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random(3c)
Standard C Library Functions random(3C)
NAME
random, srandom, initstate, setstate - pseudorandom number functions
SYNOPSIS
#include <stdlib.h>
long random(void);
void srandom(unsigned int seed);
char *initstate(unsigned int seed, char *state, size_t 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) produces 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 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 suf‐
ficient 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 argument, 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 gen‐
erator 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 near‐
est one of these values. For values smaller than 8, random() uses 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 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 lin‐
ear congruential random number generator.
Once a state has been initialized, setstate() allows switching between
state arrays. The array defined by the state argument is used for fur‐
ther random-number generation until initstate() is called or setstate()
is called again. The setstate() function returns a pointer to the pre‐
vious state array.
RETURN VALUES
The random() function returns the generated pseudo-random number.
The srandom() function returns no value.
Upon successful completion, initstate() and setstate() return a pointer
to the previous state array. Otherwise, a null pointer is returned.
ERRORS
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 functions is that the size
of the state array does not have to be saved once it is ini‐
tialized.
Programmers should use /dev/urandom or /dev/random for most random-num‐
ber generation, especially for cryptographic purposes. See getrandom(2)
and random(4D).
EXAMPLES
Example 1 Initialize an array.
The following example demonstrates the use of initstate() to initialize
an array. It also demonstrates how to initialize an array and pass it
to setstate().
# include <stdlib.h>
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());
}
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelSee NOTES below.
_ StandardSee standards(7).
SEE ALSO
getrandom(2), drand48(3C), rand(3C), random(4D), attributes(7), stan‐
dards(7)
NOTES
The random() and srandom() functions are unsafe in multithreaded appli‐
cations.
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 Oracle Developer Studio compilers.
For cryptographic uses, the random(4D) device or getrandom(2) calls
should be used instead of these routines.
Oracle Solaris 11.4 9 Jun 2018 random(3C)