1 | /* |
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2 | FUNCTION |
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3 | <<random>>, <<srandom>>---pseudo-random numbers |
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4 | |
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5 | INDEX |
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6 | random |
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7 | INDEX |
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8 | srandom |
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9 | |
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10 | SYNOPSIS |
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11 | #define _XOPEN_SOURCE 500 |
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12 | #include <stdlib.h> |
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13 | long int random(void); |
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14 | void srandom(unsigned int <[seed]>); |
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15 | |
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16 | |
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17 | |
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18 | DESCRIPTION |
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19 | <<random>> returns a different integer each time it is called; each |
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20 | integer is chosen by an algorithm designed to be unpredictable, so |
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21 | that you can use <<random>> when you require a random number. |
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22 | The algorithm depends on a static variable called the ``random seed''; |
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23 | starting with a given value of the random seed always produces the |
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24 | same sequence of numbers in successive calls to <<random>>. |
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25 | |
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26 | You can set the random seed using <<srandom>>; it does nothing beyond |
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27 | storing its argument in the static variable used by <<rand>>. You can |
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28 | exploit this to make the pseudo-random sequence less predictable, if |
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29 | you wish, by using some other unpredictable value (often the least |
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30 | significant parts of a time-varying value) as the random seed before |
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31 | beginning a sequence of calls to <<rand>>; or, if you wish to ensure |
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32 | (for example, while debugging) that successive runs of your program |
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33 | use the same ``random'' numbers, you can use <<srandom>> to set the same |
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34 | random seed at the outset. |
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35 | |
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36 | RETURNS |
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37 | <<random>> returns the next pseudo-random integer in sequence; it is a |
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38 | number between <<0>> and <<RAND_MAX>> (inclusive). |
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39 | |
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40 | <<srandom>> does not return a result. |
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41 | |
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42 | NOTES |
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43 | <<random>> and <<srandom>> are unsafe for multi-threaded applications. |
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44 | |
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45 | _XOPEN_SOURCE may be any value >= 500. |
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46 | |
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47 | PORTABILITY |
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48 | <<random>> is required by XSI. This implementation uses the same |
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49 | algorithm as <<rand>>. |
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50 | |
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51 | <<random>> requires no supporting OS subroutines. |
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52 | */ |
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53 | |
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54 | #ifndef _REENT_ONLY |
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55 | |
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56 | #include <stdlib.h> |
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57 | #include <reent.h> |
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58 | |
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59 | void |
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60 | srandom (unsigned int seed) |
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61 | { |
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62 | struct _reent *reent = _REENT; |
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63 | |
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64 | _REENT_CHECK_RAND48(reent); |
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65 | _REENT_RAND_NEXT(reent) = seed; |
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66 | } |
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67 | |
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68 | long int |
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69 | random (void) |
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70 | { |
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71 | struct _reent *reent = _REENT; |
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72 | |
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73 | /* This multiplier was obtained from Knuth, D.E., "The Art of |
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74 | Computer Programming," Vol 2, Seminumerical Algorithms, Third |
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75 | Edition, Addison-Wesley, 1998, p. 106 (line 26) & p. 108 */ |
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76 | _REENT_CHECK_RAND48(reent); |
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77 | _REENT_RAND_NEXT(reent) = |
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78 | _REENT_RAND_NEXT(reent) * __extension__ 6364136223846793005LL + 1; |
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79 | return (long int)((_REENT_RAND_NEXT(reent) >> 32) & RAND_MAX); |
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80 | } |
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81 | |
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82 | #endif /* _REENT_ONLY */ |
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