp> in your program. |
| <a name="index-stdlib_002eh-2285"></a> |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Macro: int <b>RAND_MAX</b><var><a name="index-RAND_005fMAX-2286"></a></var><br> |
| <blockquote><p>The value of this macro is an integer constant representing the largest |
| value the <code>rand</code> function can return. In the GNU library, it is |
| <code>2147483647</code>, which is the largest signed integer representable in |
| 32 bits. In other libraries, it may be as low as <code>32767</code>. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>rand</b> (<var>void</var>)<var><a name="index-rand-2287"></a></var><br> |
| <blockquote><p>The <code>rand</code> function returns the next pseudo-random number in the |
| series. The value ranges from <code>0</code> to <code>RAND_MAX</code>. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: void <b>srand</b> (<var>unsigned int seed</var>)<var><a name="index-srand-2288"></a></var><br> |
| <blockquote><p>This function establishes <var>seed</var> as the seed for a new series of |
| pseudo-random numbers. If you call <code>rand</code> before a seed has been |
| established with <code>srand</code>, it uses the value <code>1</code> as a default |
| seed. |
| |
| <p>To produce a different pseudo-random series each time your program is |
| run, do <code>srand (time (0))</code>. |
| </p></blockquote></div> |
| |
| <p>POSIX.1 extended the C standard functions to support reproducible random |
| numbers in multi-threaded programs. However, the extension is badly |
| designed and unsuitable for serious work. |
| |
| <!-- stdlib.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>rand_r</b> (<var>unsigned int *seed</var>)<var><a name="index-rand_005fr-2289"></a></var><br> |
| <blockquote><p>This function returns a random number in the range 0 to <code>RAND_MAX</code> |
| just as <code>rand</code> does. However, all its state is stored in the |
| <var>seed</var> argument. This means the RNG's state can only have as many |
| bits as the type <code>unsigned int</code> has. This is far too few to |
| provide a good RNG. |
| |
| <p>If your program requires a reentrant RNG, we recommend you use the |
| reentrant GNU extensions to the SVID random number generator. The |
| POSIX.1 interface should only be used when the GNU extensions are not |
| available. |
| </p></blockquote></div> |
| |
| </body></html> |
| |
