From Linux Man Pages
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);
DESCRIPTION
The random() function uses a non-linear 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 initial-
ized 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() and setstate() functions return a pointer to the previous state array, or NULL on error.
ERRORS
EINVAL A state array of less than 8 bytes was specified to initstate().
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.
CONFORMING TO
4.3BSD, POSIX.1-2001.
RELATED
rand(3), srand(3)
CATEGORY