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RANDOM(3) Linux Programmer's Manual RANDOM(3)
random, srandom, initstate, setstate - random number generator
#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 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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().
The random() function returns a value between 0 and RAND_MAX. The srandom()
function returns no value. The initstate() function returns a pointer to the
previous state array. The setstate() function returns a pointer to the
previous state array, or NULL on error.
EINVAL A state array of less than 8 bytes was specified to initstate().
4.3BSD, POSIX.1-2001.
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.
drand48(3), rand(3), random_r(3), srand(3)
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GNU 2010-09-20 RANDOM(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface