1random(3C) Standard C Library Functions random(3C)
2
6 random, srandom, initstate, setstate - pseudorandom number functions
7
9 #include <stdlib.h>
10 long random(void);
12 void srandom(unsigned int seed);
15 char *initstate(unsigned int seed, char *state, size_t size);
18 char *setstate(const char *state);
21
24 The random() function uses a nonlinear additive feedback random-number
25 generator employing a default state array size of 31 long integers to
26 return successive pseudo-random numbers in the range from 0 to 2^31 −1.
27 The period of this random-number generator is approximately 16 x (2^31
28 −1). The size of the state array determines the period of the random-
29 number generator. Increasing the state array size increases the period.
30 The srandom() function initializes the current state array using the
33 value of seed.
34 The random() and srandom() functions have (almost) the same calling
37 sequence and initialization properties as rand() and srand() (see
38 rand(3C)). The difference is that rand(3C) produces a much less random
39 sequence—in fact, the low dozen bits generated by rand go through a
40 cyclic pattern. All the bits generated by random() are usable.
41 The algorithm from rand() is used by srandom() to generate the 31 state
44 integers. Because of this, different srandom() seeds often produce,
45 within an offset, the same sequence of low order bits from random(). If
46 low order bits are used directly, random() should be initialized with
47 setstate() using high quality random values.
48 Unlike srand(), srandom() does not return the old seed because the
51 amount of state information used is much more than a single word. Two
52 other routines are provided to deal with restarting/changing random
53 number generators. With 256 bytes of state information, the period of
54 the random-number generator is greater than 2^69, which should be suf‐
55 ficient for most purposes.
56 Like rand(3C), random() produces by default a sequence of numbers that
59 can be duplicated by calling srandom() with 1 as the seed.
60 The initstate() and setstate() functions handle restarting and changing
63 random-number generators. The initstate() function allows a state
64 array, pointed to by the state argument, to be initialized for future
65 use. The size argument, which specifies the size in bytes of the state
66 array, is used by initstate() to decide what type of random-number gen‐
67 erator to use; the larger the state array, the more random the numbers.
68 Values for the amount of state information are 8, 32, 64, 128, and 256
69 bytes. Other values greater than 8 bytes are rounded down to the near‐
70 est one of these values. For values smaller than 8, random() uses a
71 simple linear congruential random number generator. The seed argument
72 specifies a starting point for the random-number sequence and provides
73 for restarting at the same point. The initstate() function returns a
74 pointer to the previous state information array.
75 If initstate() has not been called, then random() behaves as though
78 initstate() had been called with seed=1 and size=128.
79 If initstate() is called with size<8, then random() uses a simple lin‐
82 ear congruential random number generator.
83 Once a state has been initialized, setstate() allows switching between
86 state arrays. The array defined by the state argument is used for fur‐
87 ther random-number generation until initstate() is called or setstate()
88 is called again. The setstate() function returns a pointer to the pre‐
89 vious state array.
90
92 The random() function returns the generated pseudo-random number.
93 The srandom() function returns no value.
96 Upon successful completion, initstate() and setstate() return a pointer
99 to the previous state array. Otherwise, a null pointer is returned.
100
102 No errors are defined.
103
105 After initialization, a state array can be restarted at a different
106 point in one of two ways:
107 o The initstate() function can be used, with the desired seed,
109 state array, and size of the array.
110 o The setstate() function, with the desired state, can be
112 used, followed by srandom() with the desired seed. The
113 advantage of using both of these functions is that the size
114 of the state array does not have to be saved once it is ini‐
115 tialized.
116
118 Example 1 Initialize an array.
119 The following example demonstrates the use of initstate() to intialize
122 an array. It also demonstrates how to initialize an array and pass it
123 to setstate().
124 # include <stdlib.h>
127 static unsigned int state0[32];
128 static unsigned int state1[32] = {
129 3,
130 0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
131 0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
132 0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
133 0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
134 0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
135 0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
136 0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
137 0xf5ad9d0e, 0x8999220b, 0x27fb47b9
138 };
139 main() {
140 unsigned seed;
141 int n;
142 seed = 1;
143 n = 128;
144 (void)initstate(seed, (char *)state0, n);
145 printf("random() = %d0\n", random());
146 (void)setstate((char *)state1);
147 printf("random() = %d0\n", random());
148 }
149
152 See attributes(5) for descriptions of the following attributes:
153 ┌─────────────────────────────┬─────────────────────────────┐
158 │ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
159 ├─────────────────────────────┼─────────────────────────────┤
160 │Interface Stability │Standard │
161 ├─────────────────────────────┼─────────────────────────────┤
162 │MT-Level │See NOTES below. │
163 └─────────────────────────────┴─────────────────────────────┘
164
166 drand48(3C), rand(3C), attributes(5), standards(5)
167
169 The random() and srandom() functions are unsafe in multithreaded appli‐
170 cations.
171 Use of these functions in multithreaded applications is unsupported.
174 For initstate() and setstate(), the state argument must be aligned on
177 an int boundary.
178 Newer and better performing random number generators such as addrans()
181 and lcrans() are available with the SUNWspro package.
182SunOS 5.11 14 Aug 2002 random(3C)