• NAME
  
  rand, rand_r, srand - pseudo-random number generator
• SYNOPSIS
  

  
   #include <stdlib.h>
  
   int rand(void);
  
   int rand_r(unsigned int * seedp );
  
  
      void srand(unsigned int  seed ); 
  
    Feature Test Macro Requirements for glibc (see
  
  
    feature_test_macros(7) ):
  
   
  
      rand_r ():
  
   _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
  
  
  

• DESCRIPTION
  
  The
  rand ()
  function returns a pseudo-random integer in
  the 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 or thread-safe, 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.
  The function
  rand_r ()
  is supplied with a pointer to an
  "unsigned int" ,
  to be used as state.
  This is 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 .
  The
  srand ()
  function returns no value.
  
• CONFORMING TO
  
  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
  
  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.)
  
• EXAMPLE
  
  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 seed) {
      next = seed;
  }
  

• SEE ALSO
  
  drand48(3) , random(3)
• COLOPHON
  
  This page is part of release 3.19 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

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