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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  call-
       ing 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 gen-
       erator.  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  obso-
       lete.

NOTES
       The  versions of rand() and srand() in the Linux C Library use the same random num-
       ber 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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



                                  2008-08-29                           RAND(3)

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