Vivien Kraus wrote:
> - The library function export visibility trick is great, but the manual
> suggests to augment it with dllimport/export for MSVC:
>
> #if BUILDING_LIBFOO && HAVE_VISIBILITY
> #define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
> #elif BUILDING_LIBFOO && defined _MSC_VER
> #define LIBFOO_DLL_EXPORTED __declspec(dllexport)
> #elif defined _MSC_VER
> #define LIBFOO_DLL_EXPORTED __declspec(dllimport)
> #else
> #define LIBFOO_DLL_EXPORTED
> #endif
>
> However, using this as-is breaks for mingw (HAVE_VISIBILITY is 0 and
> _MSC_VER is not defined, so all symbols are exported, which means a
> possible collision with gnulib functions used in both libfoo and a
> program linked to libfoo). Maybe the manual should suggest to extend
> the MSVC check to mingw as well? That’s what I did in my project.
Good point. I'm making this documentation update:
2022-07-03 Bruno Haible <br...@clisp.org>
lib-symbol-visibility: Improve documentation.
Reported by Vivien Kraus <viv...@planete-kraus.eu> in
<https://lists.gnu.org/archive/html/bug-gnulib/2022-06/msg00093.html>.
* doc/lib-symbol-visibility.texi: List the platforms. Extend the
LIBFOO_DLL_EXPORTED to work also with mingw and also with
--disable-shared.
diff --git a/doc/lib-symbol-visibility.texi b/doc/lib-symbol-visibility.texi
index 60cde8dae4..9574e439dd 100644
--- a/doc/lib-symbol-visibility.texi
+++ b/doc/lib-symbol-visibility.texi
@@ -106,6 +106,24 @@ It defines a Makefile variable @code{@@CFLAG_VISIBILITY@@}
containing
as a substituted variable: @@HAVE_VISIBILITY@@. Its value is 1 when symbol
visibility control is supported, and 0 otherwise.
+As of 2022, symbol visibility control is supported on
+@itemize @bullet
+@item
+ELF platforms (glibc, Linux, *BSD, Solaris) with GCC or clang,
+@item
+macOS,
+@item
+AIX with gcc or xlclang.
+@end itemize
+@noindent
+It is not supprted on
+@itemize @bullet
+@item
+Other compilers on ELF platforms or AIX,
+@item
+Windows.
+@end itemize
+
To use this module in a library, say libfoo, you will do these steps:
@enumerate
@@ -121,7 +139,7 @@ for the compilation of the sources that make up the library.
@item
Define a macro specific to your library like this.
@smallexample
-#if BUILDING_LIBFOO && HAVE_VISIBILITY
+#if HAVE_VISIBILITY && BUILDING_LIBFOO
#define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
#else
#define LIBFOO_DLL_EXPORTED
@@ -149,13 +167,17 @@ your library.
Note about other compilers: MSVC support can be added easily, by extending
the definition of the macro mentioned above, to something like this:
@smallexample
-#if BUILDING_LIBFOO && HAVE_VISIBILITY
+#if HAVE_VISIBILITY && BUILDING_LIBFOO
#define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
-#elif BUILDING_LIBFOO && defined _MSC_VER
+#elif (defined _WIN32 && !defined __CYGWIN__) && BUILDING_SHARED &&
BUILDING_LIBFOO
#define LIBFOO_DLL_EXPORTED __declspec(dllexport)
-#elif defined _MSC_VER
+#elif (defined _WIN32 && !defined __CYGWIN__) && BUILDING_SHARED
#define LIBFOO_DLL_EXPORTED __declspec(dllimport)
#else
#define LIBFOO_DLL_EXPORTED
#endif
@end smallexample
+@noindent
+Here @code{BUILDING_SHARED} is a C macro that you have to define. It
+ought to evaluate to 1 in a build configured with @samp{--enable-shared},
+or to 0 in a build configured with @samp{--disable-shared}.
