Re: [Libguestfs] [libnbd PATCH 3/5] libev: Check for old compiler

On 5/6/21 6:30 AM, Martin Kletzander wrote: > At some point GCC added some optimisations and checks that started emitting > warnings on harmless code: > > ...dereferencing type-punned pointer will break strict-aliasing rules... > > This was later fixed, but can still break the build (or report warnings) on old > GCCs (e.g. 7.5.0) when building the libev example. > > Signed-off-by: Martin Kletzander <mkletzan(a)redhat.com&gt; > --- > configure.ac | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/configure.ac b/configure.ac > index 19b7bfdb5d2a..3b51354346cc 100644 > --- a/configure.ac > +++ b/configure.ac > @@ -247,6 +247,30 @@ PKG_CHECK_MODULES([LIBEV], [libev], [ > AC_MSG_WARN([ev.h not found, some examples will not be compiled]) > ]) > ]) > + > +AS_IF([test "x$LIBEV_LIBS" != "x"], [ > + old_CFLAGS="$CFLAGS" > + CFLAGS="-Werror=strict-aliasing -O2" > + AC_MSG_CHECKING([if the compiler is new enough for good aliasing rules]) > + AC_COMPILE_IFELSE([ > + AC_LANG_PROGRAM([ > + #include <ev.h> > + > + static void cb (struct ev_loop *l, ev_timer *t, int e) { } > + static ev_timer timer; > + ], [ > + ev_timer_init (&timer, cb, 0, .1); > + ]) > + ], [ > + AC_MSG_RESULT([yes]) > + ], [ > + AC_MSG_RESULT([no]) > + AC_SUBST([LIBEV_CFLAGS], [""]) > + AC_SUBST([LIBEV_LIBS], [""]) > + AC_MSG_WARN([compiler is probably too old to compile with libev without errors, some examples will not be compiled]) Do we really need to skip compiling the example altogether, or can we just modify CFLAGS to add -Wno-strict-aliasing to prevent the failure?

-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org