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

       rand, rand_r, srand - pseudo-random number generator

       #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

       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 hid‐
       den 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 elabo‐
       rate.  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.

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

       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.

       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.)

       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;

       drand48(3), random(3)

       This  page  is  part of release 3.22 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)

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