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



NAME
       rand, rand_r, srand - pseudo-random number generator

SYNOPSIS
       #include <&lt;stdlib.h>&gt;

       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 || _XOPEN_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 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.

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.

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 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 3.05 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/.



                                  2008-04-28                           RAND(3)