RAND(3)

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

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, 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 2^31

giving a period of 2^30 × (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, 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(3) 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(3)).

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
/src/lib9
/src/libsec/port

SEE ALSO
mp(3)

BUGS
Truerand and ntruerand maintain a static file descriptor.

To avoid name conflicts with the underlying system, rand, lrand, frand, nrand, lnrand, and srand are preprocessor macros defined as p9rand, p9lrand, and so on; see intro(3).