$home/manuals/plan9_4th/2/rand

RAND(2)                       System Calls Manual                      RAND(2)



NAME
       rand,  lrand, frand, nrand, lnrand, srand, truerand, ntruerand, genran‐
       dom, prng, fastrand, nfastrand - random number generators

SYNOPSIS
       #include <u.h>
       #include <libc.h>

       int    rand(void)

       long   lrand(void)

       double frand(void)

       int    nrand(int val)

       long   lnrand(long val)

       void   srand(long seed)

       ulong  truerand(void)

       ulong  ntruerand(ulong val)

       #include <mp.h>
       #include <libsec.h>

       void   genrandom(uchar *buf, int nbytes)

       void   prng(uchar *buf, int nbytes)

       ulong  fastrand(void)

       ulong  nfastrand(ulong val)

DESCRIPTION
       Rand returns a uniform pseudo-random number x, 0â¤x<215.

       Lrand returns a uniform long x, 0â¤x<231.

       Frand returns a uniform double x, 0.0â¤x<1.0, This function calls lrand
       twice  to generate a number with as many as 62 significant bits of man‐
       tissa.

       Nrand returns a uniform integer x, 0â¤x<val.  Lnrand is the  same,  but
       returns a long.

       The algorithm is additive feedback with:

              x[n] = (x[n−273] + x[n−607]) mod 2^31

       giving a period of 2^30 Ã (2^607 - 1).

       The  generators are initialized by calling srand with whatever you like
       as argument.  To get a different starting value each time,

              srand(time(0))

       will work as long as it is not called more often than once per  second.
       Calling

              srand(1)

       will initialize the generators to their starting state.

       Truerand  returns a random unsigned long read from /dev/random.  Due to
       the nature of /dev/random, truerand can only return a few hundred  bits
       a second.

       Ntruerand returns a uniform random integer x, 0â¤x<valâ¤232-1.

       Genrandom fills a buffer with bytes from the X9.17 pseudo-random number
       generator.  The X9.17 generator is seeded by 24 truly random bytes read
       from /dev/random.

       Prng uses the native rand(2) pseudo-random number generator to fill the
       buffer.  Used with srand, this  function  can  produce  a  reproducible
       stream of pseudo random numbers useful in testing.

       Both genrandom and prng may be passed to mprand (see mp(2)).

       Fastrand   uses   genrandom  to  return  a  uniform  unsigned  long  x,
       0â¤x<232-1.

       Nfastrand  uses  genrandom  to  return  a  uniform  unsigned  long   x,
       0â¤x<valâ¤232-1.

SOURCE
       /sys/src/libc/port/*rand.c
       /sys/src/libc/9sys/truerand.c
       /sys/src/libsec/port/genrandom.c
       /sys/src/libsec/port/prng.c
       /sys/src/libsec/port/*fastrand.c

SEE ALSO
       cons(3), mp(2)

BUGS
       Truerand and ntruerand maintain a static file descriptor.



                                                                       RAND(2)