Symbol visibility
-----------------
Key: LUCY-231
URL: https://issues.apache.org/jira/browse/LUCY-231
Project: Lucy
Issue Type: Task
Components: Clownfish
Reporter: Nick Wellnhofer
Assignee: Nick Wellnhofer
In order to get compiled extension working on Windows, we'll have to define the
visibility of our extern variables and functions. On Windows every exported
symbol of a DLL has to be marked with __declspec(dllexport) when compiling the
DLL. If you're linking against a DLL, every symbol imported from the DLL has to
be marked __declspec(dllimport)
On UNIX, every symbol is exported by default, so defining visibility is not
strictly necessary. But hiding symbols that don't have to be exported has the
benefit of reducing size and speeding up loading of a DLL. Hidden symbols also
allow the compiler to generate more optimized code.
The standard approach is to compile with the GCC option -fvisibility=hidden
which emulates the Windows behavior. Then a macro is defined roughly like that
(should probably be handled by charmonizer):
{noformat}
#if defined __GNUC__
# if defined _WIN32 || defined __CYGWIN__
# define CHY_EXPORT __attribute__ ((dllexport))
# elif __GNUC__ >= 4
# define CHY_EXPORT __attribute__ ((visibility ("default")))
# else
# define CHY_EXPORT
# endif
#elif defined _MFC_VER
# define CHY_EXPORT __declspec(dllexport)
#else
# define CHY_EXPORT
#endif
{noformat}
This macro can then be used like that:
{noformat}
CHY_EXPORT void
exported_function();
extern CHY_EXPORT int exported_variable;
{noformat}
When compiling an extension, we also have to handle __declspec(dllimport) on
Windows. For the generated headers, we could define CHY_IMPORT and use it
instead of CHY_EXPORT for "included" headers. For the code in XSBind.[ch], we
could define BUILDING_XSBIND only during compilation of XSBind.c and then use
something like that:
{noformat}
#if defined __GNUC__
# if defined _WIN32 || defined __CYGWIN__
# if BUILDING_XSBIND
# define XSBIND_EXPORT __attribute__ ((dllexport))
# else
# define XSBIND_EXPORT __attribute__ ((dllimport))
# endif
# elif __GNUC__ >= 4
# define XSBIND_EXPORT __attribute__ ((visibility ("default")))
# else
# define XSBIND_EXPORT
# endif
#elif defined _MFC_VER
# if BUILDING_XSBIND
# define XSBIND_EXPORT __declspec(dllexport)
# else
# define XSBIND_EXPORT __declspec(dllimport)
# endif
#else
# define XSBIND_EXPORT
#endif
XSBIND_EXPORT cfish_Obj*
cfish_XSBind_new_blank_obj(SV *either_sv);
{noformat}
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