.TH RAND 2
.SH NAME
rand, lrand, frand, nrand, lnrand, srand, truerand, ntruerand, fastrand, nfastrand \- random number generator
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.ta \w'\fLdouble 'u
.B
int	rand(void)
.PP
.B
long	lrand(void)
.PP
.B
double	frand(void)
.PP
.B
int	nrand(int val)
.PP
.B
long	lnrand(long val)
.PP
.B
void	srand(long seed)
.PP
.B
ulong	truerand(void)
.PP
.B
ulong	ntruerand(ulong val)
.PP
.B
ulong	fastrand(void)
.PP
.B
ulong	nfastrand(ulong val)
.sp
.B #include <mp.h>
.br
.B #include <libsec.h>
.PP
.B
void	genrandom(uchar *buf, int nbytes)
.PP
.B
void	prng(uchar *buf, int nbytes)
.SH DESCRIPTION
.I Rand
returns a uniform pseudo-random
number
.IR x ,
.RI 0≤ x <2\u\s715\s10\d.
.PP
.I Lrand
returns a uniform
.B long
.IR x ,
.RI 0≤ x <2\u\s731\s10\d.
.PP
.I Frand
returns a uniform
.B double
.IR x ,
.RI 0.0≤ x <1.0,
This function calls
.I lrand
twice to generate a number with as many as 62 significant bits of mantissa.
.PP
.I Nrand
returns a uniform integer
.IR x ,
.RI 0≤ x < val.
.I Lnrand
is the same, but returns a
.BR long .
.PP
The algorithm is additive feedback with:
.IP
x[n] = (x[n\(mi273] + x[n\(mi607]) mod
.if t 2\u\s731\s0\d
.if n 2^31
.LP
giving a period of
.if t 2\u\s730\s10\d \(mu (2\u\s7607\s10\d \- 1).
.if n 2^30 × (2^607 - 1).
.PP
The generators are initialized by calling
.I srand
with whatever you like as argument.
To get a different starting value each time,
.IP
.L
srand(time(0))
.LP
will work as long as it is not called more often
than once per second.
Calling
.IP
.L
srand(1)
.LP
will initialize the generators to their
starting state.
.PP
.I Truerand
returns a random unsigned long read from
.BR /dev/random .
Due to the nature of
.BR /dev/random ,
truerand can only return a few hundred bits a
second.
.PP
.I Ntruerand
returns a uniform random integer
.IR x ,
.RI 0≤ x < val ≤ 2\u\s732\s10\d-1.
.PP
.I Fastrand
is a pseudo random number generator which is seeded
and periodically scrambled using
.IR truerand .
It returns a uniform random integer
.IR x ,
.RI 0≤ x < 2\u\s731\s10\d-1.
It is approximately 1000 times faster than
.I truerand
and is intended for security software that requires a
larger stream of unguessable data.
.PP
.I Nfastrand
returns a uniform pseudo random integer
.IR x ,
.RI 0≤ x < val ≤ 2\u\s731\s10\d-1,
using
.IR fastrand .
.PP
.I 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
.BR /dev/random .
.PP
.I Prng
uses the native
.IR rand (2)
pseudo-random number generator to fill the buffer.  Used with
.IR srand ,
this function can produce a reproducible stream of pseudo random
numbers useful in testing.
.PP
Both functions may be passed to
.I mprand
(see
.IR mp (2)).
.SH SOURCE
.B /sys/src/libc/port/rand.c
.br
.B /sys/src/libc/9sys/truerand.c
.br
.B /sys/src/libsec/port/genrandom.c
.br
.B /sys/src/libsec/port/prng.c
.SH "SEE ALSO
.IR cons (3),
.IR mp (2),
.SH BUGS
.I Truerand
and
.I ntruerand
maintain a static file descriptor.