random, random_r, srandom, srandom_r, initstate, initstate_r,
setstate, setstate_r - Generate pseudo-random
number
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 );
Standard C Library (libc)
Specifies an initial seed value. Points to the array of
state information. Specifies the size of the state information
array. Points to a place to store the random number.
Points to a random_data structure.
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.
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.
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.
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: The retval, rand_data,
state, or seed parameters are invalid, or the state field
of the rand_data structure is invalid.
Functions: drand48(3), rand(3)
random(3)
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