srand(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

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, 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; 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.

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   ┌──────────────────────────┬───────────────┬─────────┐
   │Interface                 │ Attribute     │ Value   │
   ├──────────────────────────┼───────────────┼─────────┤
   │rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
   └──────────────────────────┴───────────────┴─────────┘

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 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 int 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>\n", 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\n", r);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO

   drand48(3), random(3)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

                              2016-03-15                           RAND(3)

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