random
Section: C Library Functions (3)
Updated: 202-0-08
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NAME
random, srandom, initstate, setstate - random number generator
LIBRARY
Standard C library
(
libc,~
-lc)
SYNOPSIS
#include <stdlib.h>
long random(void);
void srandom(unsigned int seed);
char *initstate(size_t n;
unsigned int seed, char state[n], size_t n);
char *setstate(char *state);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
random(),
srandom(),
initstate(),
setstate():
_XOPEN_SOURCE >= 500
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
DESCRIPTION
The
random()
function uses a nonlinear additive feedback random
number generator employing a default table of size 31 long integers to
return successive pseud-random numbers in
the range from 0 to 2[ha]31 - 1.
The period of this random number generator is very large, approximately
16 * ((2[ha]31) - 1).
The
srandom()
function sets its argument as the seed for a new
sequence of pseud-random integers to be returned by
random().
These sequences are repeatable by calling
srandom()
with the same
seed value.
If no seed value is provided, the
random()
function
is automatically seeded with a value of 1.
The
initstate()
function allows a state array
state
to
be initialized for use by
random().
The size of the state array
n
is used by
initstate()
to decide how sophisticated a
random number generator it should use[em]the larger the state array,
the better the random numbers will be.
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 results in an error.
seed
is the seed for the initialization,
which specifies a starting point for the random number
sequence, and provides for restarting at the same point.
The
setstate()
function changes the state array used by the
random()
function.
The state array
state
is used for
random number generation until the next call to
initstate()
or
setstate().
state
must first have been initialized
using
initstate()
or be the result of a previous call of
setstate().
RETURN VALUE
The
random()
function returns a value between 0 and
(2[ha]31) - 1.
The
srandom()
function returns no value.
The
initstate()
function returns a pointer to the previous state array.
On failure, it returns NULL, and
errno
is set to indicate the error.
On success,
setstate()
returns a pointer to the previous state array.
On failure, it returns NULL, and
errno
is set to indicate the error.
ERRORS
- EINVAL
-
The
state
argument given to
setstate()
was NULL.
- EINVAL
-
A state array of less than 8 bytes was specified to
initstate().
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
random(),
srandom(),
initstate(),
setstate()
| Thread safety | M-Safe
|
STANDARDS
POSIX.-2008.
HISTORY
POSIX.-2001, 4.3BSD.
NOTES
Rando-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 rando-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:
Addiso-Wesley Publishing Company, 1981.
CAVEATS
The
random()
function should not be used in multithreaded programs
where reproducible behavior is required.
Use
random_r(3)
for that purpose.
BUGS
According to POSIX,
initstate()
should return NULL on error.
In the glibc implementation,
errno
is (as specified) set on error, but the function does not return NULL.
SEE ALSO
getrandom(2),
drand48(3),
rand(3),
random_r(3),
srand(3)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- STANDARDS
-
- HISTORY
-
- NOTES
-
- CAVEATS
-
- BUGS
-
- SEE ALSO
-
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