Manual Pages  — RANDOM

Unless initialized with less than 32 bytes of state, 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 2³¹-1. The period of this random number generator is very large, approximately 16µ(2³¹-1).

If initialized with less than 32 bytes of state, random() uses the poor-quality 32-bit Park-Miller LCG.

The random() and srandom() functions are analagous to rand(3) and srand(3).

Like rand(3), random() is implicitly initialized as if srandom(1) had been invoked explicitly.

The srandomdev() routine initializes the state array using random numbers obtained from the kernel. This can generate states which are impossible to reproduce by calling srandom(), because the succeeding terms in the state buffer are no longer derived from the Park-Miller LCG algorithm applied to a fixed seed.

The initstate() routine initializes the provided state array of uint32_t values and uses it in future random() invocations. (Despite the char * type of state, the underlying object must be a naturally aligned array of 32-bit values.) The size of the state array (in bytes) is used by initstate() to decide how sophisticated a random number generator it should use — the more state, the better the random numbers will be. (Current "optimal" values for the amount of state information 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.) The seed is used as in srandom(). The initstate() function returns a pointer to the previous state information array.

The setstate() routine switches random() to using the provided state. It returns a pointer to the previous state.

Once a state array has been initialized, it may be restarted at a different point either by calling initstate() (with the desired seed, the state array, and its size) or by calling both setstate() (with the state array) and srandom() (with the desired seed). The advantage of calling both setstate() and srandom() is that the size of the state array does not have to be remembered after it is initialized.

With 256 bytes of state information, the period of the random number generator is greater than 2⁶⁹, which should be sufficient for most purposes.