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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | CAVEATS | BUGS | SEE ALSO | COLOPHON |
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random(3) Library Functions Manual random(3)
random, srandom, initstate, setstate - random number generator
Standard C library (libc, -lc)
#include <stdlib.h>
long random(void);
void srandom(unsigned int seed);
char *initstate(size_t n;
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
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 initstate() 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 nearest 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().
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.
EINVAL The state argument given to setstate() was NULL.
EINVAL A state array of less than 8 bytes was specified to
initstate().
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ random(), srandom(), initstate(), │ Thread safety │ MT-Safe │
│ setstate() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001, 4.3BSD.
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.
The random() function should not be used in multithreaded programs
where reproducible behavior is required. Use random_r(3) for that
purpose.
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.
getrandom(2), drand48(3), rand(3), random_r(3), srand(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-06-28 random(3)
Pages that refer to this page: drand48(3), drand48_r(3), rand(3), random_r(3)
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