random, srandom, initstate, setstate — random number generator
||unsigned int seed
||unsigned int seed,|
random() function uses a
nonlinear 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
RAND_MAX. The period of this random number
generator is very large, approximately 16 * ((2^31) − 1).
srandom() function sets
its argument as the seed for a new sequence of pseudo-random
integers to be returned by
random(). These sequences are repeatable by
srandom() with the same
seed value. If no seed value is provided, the
random() function is automatically seeded
with a value of 1.
allows a state array
state to be initialized for use
random(). The size of the
n is used
initstate() to decide how
sophisticated a random number generator it should
use—the larger the state array, the better the random
numbers will be.
is the seed for the initialization, which specifies a
starting point for the random number sequence, and provides
for restarting at the same point.
changes the state array used by the
random() function. The state array
state is used for
random number generation until the next call to
state must first have been
or be the result of a previous call of
returns a value between 0 and
srandom() function returns no value.
returns a pointer to the previous state array. On error,
errno is set to indicate the
returns a pointer to the previous state array. On error, it
returns NULL, with
errno set to
indicate the cause of the error.
argument given to
setstate() was NULL.
A state array of less than 8 bytes was specified to
For an explanation of the terms used in this section, see attributes(7).
Current "optimal" values for the size of the state array
n 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
This function should not be used in cases where multiple
random() and the
behavior should be reproducible. Use random_r(3) for that
Random-number generation is a complex topic. Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.) provides an excellent discussion of practical random-number generation issues in Chapter 7 (Random Numbers).
For a more theoretical discussion which also covers many practical issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's The Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.
According to POSIX,
initstate() should return NULL on error. In
the glibc implementation,
is (as specified) set on error, but the function does not
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Copyright 1993 David Metcalfe (davidprism.demon.co.uk)
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Linux libc source code
Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
386BSD man pages
Modified Sun Mar 28 00:25:51 1993, David Metcalfe
Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faithcs.unc.edu)
Modified Sun Aug 20 21:47:07 2000, aeb