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 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 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; this can be done using the reentrant  func‐
       tion 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.

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 gen‐
       erator 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 implementa‐
       tions 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(),  pos‐
       sibly 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 3.74 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 http://www.kernel.org/doc/man-pages/.



                                            2014-01-18                                    RAND(3)