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)