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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 non-linear 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 initstate() 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 ini-
       tialized 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 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 repro-
       ducible.  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  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  Num-
       bers) in Donald E. Knuth's The Art of Computer Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Read-
       ing, Massachusetts: Addison-Wesley Publishing Company, 1981.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                               2009-02-03                         RANDOM(3)