NAME

       random, srandom, initstate, setstate - random number generator

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

       long random(void);
       void srandom(unsigned int seed);

       char *initstate(unsigned int seed, char state[.n], size_t n);
       char *setstate(char *state);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       random(), srandom(), initstate(), setstate():
           _XOPEN_SOURCE >= 500
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

DESCRIPTION

       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  2^31 - 1.   The  period of this random number generator is very large,
       approximately 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
       automatically 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 initstate() to decide how sophisticated a  random
       number  generator  it should use—the larger the state array, the better the random numbers
       will be.  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 results in an  error.   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().

RETURN VALUE

       The  random()  function  returns a value between 0 and (2^31) - 1.  The srandom() function
       returns no value.

       The initstate() function returns a pointer to the previous state array.   On  failure,  it
       returns NULL, and errno is set to indicate the error.

       On  success,  setstate()  returns  a  pointer to the previous state array.  On failure, it
       returns NULL, and errno is set to indicate the error.

ERRORS

       EINVAL The state argument given to setstate() was NULL.

       EINVAL A state array of less than 8 bytes was specified to initstate().

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │random(), srandom(), initstate(), setstate()                   │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, 4.3BSD.

NOTES

       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 practical 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.

CAVEATS

       The random() function should not be used  in  multithreaded  programs  where  reproducible
       behavior is required.  Use random_r(3) for that purpose.

BUGS

       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.

SEE ALSO

       getrandom(2), drand48(3), rand(3), random_r(3), srand(3)