rand(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO

rand(3)                 Library Functions Manual                 rand(3)

NAME         top

       rand, rand_r, srand - pseudo-random number generator

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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         top

       The rand() and rand_r() functions return a value between 0 and
       RAND_MAX (inclusive).  The srand() function returns no value.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌─────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                           Attribute     Value   │
       ├─────────────────────────────────────┼───────────────┼─────────┤
       │ rand(), rand_r(), srand()           │ Thread safety │ MT-Safe │
       └─────────────────────────────────────┴───────────────┴─────────┘

VERSIONS         top

       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.)

STANDARDS         top

       rand()
       srand()
              C11, POSIX.1-2008.

       rand_r()
              POSIX.1-2008.

HISTORY         top

       rand()
       srand()
              SVr4, 4.3BSD, C89, POSIX.1-2001.

       rand_r()
              POSIX.1-2001.  Obsolete in POSIX.1-2008.

EXAMPLES         top

       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         top

       drand48(3), random(3)

Linux man-pages (unreleased)     (date)                          rand(3)

Pages that refer to this page: mcookie(1)arc4random(3)drand48(3)drand48_r(3)random(3)random_r(3)