TH RAND 2
SH NAME
rand, lrand, frand, nrand, lnrand, srand, truerand, n_truerand, fastrand \- 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
int n_truerand(int val)
PP
B
ulong fastrand(void)
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 N_truerand
returns a uniform integer
IR x ,
RI 0≤ x < val.
PP
I Fastrand
is a pseudo random number generator which is seeded
and periodically scrambled using
IR truerand .
It is approximately 1000 times faster than
I truerand
and is intended for security software that requires a
larger stream of unguessable data.
SH SOURCE
B /sys/src/libc/port/rand.c
br
B /sys/src/libc/9sys/truerand.c
SH "SEE ALSO
IR cons (3)
