# rand

## 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 │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## VERSIONS



           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



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


## 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);
               }
    


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