#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. 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 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.
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.