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Re: [libvirt] FreeBSD, no gcc present libvirt build issue



On Thu, Aug 29, 2013 at 09:52:52AM -0600, Eric Blake wrote:
> On 08/29/2013 09:43 AM, Eric Blake wrote:
> > On 08/29/2013 09:35 AM, Jason Helfman wrote:
> > 
> >>>>
> >>> stdlib.h:#define        RAND_MAX        0x7fffffff
> > 
> > Good.
> > 
> >>>
> >>> -jgh
> >>>
> >>
> >> And on our current head release (10) it is this:
> >>
> >> #define RAND_MAX        0x7ffffffd
> > 
> > Huh?  Why is this not 2**n-1?  That violates assumptions we have made,
> > and is WHY your compile failed.  It has nothing to do with clang vs. gcc
> > (both compilers would fail), it has to do with your changed system
> > header resulting in violating assumptions that hold in ALL OTHER
> > IMPLEMENTATIONS, that random numbers are evenly distributed within a
> > range of a power of 2.
> > 
> 
> http://lists.freebsd.org/pipermail/svn-src-head/2013-July/049076.html
> 
> makes it look like the reduction in range was _intentional_?  Yuck.  A
> non-power-of-2 random generator adds needless complexity to the user.
> 
> I think I can fix libvirt to work around the boneheaded decision;
> basically, since we cannot trust the full range of random_r to be evenly
> distributed, I will have to tweak libvirt's call to truncate every call
> to random_r to a subset of bits that are more likely to be evenly
> distributed (maybe by shifting off the most- and least-significant bits
> returned, and only using 28 instead of 31 bits of randomness per call).
>  But I would MUCH rather prefer that FreeBSD revisit their decision, and
> guarantee that random output be evenly distributed across the full 31
> bits to begin with.

Since gnulib has a working random_r() function can we just make
gnulib replace the boneheaded freebsd impl ?

> 
> I also intend to open a bug against POSIX to request that RAND_MAX be
> required to be 2**n-1, rather than relying on the assumption that
> everyone so far, until FreeBSD 10, has happened to meet that requirement.

Daniel
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