RANDOM(3) Linux Programmer's Manual RANDOM(3)
random, srandom, initstate, setstate - random number generator
long int random(void);
void srandom(unsigned int seed);
char *initstate(unsigned int seed, char *state, size_t n);
char *setstate(char *state);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
random(), srandom(), initstate(), setstate():
_SVID_SOURCE || _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
The random() function uses a nonlinear additive feedback random number generator employing a default table of size 31 long integers to
return successive pseudo-random numbers in the range from 0 to RAND_MAX. The period of this random number generator is very large, approx-
imately 16 * ((2^31) - 1).
The srandom() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by random(). These
sequences are repeatable by calling srandom() with the same seed value. If no seed value is provided, the random() function is automati-
cally seeded with a value of 1.
The initstate() function allows a state array state to be initialized for use by random(). The size of the state array n is used by init-
state() to decide how sophisticated a random number generator it should use--the larger the state array, the better the random numbers will
be. seed is the seed for the initialization, which specifies a starting point for the random number sequence, and provides for restarting
at the same point.
The setstate() function changes the state array used by the random() function. The state array state is used for random number generation
until the next call to initstate() or setstate(). state must first have been initialized using initstate() or be the result of a previous
call of setstate().
The random() function returns a value between 0 and RAND_MAX. The srandom() function returns no value.
The initstate() function returns a pointer to the previous state array. On error, errno is set to indicate the cause.
On success, setstate() returns a pointer to the previous state array. On error, it returns NULL, with errno set to indicate the cause of
EINVAL The state argument given to setstate() was NULL.
EINVAL A state array of less than 8 bytes was specified to initstate().
Current "optimal" values for the size of the state array n are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to the
nearest known amount. Using less than 8 bytes will cause an error.
This function should not be used in cases where multiple threads use random() and the behavior should be reproducible. Use random_r(3) for
Random-number generation is a complex topic. Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery,
Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 2007, 3rd ed.) provides an excellent discussion of practi-
cal random-number generation issues in Chapter 7 (Random Numbers).
For a more theoretical discussion which also covers many practical issues in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's The
Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.
According to POSIX, initstate() should return NULL on error. In the glibc implementation, errno is (as specified) set on error, but the
function does not return NULL.
drand48(3), rand(3), random_r(3), srand(3)
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