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)

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#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 <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.

Ntruerand returns a uniform random integer x, 0≤ x < val ≤ 232-1.

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

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