Re: random(3) in OpenBSD

[Bruno Haible]

[CCing bug-gnulib]

Hi Corinna,

> Looking into all the BSD variants of random(), I found that OpenBSD
> defaults the random() function to return non-deterministic output.  It
> works like this: 
> 
> - A global var random_deterministic defaults to 0.
> - Calls to srandom() set random_deterministic to 0 and ignore their
>   input.
> - Calls to srandom_deterministic() sets random_deterministic to 1 and
>   seeds the usual PRNG.
> - initstate() and setstate() set random_deterministic to 1.
> - random() returns either the next value from the PRNG if
>   random_deterministic is 1, or the next value generated by a call to
>   arc4random() if random_deterministic is 0.  So by default, it
>   returns arc4random() values.
> 
> With that behaviour, OpenBSD appears to stray from POSIX deliberately.

Yup, I observed this too when I found that the gnulib MT test for random()
yielded strange results on OpenBSD. [1]

> I was wondering if you want to add this as incompatibility to your
> document at
> https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=doc/posix-functions/random.texi
> perhaps.

I documented it as an incompatibility for the srandom() function. But you are
right, it's also an incompatibility for the random() function, since POSIX [2]
says "random() shall produce by default a sequence of numbers that can be
duplicated by calling srandom() with 1 as the seed".

[1] https://lists.gnu.org/archive/html/bug-gnulib/2023-11/msg00079.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/random.html


2023-11-14  Bruno Haible  <bruno@clisp.org>

        doc: Mention a 'random' limitation on OpenBSD.
        Reported by Corinna Vinschen <vinschen@redhat.com>.
        * doc/posix-functions/random.texi: Mention the OpenBSD limitation.

diff --git a/doc/posix-functions/random.texi b/doc/posix-functions/random.texi
index 84cacbe8b5..6f003ea3b6 100644
--- a/doc/posix-functions/random.texi
+++ b/doc/posix-functions/random.texi
@@ -25,4 +25,8 @@
 This function has a slightly incompatible declaration (the return type being
 @samp{int} instead of @samp{long}) on some platforms:
 Cygwin 1.5.25, Haiku.
+@item
+When @samp{srandom} was not called, this function returns a non-deterministic
+sequence rather than a deterministic sequence of numbers on some platforms:
+OpenBSD 7.4.
 @end itemize