1RAND(3) Linux Programmer's Manual RAND(3)
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6 rand, rand_r, srand - pseudo-random number generator
7
9 #include <stdlib.h>
10 int rand(void);
12 int rand_r(unsigned int *seedp);
14 void srand(unsigned int seed);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
20
22 The rand() function returns a pseudo-random integer in the range
23 [0, RAND_MAX].
24 The srand() function sets its argument as the seed for a new sequence
26 of pseudo-random integers to be returned by rand(). These sequences
27 are repeatable by calling srand() with the same seed value.
28 If no seed value is provided, the rand() function is automatically
30 seeded with a value of 1.
31 The function rand() is not reentrant or thread-safe, since it uses hid‐
33 den state that is modified on each call. This might just be the seed
34 value to be used by the next call, or it might be something more elabo‐
35 rate. In order to get reproducible behavior in a threaded application,
36 this state must be made explicit. The function rand_r() is supplied
37 with a pointer to an unsigned int, to be used as state. This is a very
38 small amount of state, so this function will be a weak pseudo-random
39 generator. Try drand48_r(3) instead.
40
42 The rand() and rand_r() functions return a value between 0 and
43 RAND_MAX. The srand() function returns no value.
44
46 The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
47 POSIX.1-2001. The function rand_r() is from POSIX.1-2001.
48 POSIX.1-2008 marks rand_r() as obsolete.
49
51 The versions of rand() and srand() in the Linux C Library use the same
52 random number generator as random(3) and srandom(3), so the lower-order
53 bits should be as random as the higher-order bits. However, on older
54 rand() implementations, and on current implementations on different
55 systems, the lower-order bits are much less random than the higher-
56 order bits. Do not use this function in applications intended to be
57 portable when good randomness is needed. (Use random(3) instead.)
58
60 POSIX.1-2001 gives the following example of an implementation of rand()
61 and srand(), possibly useful when one needs the same sequence on two
62 different machines.
63 static unsigned long next = 1;
65 /* RAND_MAX assumed to be 32767 */
67 int myrand(void) {
68 next = next * 1103515245 + 12345;
69 return((unsigned)(next/65536) % 32768);
70 }
71 void mysrand(unsigned seed) {
73 next = seed;
74 }
75
77 drand48(3), random(3)
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80 This page is part of release 3.25 of the Linux man-pages project. A
81 description of the project, and information about reporting bugs, can
82 be found at http://www.kernel.org/doc/man-pages/.
83 2008-08-29 RAND(3)