rand, rand_r, srand - pseudo-random number generator

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

       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 unsignedint 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.

       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);
           }

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

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

       Standard C library (libc, -lc)

       rand, rand_r, srand - pseudo-random number generator

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

drand48(3), random(3)

Linux man-pages 6.9.1                              2024-06-15                                            rand(3)

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

       rand_r()
              POSIX.1-2008.

#include<stdlib.h>intrand(void);voidsrand(unsignedintseed);[[deprecated]]intrand_r(unsignedint*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

       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.)