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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(): _SVID_SOURCE ||
       _BSD_SOURCE || _XOPEN_SOURCE >= 500

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.  seed is the seed for the initialization, which specifies a  start-
       ing  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() and setstate()
       functions return a pointer to the previous  state  array,  or  NULL  on
       error.

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

CONFORMING TO
       4.3BSD, POSIX.1-2001.

NOTES
       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 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
       that purpose.

       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
       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.

SEE ALSO
       drand48(3), rand(3), random_r(3), srand(3)

COLOPHON
       This page is part of release 3.25 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU                               2009-02-03                         RANDOM(3)
 

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