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

From: Bruno Haible
Subject: Re: [PATCH] make gl_VISIBILITY to reject unsupported configurations with newer gcc
Date: Sat, 05 Jun 2021 17:10:45 +0200
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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
        * 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 
               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) {}

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