random(3)random(3)Name
random, srandom, initstate, setstate - better random number generator;
routines for changing generators
Syntax
long random()
void srandom(seed)
int seed;
char *initstate(seed, state, n)
unsigned seed;
char *state;
int n;
char *setstate(state)
char *state;
Description
The subroutine uses a non-linear additive feedback random number gener‐
ator employing a default table of size 31 long integers to return suc‐
cessive 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 subroutines have (almost) the same calling sequence and initializa‐
tion properties as The difference is that rand(3) 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 are usable. For
example, “random()&01” will produce a random binary value.
Unlike 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 however, will by default produce a sequence
of numbers that can be duplicated by calling with 1 as the seed.
The routine allows a state array, passed in as an argument, to be ini‐
tialized for future use. The size of the state array (in bytes) is
used by 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. returns a pointer to the previous state information
array.
Once a state has been initialized, the routine provides for rapid
switching between states. The subroutine returns a pointer to the pre‐
vious state array; its argument state array is used for further random
number generation until the next call to or
Once a state array has been initialized, it may be restarted at a dif‐
ferent point either by calling (with the desired seed, the state array,
and its size) or by calling both (with the state array) and (with the
desired seed). The advantage of calling both and 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 pur‐
poses.
Diagnostics
If is called with less than 8 bytes of state information, or if detects
that the state information has been garbled, error messages are printed
on the standard error output.
See Alsorand(3)random(3)