Re: inline -Werror=missing-declarations warnings

[Pádraig Brady]

On 12/18/2012 03:16 AM, Paul Eggert wrote:
> On 12/17/12 17:35, Pádraig Brady wrote:
> > Should the check in extern-inline.m4 be expanded to all __GNUC_?
> > 39cedf6f said the issue was only with gcc 4.8, but
> > GCC 4.5.3 (on cygwin at least), seems to have the issue too.
>
> If I understand things correctly, 39cedf6f relies on
> _Pragma ("GCC diagnostic push"), which requires GCC 4.6
> or later, and so we can't apply its idea to GCC 4.5.3.

Right.

> The typical solution for older GCCs right now is to
> ignore the bogus warnings that they generate, which
> implies that with these older compilers one should either
> avoid -Werror, or avoid -Wmissing-prototypes and
> -Wmissing-declarations and -Wsuggest-attributes=const.

Well gnulib doesn't add -Werror.
It could disable that above with something like:

diff --git a/m4/manywarnings.m4 b/m4/manywarnings.m4
index f3cb23b..b02122d 100644
--- a/m4/manywarnings.m4
+++ b/m4/manywarnings.m4
@@ -90,6 +90,33 @@ AC_DEFUN([gl_MANYWARN_ALL_GCC],
       CFLAGS="$gl_save_CFLAGS"])
     AC_MSG_RESULT([$gl_cv_cc_uninitialized_supported])

+    dnl Check if the compiler has fine-grained control over where
+    dnl diagnostics apply, so help determine wether to
+    dnl disable certain warnings.
+    AC_MSG_CHECKING([whether we have fine grained diagnostics])
+    AC_CACHE_VAL([gl_cv_cc_finegrained_diagnostics], [
+      gl_save_CFLAGS="$CFLAGS"
+      CFLAGS="$CFLAGS -Werror -Wformat"
+      AC_COMPILE_IFELSE(
+        [AC_LANG_PROGRAM(
+           [[#include <stdio.h>
+             int main(void)
+             {
+               float f=0; int unused;
+               #pragma GCC diagnostic ignored "-Wunused-variable"
+               #pragma GCC diagnostic push
+               #pragma GCC diagnostic ignored "-Wformat"
+               #pragma GCC diagnostic error   "-Wunused-variable"
+               printf ("%d\n", f);
+               #pragma GCC diagnostic pop
+               return 0;
+             }
+           ]],
+           [[]])],
+        [gl_cv_cc_finegrained_diagnostics=yes],
+        [gl_cv_cc_finegrained_diagnostics=no])
+      CFLAGS="$gl_save_CFLAGS"])
+    AC_MSG_RESULT([$gl_cv_cc_finegrained_diagnostics])
   fi

   # List all gcc warning categories.
@@ -220,5 +247,9 @@ AC_DEFUN([gl_MANYWARN_ALL_GCC],
     gl_manywarn_set="$gl_manywarn_set -Wno-uninitialized"
   fi

+  if test "$gl_cv_cc_finegrained_diagnostics" = no; then
+    gl_manywarn_set="$gl_manywarn_set -Wno-missing-declarations ..."
+  fi
+
   $1=$gl_manywarn_set
 ])


However that's a bit complicated. I wouldn't mind, apart from it would
need to be updated any time part of gnulib selectively disabled warnings using
#pragmas
So I'll probably not go with this, and do the following instead...

> Z. Majeed wrote that he had to fiddle with --disable-gcc-warnings
> (I assume he meant --enable-gcc-warnings?) and perhaps
> --enable-gcc-warnings should be tweaked to disable these
> problematic warnings with older compilers.

coreutils auto does --enable-gcc-warnings if
compiling from a directory with .git/ present.
So --disable-gcc-warnings is needed to avoid that.

While explicit version checks are not as robust
as feature checks, I don't see much advantage in
adjusting the above feature check patch for use in coreutils,
so instead I'll probably go with this simpler coreutils patch:

diff --git a/configure.ac b/configure.ac
index 6504144..295802d 100644
--- a/configure.ac
+++ b/configure.ac

AC_ARG_ENABLE([gcc-warnings]
...
+   # GCC provides fine-grained control over diagnostics which
+   # is used in gnulib for example to suppress warnings from
+   # certain sections of code.  So if this is available and
+   # we're running from a git repo, then auto enable the warnings.
+   gl_gcc_warnings=no
+   gl_GCC_VERSION_IFELSE([4], [6], [test -d "$srcdir"/.git && 
gl_gcc_warnings=yes])


thanks for the info,
Pádraig.