[ovs-dev] [PATCH] acinclude.m4: Really check whether GCC support -Wno-null-pointer-arithmetic.

[Ben Pfaff]

I've noticed recently an annoying quantity of error messages like the
following in builds in various places:

    gcc: error: unrecognized command line option ‘-Wunknown-warning-option’

This didn't really make sense because OVS checks whether the compiler
supports warning options before it uses them.  Looking closer, the GCC
manual has a note that explains the issue:

     When an unrecognized warning option is requested (e.g.,
    '-Wunknown-warning'), GCC emits a diagnostic stating that the
    option is not recognized.  However, if the '-Wno-' form is used,
    the behavior is slightly different: no diagnostic is produced for
    '-Wno-unknown-warning' unless other diagnostics are being
    produced.  This allows the use of new '-Wno-' options with old
    compilers, but if something goes wrong, the compiler warns that
    an unrecognized option is present.

Thus, we can properly check only for the *positive* version of a warning
option, so this commit makes the OVS tests do that.

Fixes: a7021b08b0d5 ("configure: Disable -Wnull-pointer-arithmetic Clang warning.")
Signed-off-by: Ben Pfaff <blp at ovn.org>
---
 acinclude.m4 | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/acinclude.m4 b/acinclude.m4
index 0690baedfb9d..8d850349817f 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -996,7 +996,11 @@ AC_DEFUN([_OVS_CHECK_CC_OPTION], [dnl
      dnl    gcc: unrecognized option '-Qunused-arguments'
      dnl    0
      dnl    %
-     CFLAGS="$CFLAGS $WERROR $1"
+     dnl
+     dnl In addition, GCC does not complain about a -Wno-<foo> option that
+     dnl it does not understand, unless it has another error to report, so
+     dnl instead of testing for -Wno-<foo>, test for the positive version.
+     CFLAGS="$CFLAGS $WERROR m4_bpatsubst([$1], [-Wno-], [-W])"
      AC_COMPILE_IFELSE(
        [AC_LANG_SOURCE([int x;])],
        [if test -s conftest.err && grep "unrecognized option" conftest.err
-- 
2.16.1