NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       random, srandom, initstate, setstate - random number generator

SYNOPSIS         top

       #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 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

DESCRIPTION         top

       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
       initstate() to decide how sophisticated a random number generator it
       should useā€”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 initialized using initstate() or be the result of a
       previous call of setstate().

RETURN VALUE         top

       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         top

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

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

ATTRIBUTES         top

   Multithreading (see pthreads(7))
       The random(), srandom(), initstate(), and setstate() functions are
       thread-safe.

CONFORMING TO         top

       4.3BSD, POSIX.1-2001.

NOTES         top

       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 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
       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.

BUGS         top

       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         top

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

COLOPHON         top

       This page is part of release 3.72 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
       http://www.kernel.org/doc/man-pages/.

GNU                              2014-03-25                        RANDOM(3)