[Openvpn-devel] Fix ACL_CHECK_ADD_COMPILE_FLAGS to work with clang

Message ID 1573760407-20662-1-git-send-email-selva.nair@gmail.com
State Superseded
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  • [Openvpn-devel] Fix ACL_CHECK_ADD_COMPILE_FLAGS to work with clang
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Commit Message

Selva Nair Nov. 14, 2019, 7:40 p.m.
From: Selva Nair <selva.nair@gmail.com>

Some compilers (e.g., clang) only issue a warning for
unsupported options unless additional flags such
as -Werror are used to convert the warning to an error.

Add support for extra flags in ACL_CHECK_ADD_COMPILE_FLAGS.

Note: a similar approach is used in AX_CHECK_COMPILE_FLAG
in autoconf archives.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
---

For successful travis build result see
https://travis-ci.org/selvanair/openvpn/builds/612019849

But this + https://patchwork.openvpn.net/patch/908/
alone does not fix the travis build as clang is not
happy with struct initializers: Like this

crypto.c:1860:31: error: suggest braces around initialization of subobject
      [-Werror,-Wmissing-braces]
    struct key server_key = { 0 };

I think clang wants {{0}} there.

Darn compilers and darn -Werror :)

 configure.ac | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

Comments

Илья Шипицин Nov. 14, 2019, 8:16 p.m. | #1
Thank you for your efforts.
As you are touching this, can you try "dist: bionic" ? It might bring newer
compilers

On Thu, Nov 14, 2019, 8:41 PM <selva.nair@gmail.com> wrote:

> From: Selva Nair <selva.nair@gmail.com>
>
> Some compilers (e.g., clang) only issue a warning for
> unsupported options unless additional flags such
> as -Werror are used to convert the warning to an error.
>
> Add support for extra flags in ACL_CHECK_ADD_COMPILE_FLAGS.
>
> Note: a similar approach is used in AX_CHECK_COMPILE_FLAG
> in autoconf archives.
>
> Signed-off-by: Selva Nair <selva.nair@gmail.com>
> ---
>
> For successful travis build result see
> https://travis-ci.org/selvanair/openvpn/builds/612019849
>
> But this + https://patchwork.openvpn.net/patch/908/
> alone does not fix the travis build as clang is not
> happy with struct initializers: Like this
>
> crypto.c:1860:31: error: suggest braces around initialization of subobject
>       [-Werror,-Wmissing-braces]
>     struct key server_key = { 0 };
>
> I think clang wants {{0}} there.
>
> Darn compilers and darn -Werror :)
>
>  configure.ac | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/configure.ac b/configure.ac
> index 807804e5..e59bd91b 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -1275,18 +1275,21 @@ if test "${enable_pkcs11}" = "yes"; then
>         )
>  fi
>
> +# Usage: ACL_CHECK_ADD_COMPILE_FLAGS(flag, [extra-flags])
> +# Some compilers require extra flags to trigger an error on
> +# unsupported options. E.g., clang requires -Werror.
>  AC_DEFUN([ACL_CHECK_ADD_COMPILE_FLAGS], [
>      old_cflags="$CFLAGS"
> -    CFLAGS="$1 $CFLAGS"
> -    AC_MSG_CHECKING([whether the compiler acceppts $1])
> -    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])],
> +    CFLAGS="$2 $1 $CFLAGS"
> +    AC_MSG_CHECKING([whether the compiler accepts $1])
> +    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])];
> CFLAGS="$1 $old_cflags",
>          [AC_MSG_RESULT([no]); CFLAGS="$old_cflags"])]
>  )
>
> -ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation])
> -ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function])
> -ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter])
> -ACL_CHECK_ADD_COMPILE_FLAGS([-Wall])
> +ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation], [-Werror])
> +ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function], [-Werror])
> +ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter], [-Werror])
> +ACL_CHECK_ADD_COMPILE_FLAGS([-Wall], [-Werror])
>
>  if test "${enable_pedantic}" = "yes"; then
>         enable_strict="yes"
> --
> 2.20.1
>
>
>
> _______________________________________________
> Openvpn-devel mailing list
> Openvpn-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/openvpn-devel
>
<div dir="auto">Thank you for your efforts.<div dir="auto">As you are touching this, can you try &quot;dist: bionic&quot; ? It might bring newer compilers</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Nov 14, 2019, 8:41 PM  &lt;<a href="mailto:selva.nair@gmail.com">selva.nair@gmail.com</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">From: Selva Nair &lt;<a href="mailto:selva.nair@gmail.com" target="_blank" rel="noreferrer">selva.nair@gmail.com</a>&gt;<br>
<br>
Some compilers (e.g., clang) only issue a warning for<br>
unsupported options unless additional flags such<br>
as -Werror are used to convert the warning to an error.<br>
<br>
Add support for extra flags in ACL_CHECK_ADD_COMPILE_FLAGS.<br>
<br>
Note: a similar approach is used in AX_CHECK_COMPILE_FLAG<br>
in autoconf archives.<br>
<br>
Signed-off-by: Selva Nair &lt;<a href="mailto:selva.nair@gmail.com" target="_blank" rel="noreferrer">selva.nair@gmail.com</a>&gt;<br>
---<br>
<br>
For successful travis build result see<br>
<a href="https://travis-ci.org/selvanair/openvpn/builds/612019849" rel="noreferrer noreferrer" target="_blank">https://travis-ci.org/selvanair/openvpn/builds/612019849</a><br>
<br>
But this + <a href="https://patchwork.openvpn.net/patch/908/" rel="noreferrer noreferrer" target="_blank">https://patchwork.openvpn.net/patch/908/</a><br>
alone does not fix the travis build as clang is not<br>
happy with struct initializers: Like this<br>
<br>
crypto.c:1860:31: error: suggest braces around initialization of subobject<br>
      [-Werror,-Wmissing-braces]<br>
    struct key server_key = { 0 };<br>
<br>
I think clang wants {{0}} there.<br>
<br>
Darn compilers and darn -Werror :)<br>
<br>
 <a href="http://configure.ac" rel="noreferrer noreferrer" target="_blank">configure.ac</a> | 17 ++++++++++-------<br>
 1 file changed, 10 insertions(+), 7 deletions(-)<br>
<br>
diff --git a/<a href="http://configure.ac" rel="noreferrer noreferrer" target="_blank">configure.ac</a> b/<a href="http://configure.ac" rel="noreferrer noreferrer" target="_blank">configure.ac</a><br>
index 807804e5..e59bd91b 100644<br>
--- a/<a href="http://configure.ac" rel="noreferrer noreferrer" target="_blank">configure.ac</a><br>
+++ b/<a href="http://configure.ac" rel="noreferrer noreferrer" target="_blank">configure.ac</a><br>
@@ -1275,18 +1275,21 @@ if test &quot;${enable_pkcs11}&quot; = &quot;yes&quot;; then<br>
        )<br>
 fi<br>
<br>
+# Usage: ACL_CHECK_ADD_COMPILE_FLAGS(flag, [extra-flags])<br>
+# Some compilers require extra flags to trigger an error on<br>
+# unsupported options. E.g., clang requires -Werror.<br>
 AC_DEFUN([ACL_CHECK_ADD_COMPILE_FLAGS], [<br>
     old_cflags=&quot;$CFLAGS&quot;<br>
-    CFLAGS=&quot;$1 $CFLAGS&quot;<br>
-    AC_MSG_CHECKING([whether the compiler acceppts $1])<br>
-    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])],<br>
+    CFLAGS=&quot;$2 $1 $CFLAGS&quot;<br>
+    AC_MSG_CHECKING([whether the compiler accepts $1])<br>
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])]; CFLAGS=&quot;$1 $old_cflags&quot;,<br>
         [AC_MSG_RESULT([no]); CFLAGS=&quot;$old_cflags&quot;])]<br>
 )<br>
<br>
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation])<br>
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function])<br>
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter])<br>
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wall])<br>
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation], [-Werror])<br>
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function], [-Werror])<br>
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter], [-Werror])<br>
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wall], [-Werror])<br>
<br>
 if test &quot;${enable_pedantic}&quot; = &quot;yes&quot;; then<br>
        enable_strict=&quot;yes&quot;<br>
-- <br>
2.20.1<br>
<br>
<br>
<br>
_______________________________________________<br>
Openvpn-devel mailing list<br>
<a href="mailto:Openvpn-devel@lists.sourceforge.net" target="_blank" rel="noreferrer">Openvpn-devel@lists.sourceforge.net</a><br>
<a href="https://lists.sourceforge.net/lists/listinfo/openvpn-devel" rel="noreferrer noreferrer" target="_blank">https://lists.sourceforge.net/lists/listinfo/openvpn-devel</a><br>
</blockquote></div>
David Sommerseth Nov. 14, 2019, 9:31 p.m. | #2
On 14/11/2019 20:40, selva.nair@gmail.com wrote:
> From: Selva Nair <selva.nair@gmail.com>
> 
> Some compilers (e.g., clang) only issue a warning for
> unsupported options unless additional flags such
> as -Werror are used to convert the warning to an error.
> 
> Add support for extra flags in ACL_CHECK_ADD_COMPILE_FLAGS.
> 
> Note: a similar approach is used in AX_CHECK_COMPILE_FLAG
> in autoconf archives.

I'm glad I didn't spend more time on thinking about this earlier today, as you
have almost the same ideas as I had :)

[...snip...]
>  
> +# Usage: ACL_CHECK_ADD_COMPILE_FLAGS(flag, [extra-flags])
> +# Some compilers require extra flags to trigger an error on
> +# unsupported options. E.g., clang requires -Werror.
>  AC_DEFUN([ACL_CHECK_ADD_COMPILE_FLAGS], [
>      old_cflags="$CFLAGS"
> -    CFLAGS="$1 $CFLAGS"
> -    AC_MSG_CHECKING([whether the compiler acceppts $1])
> -    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])],
> +    CFLAGS="$2 $1 $CFLAGS"
> +    AC_MSG_CHECKING([whether the compiler accepts $1])
> +    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])]; CFLAGS="$1 $old_cflags",

My idea was just to add -Werror right in the line above, and not extend the
ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.

But I stopped when I realized that GCC has some quirks as well, as I needed to
ponder about this a bit.

Clang is fairly consistent:

* first with an unknown -Wno-* argument

$ clang-6.0 -o test test.c -Wall -Wno-non-existing-flag
warning: unknown warning option '-Wno-non-existing-flag'; did you mean
      '-Wno-non-virtual-dtor'? [-Wunknown-warning-option]

$ clang-6.0 -o test test.c -Wall -Wno-non-existing-flag  -Werror
error: unknown warning option '-Wno-non-existing-flag'; did you mean
      '-Wno-non-virtual-dtor'? [-Werror,-Wunknown-warning-option]

* then with an unknown -W* argument (without the no- prefix)

$ clang-6.0 -o test test.c -Wall -Wnon-existing-flag
warning: unknown warning option '-Wnon-existing-flag'; did you mean
      '-Wnon-virtual-dtor'? [-Wunknown-warning-option]

$ clang-6.0 -o test test.c -Wall -Wnon-existing-flag  -Werror
error: unknown warning option '-Wnon-existing-flag'; did you mean
      '-Wnon-virtual-dtor'? [-Werror,-Wunknown-warning-option]

This is consistent and I can live with this behavior, using -Werror to bail
out on warnings.

So lets running the same tests with GCC:

* first with an unknown -Wno-* argument

$ gcc -o test test.c -Wall -Wno-non-existing-flag
(no error nor warning)

$ gcc -o test test.c -Wall -Wno-non-existing-flag -Werror
(no error nor warning)

* then with an unknown -W* argument (without the no- prefix)

$ gcc -o test test.c -Wall -Wnon-existing-flag
gcc: error: unrecognized command line option ‘-Wnon-existing-flag’


I think you said it pretty well in your mail:

> Darn compilers and darn -Werror


So your change does improve Clang ... I dunno what we will do with GCC.
Selva Nair Nov. 14, 2019, 9:58 p.m. | #3
Hi David

Thanks for the comments

My idea was just to add -Werror right in the line above, and not extend the
> ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.
>

I'm fine with that approach as well. Let me know if you want a v2.


> I think you said it pretty well in your mail:
>
> > Darn compilers and darn -Werror
>
>
> So your change does improve Clang ... I dunno what we will do with GCC.
>

It doesn't affect gcc as gcc does error (or ignore -- see below)
unsupported options with or without -Werror. So I think we are okay. travis
build on my fork confirms that:
https://travis-ci.org/selvanair/openvpn/builds/612019849

However, gcc does have quirky behaviour when it comes to options like
-Wno-xx-yy. It just accepts them silently[*] with or without -Werror. That
works fine for us, but it does generate an error about that unsupported
option if any other error is encountered during the build.  As other errors
are anyway a show-stopper, I think we can live with that.

Selva

[*] I think their reasoning is that -Wno-xx-yy can be thus used to suppress
warnings added to newer versions without breaking builds with older ones.
But clang doesn't share that view.
<div dir="ltr"><div dir="ltr">Hi David<br></div><div class="gmail_quote"><br></div><div class="gmail_quote">Thanks for the comments</div><div class="gmail_quote"><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div class="gmail_quote">My idea was just to add -Werror right in the line above, and not extend the<br>
ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.</div></blockquote><div><br></div><div>I&#39;m fine with that approach as well. Let me know if you want a v2.<br></div><div> <br></div><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
I think you said it pretty well in your mail:<br>
<br>
&gt; Darn compilers and darn -Werror<br>
<br>
<br>
So your change does improve Clang ... I dunno what we will do with GCC.<br></blockquote><div><br></div><div>It doesn&#39;t affect gcc as gcc does error (or ignore -- see below) unsupported options with or without -Werror. So I think we are okay. travis build on my fork confirms that:</div><div><a href="https://travis-ci.org/selvanair/openvpn/builds/612019849" rel="noreferrer" target="_blank">https://travis-ci.org/selvanair/openvpn/builds/612019849</a></div><div><br></div><div>However, gcc does have quirky behaviour when it comes to options like -Wno-xx-yy. It just accepts them silently[*] with or without -Werror. That works fine for us, but it does generate an error about that unsupported option if any other error is encountered during the build.  As other errors are anyway a show-stopper, I think we can live with that.</div><div><br></div><div>Selva</div><div><br></div><div>[*] I think their reasoning is that -Wno-xx-yy can be thus used to suppress warnings added to newer versions without breaking builds with older ones. But clang doesn&#39;t share that view.<br></div></div></div>
Selva Nair Nov. 14, 2019, 10:32 p.m. | #4
Hi,

On Thu, Nov 14, 2019 at 3:16 PM Илья Шипицин <chipitsine@gmail.com> wrote:

> Thank you for your efforts.
> As you are touching this, can you try "dist: bionic" ? It might bring
> newer compilers
>

I don't expect newer compilers on bionic break this patch. But fwiw, I've
started a travis build with dist: bionic. For results see
https://travis-ci.org/selvanair/openvpn/jobs/612099524

Selva
<div dir="ltr"><div>Hi,<br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Nov 14, 2019 at 3:16 PM Илья Шипицин &lt;<a href="mailto:chipitsine@gmail.com" target="_blank">chipitsine@gmail.com</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto">Thank you for your efforts.<div dir="auto">As you are touching this, can you try &quot;dist: bionic&quot; ? It might bring newer compilers</div></div></blockquote><div><br></div><div>I don&#39;t expect newer compilers on bionic break this patch. But fwiw, I&#39;ve started a travis build with dist: bionic. For results see <a href="https://travis-ci.org/selvanair/openvpn/jobs/612099524" target="_blank">https://travis-ci.org/selvanair/openvpn/jobs/612099524<br></a></div><div><br></div><div>Selva<br></div></div></div>
Arne Schwabe Nov. 17, 2019, 7:08 p.m. | #5
Am 14.11.19 um 22:58 schrieb Selva Nair:
> Hi David
> 
> Thanks for the comments
> 
>     My idea was just to add -Werror right in the line above, and not
>     extend the
>     ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.
> 
> 
> I'm fine with that approach as well. Let me know if you want a v2.
> 
> 
>     I think you said it pretty well in your mail:
> 
>     > Darn compilers and darn -Werror
> 
> 
>     So your change does improve Clang ... I dunno what we will do with GCC.
> 
> 
> It doesn't affect gcc as gcc does error (or ignore -- see below)
> unsupported options with or without -Werror. So I think we are okay.
> travis build on my fork confirms that:
> https://travis-ci.org/selvanair/openvpn/builds/612019849
> 
> However, gcc does have quirky behaviour when it comes to options like
> -Wno-xx-yy. It just accepts them silently[*] with or without -Werror.
> That works fine for us, but it does generate an error about that
> unsupported option if any other error is encountered during the build. 
> As other errors are anyway a show-stopper, I think we can live with that.
> 
> Selva
> 
> [*] I think their reasoning is that -Wno-xx-yy can be thus used to
> suppress warnings added to newer versions without breaking builds with
> older ones. But clang doesn't share that view.

I agree with Selva here. If we cannot get rid of the warnings themselves
this seems to be the only sane way. So I am not totally with this stuff
but it is better than the altneratives at the moment. And I really
annoyed by all these new warnings of Clang

Acked-By: Arne Schwabe <arne@rfc2549.org>

Arne
David Sommerseth Nov. 19, 2019, 2:09 p.m. | #6
On 14/11/2019 22:58, Selva Nair wrote:
> Hi David
> 
> Thanks for the comments
> 
>     My idea was just to add -Werror right in the line above, and not extend the
>     ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.
> 
> 
> I'm fine with that approach as well. Let me know if you want a v2.

Sorry for the delay responding.  I've done some pondering, and I have come to
the same conclusion as Arne, so this is a reasonable approach to add -Werror.

That said, is there any specific reasons why adding the [-Werror] to
essentially all ACL_CHECK_ADD_COMPILE_FLAGS() calls?  Our ./configure script
is the only one implementing and supporting and using this macro, and it is
defined in configure.ac.

If there are no foreseeable reason for that now, I think it's just fine to
change the AC_COMPILE_IFELSE() call to use CFLAGS="$1 -Werror $old_cflags".
Just add a comment in that area why we add the -Werror explicitly.  This makes
the whole change smaller and clearer.  If we need the flexibility later on, we
can adjust this when that time arrives.
Selva Nair Nov. 19, 2019, 4:50 p.m. | #7
Hi,

On Tue, Nov 19, 2019 at 9:09 AM David Sommerseth <
openvpn@sf.lists.topphemmelig.net> wrote:

> On 14/11/2019 22:58, Selva Nair wrote:
> > Hi David
> >
> > Thanks for the comments
> >
> >     My idea was just to add -Werror right in the line above, and not
> extend the
> >     ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.
> >
> >
> > I'm fine with that approach as well. Let me know if you want a v2.
>
> Sorry for the delay responding.  I've done some pondering, and I have come
> to
> the same conclusion as Arne, so this is a reasonable approach to add
> -Werror.
>
> That said, is there any specific reasons why adding the [-Werror] to
> essentially all ACL_CHECK_ADD_COMPILE_FLAGS() calls?  Our ./configure
> script
> is the only one implementing and supporting and using this macro, and it is
> defined in configure.ac.
>
> If there are no foreseeable reason for that now, I think it's just fine to
> change the AC_COMPILE_IFELSE() call to use CFLAGS="$1 -Werror $old_cflags".
> Just add a comment in that area why we add the -Werror explicitly.  This
> makes
> the whole change smaller and clearer.  If we need the flexibility later
> on, we
> can adjust this when that time arrives.
>

Agreed. v2 follows.

Selva
<div dir="ltr"><div>Hi,<br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 19, 2019 at 9:09 AM David Sommerseth &lt;<a href="mailto:openvpn@sf.lists.topphemmelig.net">openvpn@sf.lists.topphemmelig.net</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On 14/11/2019 22:58, Selva Nair wrote:<br>
&gt; Hi David<br>
&gt; <br>
&gt; Thanks for the comments<br>
&gt; <br>
&gt;     My idea was just to add -Werror right in the line above, and not extend the<br>
&gt;     ACL_CHECK_ADD_COMPILE_FLAGS macro with another argument.<br>
&gt; <br>
&gt; <br>
&gt; I&#39;m fine with that approach as well. Let me know if you want a v2.<br>
<br>
Sorry for the delay responding.  I&#39;ve done some pondering, and I have come to<br>
the same conclusion as Arne, so this is a reasonable approach to add -Werror.<br>
<br>
That said, is there any specific reasons why adding the [-Werror] to<br>
essentially all ACL_CHECK_ADD_COMPILE_FLAGS() calls?  Our ./configure script<br>
is the only one implementing and supporting and using this macro, and it is<br>
defined in <a href="http://configure.ac" rel="noreferrer" target="_blank">configure.ac</a>.<br>
<br>
If there are no foreseeable reason for that now, I think it&#39;s just fine to<br>
change the AC_COMPILE_IFELSE() call to use CFLAGS=&quot;$1 -Werror $old_cflags&quot;.<br>
Just add a comment in that area why we add the -Werror explicitly.  This makes<br>
the whole change smaller and clearer.  If we need the flexibility later on, we<br>
can adjust this when that time arrives.<br></blockquote><div><br></div><div>Agreed. v2 follows.</div><div><br></div><div>Selva</div></div></div>

Patch

diff --git a/configure.ac b/configure.ac
index 807804e5..e59bd91b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1275,18 +1275,21 @@  if test "${enable_pkcs11}" = "yes"; then
 	)
 fi
 
+# Usage: ACL_CHECK_ADD_COMPILE_FLAGS(flag, [extra-flags])
+# Some compilers require extra flags to trigger an error on
+# unsupported options. E.g., clang requires -Werror.
 AC_DEFUN([ACL_CHECK_ADD_COMPILE_FLAGS], [
     old_cflags="$CFLAGS"
-    CFLAGS="$1 $CFLAGS"
-    AC_MSG_CHECKING([whether the compiler acceppts $1])
-    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])],
+    CFLAGS="$2 $1 $CFLAGS"
+    AC_MSG_CHECKING([whether the compiler accepts $1])
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [AC_MSG_RESULT([yes])]; CFLAGS="$1 $old_cflags",
         [AC_MSG_RESULT([no]); CFLAGS="$old_cflags"])]
 )
 
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation])
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function])
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter])
-ACL_CHECK_ADD_COMPILE_FLAGS([-Wall])
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-stringop-truncation], [-Werror])
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-function], [-Werror])
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wno-unused-parameter], [-Werror])
+ACL_CHECK_ADD_COMPILE_FLAGS([-Wall], [-Werror])
 
 if test "${enable_pedantic}" = "yes"; then
 	enable_strict="yes"