rand(3) — Linux manual page


RAND(3)                 Linux Programmer's Manual                RAND(3)

NAME         top

       rand, rand_r, srand - pseudo-random number generator

SYNOPSIS         top

       #include <stdlib.h>

       int rand(void);
       int rand_r(unsigned int *seedp);
       void srand(unsigned int seed);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.24:
               _POSIX_C_SOURCE >= 199506L
           Glibc 2.23 and earlier

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

       │Interface                             Attribute     Value   │
       │rand(), rand_r(), srand()             │ Thread safety │ MT-Safe │

CONFORMING TO         top

       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         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.)

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.

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

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

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

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

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


SEE ALSO         top

       drand48(3), random(3)

COLOPHON         top

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                               2021-03-22                        RAND(3)

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