random(3) Library Functions Manual random(3)
NAME
random, srandom, initstate, setstate - random number generator
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
long random(void);
void srandom(unsigned int seed);
char *initstate(unsigned int seed, char state[.n], size_t n);
char *setstate(char *state);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
random(), srandom(), initstate(), setstate():
_XOPEN_SOURCE >= 500
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
DESCRIPTION
The random() function uses a nonlinear additive feedback random number
generator employing a default table of size 31 long integers to return
successive pseudo-random numbers in the range from 0 to 2^31 - 1. The
period of this random number generator is very large, approximately
16 * ((2^31) - 1).
The srandom() function sets its argument as the seed for a new sequence
of pseudo-random integers to be returned by random(). These sequences
are repeatable by calling srandom() with the same seed value. If no
seed value is provided, the random() function is automatically seeded
with a value of 1.
The initstate() function allows a state array state to be initialized
for use by random(). The size of the state array n is used by init-
state() to decide how sophisticated a random number generator it should
use—the larger the state array, the better the random numbers will be.
Current "optimal" values for the size of the state array n are 8, 32,
64, 128, and 256 bytes; other amounts will be rounded down to the near-
est known amount. Using less than 8 bytes results in an error. seed is
the seed for the initialization, which specifies a starting point for
the random number sequence, and provides for restarting at the same
point.
The setstate() function changes the state array used by the random()
function. The state array state is used for random number generation
until the next call to initstate() or setstate(). state must first have
been initialized using initstate() or be the result of a previous call
of setstate().
RETURN VALUE
The random() function returns a value between 0 and (2^31) - 1. The
srandom() function returns no value.
The initstate() function returns a pointer to the previous state array.
On failure, it returns NULL, and errno is set to indicate the error.
On success, setstate() returns a pointer to the previous state array.
On failure, it returns NULL, and errno is set to indicate the error.
ERRORS
EINVAL The state argument given to setstate() was NULL.
EINVAL A state array of less than 8 bytes was specified to initstate().
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ random(), srandom(), initstate(), │ Thread safety │ MT-Safe │
│ setstate() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 4.3BSD.
NOTES
Random-number generation is a complex topic. Numerical Recipes in C:
The Art of Scientific Computing (William H. Press, Brian P. Flannery,
Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University
Press, 2007, 3rd ed.) provides an excellent discussion of practical
random-number generation issues in Chapter 7 (Random Numbers).
For a more theoretical discussion which also covers many practical is-
sues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's The
Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd
ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.
CAVEATS
The random() function should not be used in multithreaded programs where
reproducible behavior is required. Use random_r(3) for that purpose.
BUGS
According to POSIX, initstate() should return NULL on error. In the
glibc implementation, errno is (as specified) set on error, but the
function does not return NULL.
SEE ALSO
getrandom(2), drand48(3), rand(3), random_r(3), srand(3)
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