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RAND(2)                                                   RAND(2)

NAME
rand, lrand, frand, nrand, lnrand, srand, truerand,
ntruerand, genrandom, 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 <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, 0≤ x <2^15.

Lrand returns a uniform long x, 0≤ x <2^31.

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 2^31

Page 1                       Plan 9              (printed 9/8/24)

RAND(2)                                                   RAND(2)

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

The generators are initialized by calling srand with what-
ever 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.

Ntruerand returns a uniform random integer x, 0≤ x < val ≤
2^32-1.

Genrandom fills a buffer with bytes from the cryptographic
pseudo-random number generator. The generator is automati-
cally seeded by truerand.

Prng uses the native rand(2) pseudo-random number generator
to fill the buffer.  Used with srand, this function can pro-
duce 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 <2^32-1.

Nfastrand uses genrandom to return a uniform unsigned long
x, 0≤ x < val ≤ 2^32-1.

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