NAME

       rand, rand_r, srand - pseudo-random number generator

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

       int rand(void);
       void srand(unsigned int seed);

       [[deprecated]] int rand_r(unsigned int *seedp);

   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-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 │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       The functions rand() and srand() conform to SVr4, 4.3BSD, 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.)

EXAMPLES

       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.  When the seed is -1, the program uses a random seed.

           #include <stdio.h>
           #include <stdlib.h>

           int
           main(int argc, char *argv[])
           {
               int           r;
               unsigned int  seed, nloops;

               if (argc != 3) {
                   fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
                   exit(EXIT_FAILURE);
               }

               seed = atoi(argv[1]);
               nloops = atoi(argv[2]);

               if (seed == -1) {
                   seed = arc4random();
                   printf("seed: %u\n", seed);
               }

               srand(seed);
               for (unsigned int j = 0; j < nloops; j++) {
                   r =  rand();
                   printf("%d\n", r);
               }

               exit(EXIT_SUCCESS);
           }

SEE ALSO

       drand48(3), random(3)