RANDOM(3) BSD Programmer's Manual RANDOM(3)

random,srandom,srandomdev,initstate,setstate- better random number generator; routines for changing generators

#include <stdlib.h>longrandom(void);voidsrandom(unsigned int seed);voidsrandomdev(void);char*initstate(unsigned int seed,char *state,size_t n);char*setstate(const char *state);

Therandom() 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 (2**31)-1. The period of this random number generator is very large, approximately 16*((2**31)-1). Therandom() andsrandom() functions have (almost) the same calling se- quence and initialization properties as rand(3)/srand(3). The difference is that rand produces a much less random sequence - in fact, the low dozen bits generated by rand go through a cyclic pattern. All the bits generated byrandom() are usable. For example, 'random()&01' will produce a random binary value. Like rand(3),random() will by default produce a sequence of numbers that can be duplicated by callingsrandom() with '1' as the seed. Thesrandomdev() routine initializes a state array using the arandom(4) random number device which returns good random numbers, suitable for cryptographic use. Note that this particular seeding procedure can gen- erate states which are impossible to reproduce by callingsrandom() with any value, since the succeeding terms in the state buffer are no longer derived from the LC algorithm applied to a fixed seed. Theinitstate() routine allows a state array, passed in as an argument, to be initialized for future use. The size of the state array (in bytes) is used byinitstate() to decide how sophisticated a random number gen- erator 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 for the initialization (which specifies a starting point for the random number sequence, and provides for restarting at the same point) is also an argument. Theinitstate() function returns a pointer to the pre- vious state information array. Once a state has been initialized, thesetstate() routine provides for rapid switching between states. Thesetstate() function returns a pointer to the previous state array; its argument state array is used for further random number generation until the next call toinitstate() orset-state(). Once a state array has been initialized, it may be restarted at a dif- ferent point either by callinginitstate() (with the desired seed, the state array, and its size) or by calling bothsetstate() (with the state array) andsrandom() (with the desired seed). The advantage of calling bothsetstate() andsrandom() 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 gen- erator is greater than 2**69 which should be sufficient for most pur- poses.

Ifinitstate() is called with less than 8 bytes of state information, or ifsetstate() detects that the state information has been garbled, error messages are printed on the standard error output.

arc4random(3), drand48(3), rand(3), random(4)

Therandom(),srandom(),initstate(), andsetstate() functions conform to X/Open Portability Guide Issue 4.2 ("XPG4.2"). Thesrandomdev() function is an extension.

These functions appeared in 4.2BSD.

Earl T. Cohen

About 2/3 the speed of rand(3). MirOS BSD #10-current April 19, 1991 1

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