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(unsigned int seed, char state[.n], size_t n);
char *setstate(char *state);
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 (), setstate () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
The
random() function should not be used in multithreaded programs where
reproducible behavior is required. 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.
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)