Re: [PATCH] make gl_VISIBILITY to reject unsupported configurations with newer gcc

[Ozkan Sezer]

On 6/5/21, Bruno Haible <bruno@clisp.org> wrote:
> Ozkan Sezer wrote:
>> >> The attached patch makes gl_VISIBILITY to reject unsupported
>> >> configurations
>> >> with newer gcc: In its current form, mingw gcc-4.5.4 rejects the
>> >> attributes
>> >> but gcc-7.5 does not, apparently because the attributes are applied to
>> >> func
>> >> prototypes without their definitions.  Attaching a hidden visibility
>> >> attrib
>> >> to dummyfunc() makes gcc-7.5 to reject it properly.
>> >
>> > I don't fully understand why we should apply the patch.
>> >
>> > With gcc 7.5 on mingw and the visibility.m4 that is in gnulib now:
>> > What is the result of the
>> >   checking for simple visibility declarations...
>> > test? What are the effects: warnings? compilation errors? link errors?
>> > too
>> > many exported symbols? too few exported symbols?
>>
>> No warnings from the testcase in m4. In real-life, it results in
>> warnings,
>> see e.g.: https://github.com/libsndfile/libsamplerate/issues/154
>
> OK, I understand better now.
>
> In the GCC source code I see that the
>   "visibility attribute not supported in this configuration; ignored"
> warning is emitted when a definition is emitted. Therefore I think the
> safest patch is to add a definition for each declaration in the test code.
>
>
> 2021-06-05  Bruno Haible  <bruno@clisp.org>
>
>       lib-symbol-visibility: Make configure check work for newer GCC.
>       Reported by Ozkan Sezer <sezeroz@gmail.com> in
>       <https://lists.gnu.org/archive/html/bug-gnulib/2021-05/msg00117.html>.
>       * m4/visibility.m4 (gl_VISIBILITY): Add a function definition for each
>       declaration in the test program.
>
> diff --git a/m4/visibility.m4 b/m4/visibility.m4
> index 8f27a12..d161bd7 100644
> --- a/m4/visibility.m4
> +++ b/m4/visibility.m4
> @@ -1,4 +1,4 @@
> -# visibility.m4 serial 7
> +# visibility.m4 serial 8
>  dnl Copyright (C) 2005, 2008, 2010-2021 Free Software Foundation, Inc.
>  dnl This file is free software; the Free Software Foundation
>  dnl gives unlimited permission to copy and/or distribute it,
> @@ -59,6 +59,10 @@ AC_DEFUN([gl_VISIBILITY],
>                extern __attribute__((__visibility__("hidden"))) int
> hiddenfunc (void);
>                extern __attribute__((__visibility__("default"))) int
> exportedfunc (void);
>                void dummyfunc (void);
> +              int hiddenvar;
> +              int exportedvar;
> +              int hiddenfunc (void) { return 51; }
> +              int exportedfunc (void) { return 1225736919; }
>                void dummyfunc (void) {}
>              ]],
>              [[]])],
>
>

Works for me:

configure:13230: checking for simple visibility declarations
configure:13261: x86_64-w64-mingw32-gcc -c -g -O2 -Wall
-fvisibility=hidden -Werror  conftest.c >&5
conftest.c: In function 'hiddenfunc':
conftest.c:37:15: error: visibility attribute not supported in this
configuration; ignored [-Werror=attributes]
               int hiddenfunc (void) { return 51; }
               ^~~
conftest.c: At top level:
conftest.c:47:1: error: visibility attribute not supported in this
configuration; ignored [-Werror=attributes]
 }
 ^
cc1: all warnings being treated as errors
configure:13261: $? = 1