NAME
initstate, random, setstate, srandom - pseudo-random number functions
SYNOPSIS
#include <stdlib.h>
char *initstate(unsigned seed, char *state, size_t size);
long random(void);
char *setstate(const char *state);
void srandom(unsigned seed);
DESCRIPTION
The random() function shall use a non-linear additive feedback random-
number generator employing a default state array size of 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
approximately 16 x (2**31-1). The size of the state array determines
the period of the random-number generator. Increasing the state array
size shall increase the period.
With 256 bytes of state information, the period of the random-number
generator shall be greater than 2**69.
Like rand(), random() shall produce by default a sequence of numbers
that can be duplicated by calling srandom() with 1 as the seed.
The srandom() function shall initialize the current state array using
the value of seed.
The initstate() and setstate() functions handle restarting and changing
random-number generators. The initstate() function allows a state
array, pointed to by the state argument, to be initialized for future
use. The size argument, which specifies the size in bytes of the state
array, shall be used by initstate() to decide what type of random-
number generator to use; the larger the state array, the more random
the numbers. Values for the amount of state information are 8, 32, 64,
128, and 256 bytes. Other values greater than 8 bytes are rounded down
to the nearest one of these values. If initstate() is called with
8<=size<32, then random() shall use a simple linear congruential random
number generator. The seed argument specifies a starting point for the
random-number sequence and provides for restarting at the same point.
The initstate() function shall return a pointer to the previous state
information array.
If initstate() has not been called, then random() shall behave as
though initstate() had been called with seed=1 and size=128.
Once a state has been initialized, setstate() allows switching between
state arrays. The array defined by the state argument shall be used for
further random-number generation until initstate() is called or
setstate() is called again. The setstate() function shall return a
pointer to the previous state array.
RETURN VALUE
If initstate() is called with size less than 8, it shall return NULL.
The random() function shall return the generated pseudo-random number.
The srandom() function shall not return a value.
Upon successful completion, initstate() and setstate() shall return a
pointer to the previous state array; otherwise, a null pointer shall be
returned.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
After initialization, a state array can be restarted at a different
point in one of two ways:
1. The initstate() function can be used, with the desired seed, state
array, and size of the array.
2. The setstate() function, with the desired state, can be used,
followed by srandom() with the desired seed. The advantage of using
both of these functions is that the size of the state array does
not have to be saved once it is initialized.
Although some implementations of random() have written messages to
standard error, such implementations do not conform to
IEEE Std 1003.1-2001.
Issue 5 restored the historical behavior of this function.
Threaded applications should use erand48(), nrand48(), or jrand48()
instead of random() when an independent random number sequence in
multiple threads is required.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
drand48() , rand() , the Base Definitions volume of
IEEE Std 1003.1-2001, <stdlib.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .