1RAND(3) Linux Programmer's Manual RAND(3)
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6 rand, rand_r, srand - pseudo-random number generator
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9 #include <stdlib.h>
10 int rand(void);
12 int rand_r(unsigned int *seedp);
14 void srand(unsigned int seed);
16
18 The rand() function returns a pseudo-random integer between 0 and
19 RAND_MAX.
20 The srand() function sets its argument as the seed for a new sequence
22 of pseudo-random integers to be returned by rand(). These sequences
23 are repeatable by calling srand() with the same seed value.
24 If no seed value is provided, the rand() function is automatically
26 seeded with a value of 1.
27 The function rand() is not reentrant or thread-safe, since it uses hid‐
29 den state that is modified on each call. This might just be the seed
30 value to be used by the next call, or it might be something more elabo‐
31 rate. In order to get reproducible behaviour in a threaded application,
32 this state must be made explicit. The function rand_r() is supplied
33 with a pointer to an unsigned int, to be used as state. This is a very
34 small amount of state, so this function will be a weak pseudo-random
35 generator. Try drand48_r(3) instead.
36
38 The rand() and rand_r() functions return a value between 0 and
39 RAND_MAX. The srand() function returns no value.
40
42 POSIX.1-2001 gives the following example of an implementation of rand()
43 and srand(), possibly useful when one needs the same sequence on two
44 different machines.
45 static unsigned long next = 1;
47 /* RAND_MAX assumed to be 32767 */
49 int myrand(void) {
50 next = next * 1103515245 + 12345;
51 return((unsigned)(next/65536) % 32768);
52 }
53 void mysrand(unsigned seed) {
55 next = seed;
56 }
57
59 The versions of rand() and srand() in the Linux C Library use the same
60 random number generator as random() and srandom(), so the lower-order
61 bits should be as random as the higher-order bits.
62 Most modern rand() functions, including the one provided on Linux sys‐
64 tems, do not suffer from this problem, so using expressions like
65 rand() % 10
66 works fine.
67 However, on older rand() implementations, and on current implementa‐
69 tions on different systems, the lower-order bits are much less random
70 than the higher-order bits. Do not use this function in applications
71 intended to be portable when good randomness is needed.
72 In Numerical Recipes in C: The Art of Scientific Computing (William H.
74 Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New
75 York: Cambridge University Press, 1992 (2nd ed., p. 277)), the follow‐
76 ing comments are made:
77 "If you want to generate a random integer between 1 and 10, you
78 should always do it by using high-order bits, as in
79 j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));
81 and never by anything resembling
83 j = 1 + (rand() % 10);
85 (which uses lower-order bits)."
87 Random-number generation is a complex topic. The Numerical Recipes in
89 C book (see reference above) provides an excellent discussion of prac‐
90 tical random-number generation issues in Chapter 7 (Random Numbers).
91 For a more theoretical discussion which also covers many practical
93 issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
94 The Art of Computer Programming, volume 2 (Seminumerical Algorithms),
95 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company,
96 1981.
97
99 The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
100 POSIX.1-2001. The function rand_r() is from POSIX.1-2001.
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103 drand48(3), random(3)
104 2003-11-15 RAND(3)