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():
_XOPEN_SOURCE >= 500
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 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 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() function returns a pointer to the previous state array.
On error, errno is set to indicate the cause.
On success, setstate() returns a pointer to the previous state array.
On error, it returns NULL, with errno set to indicate the cause of 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(), │ Thread safety │ MT-Safe │
│initstate(), setstate() │ │ │
└────────────────────────┴───────────────┴─────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
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.
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)
COLOPHON
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