Standard C Library Functions random(3C)
NAME
random, srandom, initstate, setstate - pseudorandom number
functions
SYNOPSIS
#include
long random(void);
void srandom(unsigned int seed);
char *initstate(unsigned int seed, char *state, sizet size);
char *setstate(const char *state);
DESCRIPTION
The random() function uses a nonlinear 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
increases the period.
The srandom() function initializes the current state array
using the value of seed.
The random() and srandom() functions have (almost) the same
calling sequence and initialization properties as rand() and
srand() (see rand(3C)). The difference is that rand(3C) pro-
duces a much less random sequence-in fact, the low dozen
bits generated by rand go through a cyclic pattern. All the
bits generated by random() are usable.
The algorithm from rand() is used by srandom() to generate
the 31 state integers. Because of this, different srandom()
seeds often produce, within an offset, the same sequence of
low order bits from random(). If low order bits are used
directly, random() should be initialized with setstate()
using high quality random values.
Unlike srand(), srandom() does not return the old seed
because the amount of state information used is much more
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Standard C Library Functions random(3C)
than a single word. Two other routines are provided to deal
with restarting/changing random number generators. With 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 rand(3C), random() produces by default a sequence of
numbers that can be duplicated by calling srandom() with 1
as the 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 argu-
ment, to be initialized for future use. The size argument,
which specifies the size in bytes of the state array, is
used by initstate() to decide what type of random-number
generator to use; the larger the state array, the more ran-
dom 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. For values smaller than 8, random() uses a simple
linear congruential random number generator. The seed argu-
ment 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.
If initstate() has not been called, then random() behaves as
though initstate() had been called with seed=1 and size=128.
If initstate() is called with size<8, then random() uses a
simple linear congruential random number generator.
Once a state has been initialized, setstate() allows switch-
ing between state arrays. The array defined by the state
argument is used for further random-number generation until
initstate() is called or setstate() is called again. The
setstate() function returns a pointer to the previous state
array.
RETURN VALUES
The random() function returns the generated pseudo-random
number.
The srandom() function returns no value.
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Standard C Library Functions random(3C)
Upon successful completion, initstate() and setstate()
return a pointer to the previous state array. Otherwise, a
null pointer is returned.
ERORS
No errors are defined.
USAGE
After initialization, a state array can be restarted at a
different point in one of two ways:
o The initstate() function can be used, with the
desired seed, state array, and size of the array.
o The setstate() function, with the desired state,
can be used, followed by srandom() with the desired
seed. The advantage of using both of these func-
tions is that the size of the state array does not
have to be saved once it is initialized.
EXAMPLES
Example 1 Initialize an array.
The following example demonstrates the use of initstate() to
intialize an array. It also demonstrates how to initialize
an array and pass it to setstate().
# include
static unsigned int state0[32];
static unsigned int state1[32] = {
3,
0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342,
0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86,
0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7,
0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc,
0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb,
0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b,
0xf5ad9d0e, 0x8999220b, 0x27fb47b9
};
main() {
unsigned seed;
int n;
seed = 1;
n = 128;
(void)initstate(seed, (char *)state0, n);
printf("random() = %d0\n", random());
(void)setstate((char *)state1);
printf("random() = %d0\n", random());
}
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Standard C Library Functions random(3C)
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Standard
MT-Level See NOTES below.
SEE ALSO
drand48(3C), rand(3C), attributes(5), standards(5)
NOTES
The random() and srandom() functions are unsafe in mul-
tithreaded applications.
Use of these functions in multithreaded applications is
unsupported.
For initstate() and setstate(), the state argument must be
aligned on an int boundary.
Newer and better performing random number generators such as
addrans() and lcrans() are available with the SUNWspro pack-
age.
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