srandom

RANDOM(3)                  Linux Programmer's Manual                 RANDOM(3)

NAME
       random, srandom, initstate, setstate - random number generator

SYNOPSIS
       #include <stdlib.h>

       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():
           _XOPEN_SOURCE >= 500
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 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 RAND_MAX.  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  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.
       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 near-
       est  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  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 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  at-
       tributes(7).

       +------------------------+---------------+---------+
       |Interface               | Attribute     | Value   |
       +------------------------+---------------+---------+
       |random(), srandom(),    | Thread safety | MT-Safe |
       |initstate(), setstate() |               |         |
       +------------------------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.3BSD.

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

       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 Univer-
       sity 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 is-
       sues 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.

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)

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-03-06                         RANDOM(3)