RANDOM(3B)RANDOM(3B)NAME
random, srandom, initstate, setstate - better random number generator;
routines for changing generators
SYNOPSIS
#include <stdlib.h>
long random(void);
void srandom(unsigned seed);
char *initstate(unsigned int seed, char *state, size_t n);
char *setstate(const char *state);
DESCRIPTION
Random 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).
Random/srandom have (almost) the same calling sequence and initialization
properties as rand/srand. The difference is that rand(3C) produces a much
less random sequence - in fact, the low dozen bits generated by rand go
through a cyclic pattern. All the bits generated by random are usable.
For example, ``random()&01'' will produce a random binary value.
Srandom does not return the old seed; the reason for this is that the
amount of state information used is much more than a single word. (Two
other routines are provided to deal with restarting/changing random
number generators). Like rand(3C), however, random will by default
produce a sequence of numbers that can be duplicated by calling srandom
with 1 as the seed.
The initstate 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 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
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. Initstate returns a pointer to the previous state
information array.
Once a state has been initialized, the setstate routine provides for
rapid switching between states. Setstate returns a pointer to the
previous state array; its argument state array is used for further random
number generation until the next call to initstate or setstate.
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RANDOM(3B)RANDOM(3B)
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 2**69 which should be sufficient for most
purposes.
DIAGNOSTICS
If initstate is called with less than 8 bytes of state information, or if
setstate detects that the state information has been garbled, error
messages are printed on the standard error output.
SEE ALSOdrand48(3C), rand(3C)BUGS
About 2/3 the speed of rand(3C).
When initstate is used in the 64bit ABI, it misinterprets the size of the
state array, causing it to overflowing the array. A simple workaround is
to declare the state array to be twice as large as the size specified by
the third parameter.
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