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 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 sran-
       dom(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 3.25 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)