random(3)
NAME
random, srandom, initstate, setstate - random number generator
SYNOPSIS
#include <stdlib.h> long int random(void); void srandom(unsigned int seed); char *initstate(unsigned int seed, char *state, size_t n); char *setstate(char *state); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): random(), srandom(), initstate(), setstate(): _SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500
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 RAND_MAX. 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 initstate() to decide how sophisticated a random number generator it should
use -- the larger the state array, the better the random numbers will
be. 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 RAND_MAX. The
srandom() function returns no value. The initstate() and setstate()
functions return a pointer to the previous state array, or NULL on
error.
ERRORS
EINVAL A state array of less than 8 bytes was specified to initstate().
CONFORMING TO
4.3BSD, POSIX.1-2001.
NOTES
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 nearest known amount. Using less than 8 bytes will cause an error.
This function should not be used in cases where multiple threads use
random() and the behavior should be reproducible. Use random_r(3) for
that purpose.
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
issues 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.
SEE ALSO
drand48(3), rand(3), random_r(3), srand(3)
COLOPHON
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