plan9fox/sys/man/2/rand

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.TH RAND 2
.SH NAME
rand, lrand, frand, nrand, lnrand, srand, truerand, ntruerand, genrandom, prng, fastrand, nfastrand \- random number generators
.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)
.sp
.B #include <libsec.h>
.PP
.B
void genrandom(uchar *buf, int nbytes)
.PP
.B
void prng(uchar *buf, int nbytes)
.PP
.B
ulong fastrand(void)
.PP
.B
ulong nfastrand(ulong val)
.SH DESCRIPTION
.I Rand
returns a uniform pseudo-random
number
.IR x ,
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.if t 0≤ \fIx\fR <2\u\s715\s10\d.
.if n 0≤ x <2^15.
.PP
.I Lrand
returns a uniform
.B long
.IR x ,
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.if t 0≤ \fIx\fR <2\u\s731\s10\d.
.if n 0≤ x <2^31.
.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 .
.PP
.I Ntruerand
returns a uniform random integer
.IR x ,
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.if t 0≤ \fIx\fR < \fIval\fR ≤ 2\u\s732\s10\d-1.
.if n 0≤ x < val ≤ 2^32-1.
.PP
.I Genrandom
fills a buffer with bytes from the cryptographic pseudo-random
number generator. The generator is automatically seeded by
.IR truerand .
.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
.I genrandom
and
.I prng
may be passed to
.I mprand
(see
.IR mp (2)).
.PP
.I Fastrand
uses
.I genrandom
to return a uniform
.B "unsigned long
.IR x ,
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.if t 0≤ \fIx\fR <2\u\s732\s10\d-1.
.if n 0≤ x <2^32-1.
.PP
.I Nfastrand
uses
.I genrandom
to return a uniform
.B "unsigned long
.IR x ,
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.if t 0≤ \fIx\fR < \fIval\fR ≤ 2\u\s732\s10\d-1.
.if n 0≤ x < val ≤ 2^32-1.
.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
.br
.B /sys/src/libsec/port/*fastrand.c
.SH "SEE ALSO
.IR cons (3),
.IR mp (2)
.SH BUGS
.I Truerand
and
.I ntruerand
maintain a static file descriptor.