random(3)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 ran‐
dom_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
Specifies an initial seed value. Points to the array of state informa‐
tion. Specifies the size of the state information array. Points to a
place to store the random number. 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, ran‐
dom()&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 restart‐
ing 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() func‐
tion is called or the setstate() function is called again. The set‐
state() 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 sran‐
dom() 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() func‐
tions 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 init‐
state_r() or setstate_r() functions are called.
RETURN VALUES
Upon successful completion, the random() function returns a random num‐
ber.
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 correspond‐
ing value: 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)random(3)
