centos man page for rand

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

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 to RAND_MAX inclusive (i.e., the mathematical 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 applica-
       tion, this state must be made explicit; this can be done using the reentrant function rand_r().

       Like rand(), rand_r() returns a pseudo-random integer in the range [0, RAND_MAX].  The seedp argument is a pointer to an unsigned int  that
       is  used  to  store  state  between calls.  If rand_r() is called with the same initial value for the integer pointed to by seedp, and that
       value is not modified between calls, then the same pseudo-random sequence will result.

       The value pointed to by the seedp argument of rand_r() provides only 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 (inclusive).  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 dif-
       ferent 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;
	   }

       The following program can be used to display the pseudo-random sequence produced by rand() when given a particular seed.

	   #include <stdlib.h>
	   #include <stdio.h>

	   int
	   main(int argc, char *argv[])
	   {
	       int j, r, nloops;
	       unsigned int seed;

	       if (argc != 3) {
		   fprintf(stderr, "Usage: %s <seed> <nloops>
", argv[0]);
		   exit(EXIT_FAILURE);
	       }

	       seed = atoi(argv[1]);
	       nloops = atoi(argv[2]);

	       srand(seed);
	       for (j = 0; j < nloops; j++) {
		   r =	rand();
		   printf("%d
", r);
	       }

	       exit(EXIT_SUCCESS);
	   }

SEE ALSO
       drand48(3), random(3)

COLOPHON
       This page is part of release 3.53 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/.

								    2010-10-01								   RAND(3)