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| <h3 class="section">19.8 Pseudo-Random Numbers</h3> |
| |
| <p><a name="index-random-numbers-2282"></a><a name="index-pseudo_002drandom-numbers-2283"></a><a name="index-seed-_0028for-random-numbers_0029-2284"></a> |
| This section describes the GNU facilities for generating a series of |
| pseudo-random numbers. The numbers generated are not truly random; |
| typically, they form a sequence that repeats periodically, with a period |
| so large that you can ignore it for ordinary purposes. The random |
| number generator works by remembering a <dfn>seed</dfn> value which it uses |
| to compute the next random number and also to compute a new seed. |
| |
| <p>Although the generated numbers look unpredictable within one run of a |
| program, the sequence of numbers is <em>exactly the same</em> from one run |
| to the next. This is because the initial seed is always the same. This |
| is convenient when you are debugging a program, but it is unhelpful if |
| you want the program to behave unpredictably. If you want a different |
| pseudo-random series each time your program runs, you must specify a |
| different seed each time. For ordinary purposes, basing the seed on the |
| current time works well. |
| |
| <p>You can obtain repeatable sequences of numbers on a particular machine type |
| by specifying the same initial seed value for the random number |
| generator. There is no standard meaning for a particular seed value; |
| the same seed, used in different C libraries or on different CPU types, |
| will give you different random numbers. |
| |
| <p>The GNU library supports the standard ISO C<!-- /@w --> random number functions |
| plus two other sets derived from BSD and SVID. The BSD and ISO C<!-- /@w --> |
| functions provide identical, somewhat limited functionality. If only a |
| small number of random bits are required, we recommend you use the |
| ISO C<!-- /@w --> interface, <code>rand</code> and <code>srand</code>. The SVID functions |
| provide a more flexible interface, which allows better random number |
| generator algorithms, provides more random bits (up to 48) per call, and |
| can provide random floating-point numbers. These functions are required |
| by the XPG standard and therefore will be present in all modern Unix |
| systems. |
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| <li><a accesskey="1" href="ISO-Random.html#ISO-Random">ISO Random</a>: <code>rand</code> and friends. |
| <li><a accesskey="2" href="BSD-Random.html#BSD-Random">BSD Random</a>: <code>random</code> and friends. |
| <li><a accesskey="3" href="SVID-Random.html#SVID-Random">SVID Random</a>: <code>drand48</code> and friends. |
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