rand_r(3c) [sunos man page]

rand(3C)						   Standard C Library Functions 						  rand(3C)

NAME

       rand, srand, rand_r - simple random-number generator

SYNOPSIS

       #include <stdlib.h>

       int rand(void);

       void srand(unsigned int seed);

       int rand_r(unsigned int *seed);

DESCRIPTION

       The rand() function uses a multiplicative congruential random-number generator with period 2**32 that returns successive pseudo-random num-
       bers in the range of 0 to RAND_MAX (defined in <stdlib.h>).

       The  srand() function uses the argument seed as a seed for a new sequence of pseudo-random numbers to be returned by  subsequent  calls	to
       rand().	If  srand()  is then called with the same seed value, the sequence of pseudo-random numbers will be repeated.  If rand() is called
       before any calls to srand() have been made, the same sequence will be generated as when srand() is first called with a seed value of 1.

       The rand_r() function has the same functionality as rand() except that a pointer to a seed   seed  must	be  supplied  by  the  caller.	If
       rand_r()  is  called with the same initial value for the object pointed to by seed and that object is not modified between successive calls
       to rand_r(), the same sequence as that produced by calls to rand() will be generated.

       The rand() and srand() functions provide per-process pseudo-random streams shared by all threads. The same effect can be  achieved  if  all
       threads	call  rand_r()	with  a pointer to the same seed object. The rand_r() function allows a thread to generate a private pseudo-random
       stream by having the seed object be private to the thread.

USAGE

       The spectral properties of rand() are limited.  The drand48(3C) function provides a better, more elaborate random-number generator.

       When compiling multithreaded applications, the  _REENTRANT flag must be defined on the compile line.  This flag should be used only in mul-
       tithreaded applications.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       drand48(3C), attributes(5), standards(5)

SunOS 5.10							    19 May 2004 							  rand(3C)

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, 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.	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 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;
	   }

SEE ALSO

       drand48(3), random(3)

COLOPHON

       This page is part of release 3.25 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)