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random(3)
NAME
random, random_r, srandom, srandom_r, initstate, initstate_r, setstate,
setstate_r - Generate pseudo-random number
SYNOPSIS
int random(
void );
int srandom(
unsigned seed );
char *initstate(
unsigned int seed,
char *state,
size_t size );
char *setstate(
const char *state );
The following functions are supported in order to maintain backward
compatibility with previous versions of the operating system.
int random_r(
int *retval,
struct random_data *rand_data );
int srandom_r(
unsigned seed,
struct random_data *rand_data );
int initstate_r(
unsigned seed,
char *state,
int size,
char **retval,
struct random_data *rand_data );
int setstate_r(
char *state,
char **retval,
struct random_data *rand_data );
LIBRARY
Standard C Library (libc)
PARAMETERS
seed
Specifies an initial seed value.
state
Points to the array of state information.
size
Specifies the size of the state information array.
retval
Points to a place to store the random number.
rand_data
Points to a random_data structure.
DESCRIPTION
The random() and srandom() functions are random number generators that have
virtually the same calling sequence and initialization properties as the
rand() and srand() functions, but produce sequences that are more random.
The low 12 bits generated by rand() go through a cyclic pattern. All bits
generated by random() are usable. For example, random()&01 produces a
random binary value.
The random() function uses a nonlinear additive feedback random number
generator employing a default state array size of 31 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*((2^31)-1). The
size of the state array determines the period of the random number
generator. Increasing the state array size increases the period.
With a full 256 bytes of state information, the period of the random-number
generator is greater than 2^69, which should be sufficient for most
purposes.
Like the rand() function, the random() function produces by default a
sequence of numbers that can be duplicated by calling the srandom()
function with a value of 1 as the seed. The srandom() function, unlike the
srand() function, does not return the old seed because the amount of state
information used is more than a single word.
The initstate() and setstate() functions handle restarting and changing
random-number generators. The initstate() function allows a state array,
passed in as an argument, to be initialized for future use. The size in
bytes of the state array is used by the initstate() function to decide how
sophisticated a 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. Amounts are rounded down to the nearest
known value. The seed parameter specifies a starting point for the random-
number sequence and provides for restarting at the same point. The
initstate() function returns a pointer to the previous state information
array.
Once a state has been initialized, the setstate() function allows rapid
switching between states. The array defined by the state parameter is used
for further random-number generation until the initstate() function is
called or the setstate() function is called again. The setstate() function
returns a pointer to the previous state array.
After initialization, a state array can be restarted at a different point
in one of two ways:
· The initstate() function can be used, with the desired seed, state
array, and size of the array.
· The setstate() function, with the desired state, can be used, followed
by the srandom() function 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 after it is initialized.
NOTES
The random_r(), srandom_r(), initstate_r(), and setstate_r() functions are
the reentrant versions of the random(), srandom(), initstate(), and
setstate() functions. They are supported in order to maintain backward
compatibility with previous versions of the operating system.
Upon successful completion, the initstate_r() and setstate_r() functions
provide a pointer to the returned state in retval. The random_r() function
provides a pointer to the returned random number in retval. Upon successful
completion, the random_r(), srandom_r(), initstate_r(), and setstate_r()
functions return a value of 0 (zero). Upon error, they return a value of
-1 and may set errno.
Note that the srandom_r() function takes the rand_data structure, which
should first be initialized by the initstate_r() function. Note also that
the rand_data.state parameter needs to be NULL before the initstate_r() or
setstate_r() functions are called.
RETURN VALUES
Upon successful completion, the random() function returns a random number.
Upon successful completion, the initstate() and setstate() functions return
a pointer to the previous state information array. Upon error, a value of 0
(zero) is returned. If initstate() is called with size less than 8, NULL is
returned.
Upon successful completion, the srandom() function returns success with a
value of 0 (zero). Upon failure, it returns -1 and may set errno. The
srandom() function initializes the state seed.
ERRORS
If the setstate() function detects that the state information has been
damaged, an error message is written to standard error.
If any of the following conditions occurs, the random_r(), srandom_r(),
setstate_r(), and initstate_r() functions set errno to the corresponding
value:
[EINVAL]
The retval, rand_data, state, or seed parameters are invalid, or the
state field of the rand_data structure is invalid.
SEE ALSO
Functions: drand48(3), rand(3)
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