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NAME

       rand, rand_r, srand - pseudo-random number generator

SYNOPSIS

       #include <stdlib.h>

       int rand(void);

       int rand_r(unsigned int *seedp);

       void srand(unsigned int seed);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

DESCRIPTION

       The  rand()  function  returns  a  pseudo-random  integer  in the range
       [0, RAND_MAX].

       The srand() function sets its argument as the seed for a  new  sequence
       of  pseudo-random  integers  to be returned by rand().  These sequences
       are repeatable by calling srand() with the same seed value.

       If no seed value is provided,  the  rand()  function  is  automatically
       seeded with a value of 1.

       The  function  rand()  is  not  reentrant or thread-safe, since it uses
       hidden state that is modified on each call.  This  might  just  be  the
       seed  value  to be used by the next call, or it might be something more
       elaborate.  In  order  to  get  reproducible  behavior  in  a  threaded
       application,  this  state must be made explicit.  The function rand_r()
       is supplied with a pointer to an unsigned int, to  be  used  as  state.
       This  is  a very small amount of state, so this function will be a weak
       pseudo-random generator.  Try drand48_r(3) instead.

RETURN VALUE

       The rand()  and  rand_r()  functions  return  a  value  between  0  and
       RAND_MAX.  The srand() function returns no value.

CONFORMING TO

       The  functions  rand()  and  srand() conform to SVr4, 4.3BSD, C89, C99,
       POSIX.1-2001.    The   function   rand_r()   is   from    POSIX.1-2001.
       POSIX.1-2008 marks rand_r() as obsolete.

NOTES

       The  versions of rand() and srand() in the Linux C Library use the same
       random number generator as random(3) and srandom(3), so the lower-order
       bits  should  be as random as the higher-order bits.  However, on older
       rand() implementations, and on  current  implementations  on  different
       systems,  the  lower-order  bits  are much less random than the higher-
       order bits.  Do not use this function in applications  intended  to  be
       portable when good randomness is needed.  (Use random(3) instead.)

EXAMPLE

       POSIX.1-2001 gives the following example of an implementation of rand()
       and srand(), possibly useful when one needs the same  sequence  on  two
       different machines.

           static unsigned long next = 1;

           /* RAND_MAX assumed to be 32767 */
           int myrand(void) {
               next = next * 1103515245 + 12345;
               return((unsigned)(next/65536) % 32768);
           }

           void mysrand(unsigned seed) {
               next = seed;
           }

SEE ALSO

       drand48(3), random(3)

COLOPHON

       This  page  is  part of release 3.24 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/.

                                  2008-08-29                           RAND(3)