RANDOM(3) Linux Programmer's Manual RANDOM(3)
random, srandom, initstate, setstate - random number generator
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 automat-
ically 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. 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.
EINVAL The state argument given to setstate() was NULL.
EINVAL A state array of less than 8 bytes was specified to initstate().
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 Scien-
tific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vet-
terling; New York: Cambridge University Press, 2007, 3rd ed.) provides an excellent dis-
cussion 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
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.
drand48(3), rand(3), random_r(3), srand(3)
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GNU 2013-04-19 RANDOM(3)