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| <h4 class="subsection">19.8.2 BSD Random Number Functions</h4> |
| |
| <p>This section describes a set of random number generation functions that |
| are derived from BSD. There is no advantage to using these functions |
| with the GNU C library; we support them for BSD compatibility only. |
| |
| <p>The prototypes for these functions are in <samp><span class="file">stdlib.h</span></samp>. |
| <a name="index-stdlib_002eh-2290"></a> |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| |
| <div class="defun"> |
| — Function: long int <b>random</b> (<var>void</var>)<var><a name="index-random-2291"></a></var><br> |
| <blockquote><p>This function returns the next pseudo-random number in the sequence. |
| The value returned ranges from <code>0</code> to <code>RAND_MAX</code>. |
| |
| <p><strong>NB:</strong> Temporarily this function was defined to return a |
| <code>int32_t</code> value to indicate that the return value always contains |
| 32 bits even if <code>long int</code> is wider. The standard demands it |
| differently. Users must always be aware of the 32-bit limitation, |
| though. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>srandom</b> (<var>unsigned int seed</var>)<var><a name="index-srandom-2292"></a></var><br> |
| <blockquote><p>The <code>srandom</code> function sets the state of the random number |
| generator based on the integer <var>seed</var>. If you supply a <var>seed</var> value |
| of <code>1</code>, this will cause <code>random</code> to reproduce the default set |
| of random numbers. |
| |
| <p>To produce a different set of pseudo-random numbers each time your |
| program runs, do <code>srandom (time (0))</code>. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void * <b>initstate</b> (<var>unsigned int seed, void *state, size_t size</var>)<var><a name="index-initstate-2293"></a></var><br> |
| <blockquote><p>The <code>initstate</code> function is used to initialize the random number |
| generator state. The argument <var>state</var> is an array of <var>size</var> |
| bytes, used to hold the state information. It is initialized based on |
| <var>seed</var>. The size must be between 8 and 256 bytes, and should be a |
| power of two. The bigger the <var>state</var> array, the better. |
| |
| <p>The return value is the previous value of the state information array. |
| You can use this value later as an argument to <code>setstate</code> to |
| restore that state. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void * <b>setstate</b> (<var>void *state</var>)<var><a name="index-setstate-2294"></a></var><br> |
| <blockquote><p>The <code>setstate</code> function restores the random number state |
| information <var>state</var>. The argument must have been the result of |
| a previous call to <var>initstate</var> or <var>setstate</var>. |
| |
| <p>The return value is the previous value of the state information array. |
| You can use this value later as an argument to <code>setstate</code> to |
| restore that state. |
| |
| <p>If the function fails the return value is <code>NULL</code>. |
| </p></blockquote></div> |
| |
| <p>The four functions described so far in this section all work on a state |
| which is shared by all threads. The state is not directly accessible to |
| the user and can only be modified by these functions. This makes it |
| hard to deal with situations where each thread should have its own |
| pseudo-random number generator. |
| |
| <p>The GNU C library contains four additional functions which contain the |
| state as an explicit parameter and therefore make it possible to handle |
| thread-local PRNGs. Beside this there is no difference. In fact, the |
| four functions already discussed are implemented internally using the |
| following interfaces. |
| |
| <p>The <samp><span class="file">stdlib.h</span></samp> header contains a definition of the following type: |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Data Type: <b>struct random_data</b><var><a name="index-struct-random_005fdata-2295"></a></var><br> |
| <blockquote> |
| <p>Objects of type <code>struct random_data</code> contain the information |
| necessary to represent the state of the PRNG. Although a complete |
| definition of the type is present the type should be treated as opaque. |
| </p></blockquote></div> |
| |
| <p>The functions modifying the state follow exactly the already described |
| functions. |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>random_r</b> (<var>struct random_data *restrict buf, int32_t *restrict result</var>)<var><a name="index-random_005fr-2296"></a></var><br> |
| <blockquote><p>The <code>random_r</code> function behaves exactly like the <code>random</code> |
| function except that it uses and modifies the state in the object |
| pointed to by the first parameter instead of the global state. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>srandom_r</b> (<var>unsigned int seed, struct random_data *buf</var>)<var><a name="index-srandom_005fr-2297"></a></var><br> |
| <blockquote><p>The <code>srandom_r</code> function behaves exactly like the <code>srandom</code> |
| function except that it uses and modifies the state in the object |
| pointed to by the second parameter instead of the global state. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>initstate_r</b> (<var>unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf</var>)<var><a name="index-initstate_005fr-2298"></a></var><br> |
| <blockquote><p>The <code>initstate_r</code> function behaves exactly like the <code>initstate</code> |
| function except that it uses and modifies the state in the object |
| pointed to by the fourth parameter instead of the global state. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>setstate_r</b> (<var>char *restrict statebuf, struct random_data *restrict buf</var>)<var><a name="index-setstate_005fr-2299"></a></var><br> |
| <blockquote><p>The <code>setstate_r</code> function behaves exactly like the <code>setstate</code> |
| function except that it uses and modifies the state in the object |
| pointed to by the first parameter instead of the global state. |
| </p></blockquote></div> |
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