NAME

rand, lrand, frand, nrand, lnrand, srand, truerand, ntruerand, fastrand, nfastrand − random number generator

SYNOPSIS

­#include <u.h>
­#include <libc.h>

intrand(void)

longlrand(void)

doublefrand(void)

intnrand(int val)

longlnrand(long val)

voidsrand(long seed)

ulongtruerand(void)

ulongntruerand(ulong val) ­#include <mp.h>
­#include <libsec.h>

voidgenrandom(uchar ∗buf, int nbytes)

voidprng(uchar ∗buf, int nbytes)

ulongfastrand(void)

ulongnfastrand(ulong val)

DESCRIPTION

­Rand returns a uniform pseudo-random number x, 0≤x<215. 

­Lrand returns a uniform ­long x, 0≤x<231. 

­Frand returns a uniform ­double x, 0.0≤x<1.0, This function calls ­lrand twice to generate a number with as many as 62 significant bits of mantissa. 

­Nrand returns a uniform integer x, 0≤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 231

giving a period of 230 × (2607 − 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, 0≤x<val≤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, 0≤x<232-1. 

­Nfastrand uses ­genrandom to return a uniform ­unsigned long x, 0≤x<val≤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. 

Plan 9  —  October 28, 2002