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.Dd February 1, 2020
.Dt RAND 3
.Os
.Sh NAME
.Nm rand ,
.Nm srand ,
.Nm rand_r
.Nd bad random number generator
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft void
.Fn srand "unsigned seed"
.Ft int
.Fn rand void
.Ft int
.Fn rand_r "unsigned *ctx"
.Sh DESCRIPTION
.Bf -symbolic
The functions described in this manual page are not cryptographically
secure.
Applications which require unpredictable random numbers should use
.Xr arc4random 3
instead.
.Ef
.Pp
The
.Fn rand
function computes a sequence of pseudo-random integers in the range
of 0 to
.Dv RAND_MAX ,
inclusive.
.Pp
The
.Fn srand
function seeds the algorithm with the
.Fa seed
parameter.
Repeatable sequences of
.Fn rand
output may be obtained by calling
.Fn srand
with the same
.Fa seed .
.Fn rand
is implicitly initialized as if
.Fn srand "1"
had been invoked explicitly.
.Pp
In
.Fx 13 ,
.Fn rand
is implemented using the same 128-byte state LFSR generator algorithm as
.Xr random 3 .
However, the legacy
.Fn rand_r
function is not (and can not be, because of its limited
.Fa *ctx
size).
.Fn rand_r
implements the historical, poor-quality Park-Miller 32-bit LCG and should not
be used in new designs.
.Sh IMPLEMENTATION NOTES
Since
.Fx 13 ,
.Fn rand
is implemented with the same generator as
.Xr random 3 ,
so the low-order bits should no longer be significantly worse than the
high-order bits.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr random 3 ,
.Xr random 4
.Sh STANDARDS
The
.Fn rand
and
.Fn srand
functions
conform to
.St -isoC .
.Pp
The
.Fn rand_r
function is not part of
.St -isoC
and is marked obsolescent in
.St -p1003.1-2008 .
It may be removed in a future revision of POSIX.
.Sh CAVEATS
Prior to
.Fx 13 ,
.Fn rand
used the historical Park-Miller generator with 32 bits of state and produced
poor quality output, especially in the lower bits.
.Fn rand
in earlier versions of
.Fx ,
as well as other standards-conforming implementations, may continue to produce
poor quality output.
.Pp
.Em These functions should not be used in portable applications that want a
.Em high quality or high performance pseudorandom number generator .
One possible replacement,
.Xr random 3 ,
is portable to Linux — but it is not especially fast, nor standardized.
.Pp
If broader portability or better performance is desired, any of the widely
available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or SplitMix64
algorithm implementations may be embedded in your application.
These algorithms have the benefit of requiring less space than
.Xr random 3
and being quite fast (in header inline implementations).