1random(3) Library Functions Manual random(3)
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6 random, srandom, initstate, setstate - random number generator
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9 Standard C library (libc, -lc)
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12 #include <stdlib.h>
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14 long random(void);
15 void srandom(unsigned int seed);
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17 char *initstate(unsigned int seed, char state[.n], size_t n);
18 char *setstate(char *state);
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20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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22 random(), srandom(), initstate(), setstate():
23 _XOPEN_SOURCE >= 500
24 || /* glibc >= 2.19: */ _DEFAULT_SOURCE
25 || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
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28 The random() function uses a nonlinear additive feedback random number
29 generator employing a default table of size 31 long integers to return
30 successive pseudo-random numbers in the range from 0 to 2^31 - 1. The
31 period of this random number generator is very large, approximately
32 16 * ((2^31) - 1).
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34 The srandom() function sets its argument as the seed for a new sequence
35 of pseudo-random integers to be returned by random(). These sequences
36 are repeatable by calling srandom() with the same seed value. If no
37 seed value is provided, the random() function is automatically seeded
38 with a value of 1.
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40 The initstate() function allows a state array state to be initialized
41 for use by random(). The size of the state array n is used by init‐
42 state() to decide how sophisticated a random number generator it should
43 use—the larger the state array, the better the random numbers will be.
44 Current "optimal" values for the size of the state array n are 8, 32,
45 64, 128, and 256 bytes; other amounts will be rounded down to the near‐
46 est known amount. Using less than 8 bytes results in an error. seed
47 is the seed for the initialization, which specifies a starting point
48 for the random number sequence, and provides for restarting at the same
49 point.
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51 The setstate() function changes the state array used by the random()
52 function. The state array state is used for random number generation
53 until the next call to initstate() or setstate(). state must first
54 have been initialized using initstate() or be the result of a previous
55 call of setstate().
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58 The random() function returns a value between 0 and (2^31) - 1. The
59 srandom() function returns no value.
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61 The initstate() function returns a pointer to the previous state array.
62 On failure, it returns NULL, and errno is set to indicate the error.
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64 On success, setstate() returns a pointer to the previous state array.
65 On failure, it returns NULL, and errno is set to indicate the error.
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68 EINVAL The state argument given to setstate() was NULL.
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70 EINVAL A state array of less than 8 bytes was specified to initstate().
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73 For an explanation of the terms used in this section, see at‐
74 tributes(7).
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76 ┌────────────────────────────────────────────┬───────────────┬─────────┐
77 │Interface │ Attribute │ Value │
78 ├────────────────────────────────────────────┼───────────────┼─────────┤
79 │random(), srandom(), initstate(), │ Thread safety │ MT-Safe │
80 │setstate() │ │ │
81 └────────────────────────────────────────────┴───────────────┴─────────┘
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84 POSIX.1-2008.
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87 POSIX.1-2001, 4.3BSD.
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90 Random-number generation is a complex topic. Numerical Recipes in C:
91 The Art of Scientific Computing (William H. Press, Brian P. Flannery,
92 Saul A. Teukolsky, William T. Vetterling; New York: Cambridge
93 University Press, 2007, 3rd ed.) provides an excellent discussion of
94 practical random-number generation issues in Chapter 7 (Random
95 Numbers).
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97 For a more theoretical discussion which also covers many practical
98 issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
99 The Art of Computer Programming, volume 2 (Seminumerical Algorithms),
100 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company,
101 1981.
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104 The random() function should not be used in multithreaded programs
105 where reproducible behavior is required. Use random_r(3) for that
106 purpose.
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109 According to POSIX, initstate() should return NULL on error. In the
110 glibc implementation, errno is (as specified) set on error, but the
111 function does not return NULL.
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114 getrandom(2), drand48(3), rand(3), random_r(3), srand(3)
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118Linux man-pages 6.05 2023-07-20 random(3)