Re: configure tests for gcc builtins with CFLAGS=-Werror

[Eric Blake]

On 05/25/2016 02:05 AM, Chris Packham wrote:
> Hi,
> 
> We've run into a problem trying to compile openvswitch with CFLAGS=-Werror.

That's never a good thing to do. We explicitly recommend that you avoid
CFLAGS=-Werror during configure tests, and that configure files that
WANT to (possibly conditionally) enable error flags during the main
compile should probe that setting independently during configure.ac.
GNU Coreutils is a good example for how to do this, using gnulib's
gl_WARN_ADD macro

http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=blob;f=configure.ac;h=a4846011#l108

http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/local.mk;h=12a0d55d#l20

> 
> I don't think it's an openvswitch problem. It appears to be a 
> autoconf/gcc issue.

On the contrary, it IS an openvswitch problem, as you should never run
configure with -Werror set for the majority of the probes.

> 
> Any thoughts on how we can get -latomic detected and -Werror passed 
> through to the build?

As suggested above, make the probing of whether -Werror works
independent from the rest of configure, and set it in a separate
variable (not CFLAGS), then use that separate variable in your Makefiles
but not the rest of configure.

That said, there is an open patch to autoconf to promote gnulib's
gl_WARN_ADD logic into a formal autoconf macro, and that's one of the
remaining things that needs to be merged in before I can release
autoconf 2.70.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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