srand

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():
           Since glibc 2.24:
               _POSIX_C_SOURCE >= 199506L
           Glibc 2.23 and earlier
               _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-ran-
       dom 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 at-
       tributes(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-or-
       der bits.  Do not use this function in applications intended to be por-
       table 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 5.05 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/.

                                  2019-03-06                           RAND(3)