random, srandom, srandomdev, initstate, setstate - better
random number
generator; routines for changing generators
#include <stdlib.h>
long
random(void);
void
srandom(unsigned int seed);
void
srandomdev(void);
char *
initstate(unsigned int seed, char *state, size_t n);
char *
setstate(const char *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
(2**31)-1. The
period of this random number generator is very large, approximately
16*((2**31)-1).
The random() and srandom() functions have (almost) the same
calling sequence
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 by random() 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 calling srandom() with `1' as the seed.
The srandomdev() 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 generate
states which are impossible to reproduce by calling
srandom() with
any value, since the succeeding terms in the state buffer
are no longer
derived from the LC algorithm applied to a fixed 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. The initstate() function returns a pointer to the previous
state information array.
Once a state has been initialized, the setstate() routine
provides for
rapid switching between states. The setstate() function 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().
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.
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.
arc4random(3), drand48(3), rand(3), random(4)
The random(), srandom(), initstate(), and setstate() functions conform to
X/Open Portability Guide Issue 4.2 (``XPG4.2'').
The srandomdev() function is an extension.
These functions appeared in 4.2BSD.
Earl T. Cohen
About 2/3 the speed of rand(3).
OpenBSD 3.6 April 19, 1991
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