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 19.8.1 ISO C Random Number Functions
 ------------------------------------
 
 This section describes the random number functions that are part of the
 ISO C standard.
 
    To use these facilities, you should include the header file
 `stdlib.h' in your program.  
 
  -- Macro: int RAND_MAX
      The value of this macro is an integer constant representing the
      largest value the `rand' function can return.  In the GNU library,
      it is `2147483647', which is the largest signed integer
      representable in 32 bits.  In other libraries, it may be as low as
      `32767'.
 
  -- Function: int rand (void)
      The `rand' function returns the next pseudo-random number in the
      series.  The value ranges from `0' to `RAND_MAX'.
 
  -- Function: void srand (unsigned int SEED)
      This function establishes SEED as the seed for a new series of
      pseudo-random numbers.  If you call `rand' before a seed has been
      established with `srand', it uses the value `1' as a default seed.
 
      To produce a different pseudo-random series each time your program
      is run, do `srand (time (0))'.
 
    POSIX.1 extended the C standard functions to support reproducible
 random numbers in multi-threaded programs.  However, the extension is
 badly designed and unsuitable for serious work.
 
  -- Function: int rand_r (unsigned int *SEED)
      This function returns a random number in the range 0 to `RAND_MAX'
      just as `rand' does.  However, all its state is stored in the SEED
      argument.  This means the RNG's state can only have as many bits
      as the type `unsigned int' has.  This is far too few to provide a
      good RNG.
 
      If your program requires a reentrant RNG, we recommend you use the
      reentrant GNU extensions to the SVID random number generator.  The
      POSIX.1 interface should only be used when the GNU extensions are
      not available.
 

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(libc.info.gz) Pseudo-Random Numbers

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