#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():
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().
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.