[ovs-dev] [PATCH] m4: Fix 32bit build with 64bit version of libunwind-dev installed.

[Ben Pfaff]

On Tue, Oct 01, 2019 at 07:46:10PM +0300, Ilya Maximets wrote:
> On 01.10.2019 19:17, Ben Pfaff wrote:
> > On Tue, Oct 01, 2019 at 02:22:20PM +0300, Ilya Maximets wrote:
> > > 32 and 64 bit versions of libunwind-dev package can not be installed
> > > at the same time at least in Debian and Ubuntu.  AC_CHECK_LIB macro
> > > doesn't know if we have '-m32' in OVS_CFLAGS, so we need to
> > > additionally check for actual compiling possibility before enabling
> > > the feature.  Otherwise 'make' will fail due to inability to
> > > build with 64bit libunwind headers:
> > > 
> > >      https://travis-ci.org/openvswitch/ovs/jobs/590580374
> > > 
> > >      /usr/include/libunwind.h:23:28: fatal error:
> > >          libunwind-x86.h: No such file or directory
> > > 
> > > CC: William Tu <u9012063 at gmail.com>
> > > Fixes: e2ed6fbeb18c ("fatal-signal: Catch SIGSEGV and print backtrace.")
> > > Signed-off-by: Ilya Maximets <i.maximets at ovn.org>
> > 
> > I think there's some confusion here on the use of options.  CFLAGS is
> > for the user, OVS_CFLAGS is for the OVS configure script to use itself.
> > .cirrus.yml and .travis/linux-build.sh are setting OVS_CFLAGS, but they
> > should not because they are the "user", not the configure script.  If
> > they set CFLAGS, instead, I believe that there would be no problem here.
> > 
> > See commit 95626395a3db ("build: Respect CFLAGS and LDFLAGS passed to
> > make") and the documentation it references.
> 
> That's an interesting point.
> However, 'configure' script makes decision about building with libunwind
> and it needs to know  if we're going to build 32 or 64 bit binary to
> enable or disable the feature. Setting CFLAGS for configure script breaks
> it in a following way:
> 
> $ CFLAGS='-m32' ./configure
> ...
> checking for gcc... gcc
> checking whether the C compiler works... no

Is this because the 32-bit C library isn't installed?  It should be easy
to fix with "sudo dnf install glibc-devel.i686" or the distro-specific
equivalent.

I don't see how it's reasonable to run "configure" with a toolchain
different from the one we're going to use for building.  Adding -m32
changes the ABI and the API.

My usual way to do this is actually to use CC="gcc -m32".  That way the
-m32, which is important, can't get separated at all!