Re: [xml] Creating Win32 wrapper function for exported variables

["Robert Collins" <robert collins itdomain com au>]

----- Original Message -----
From: "Vakoc, Mark" <Mark_Vakoc jdedwards com>
To: <xml gnome org>
Sent: Thursday, July 19, 2001 8:34 AM
Subject: RE: [xml] Creating Win32 wrapper function for exported variables


> Hopefully this will put this to rest...
>
> Exporting data from a library requires an additional layer of indirection
> because you have to go through the import address table.  This would make
> ugly code because every exported variable would look like
>
> #ifdef _DLL
>  int someVar; /* when accessed from within the dll */
> #else
>  int *someVar; /* when accessed outside the dll */
> #endif
>
> Obviously this is to be avoided at all costs.  Using __declspec(dllexport)
> and __declspec(dllimport) handle the indirection automatically.
>
> For our solution:  If we export data entirely through the .def file (not
> using any LIBXML_DLL_IMPORTS macro) which also needs the tag DATA appended
> to the variable names in the .DEF file, any program using wanting to use
> this data in libxml2.dll (such as xmllint) needs to still declare the
> variable with __declspec(dllimport) int someVar; --  Using extern int
> someVar alone will not work as that will return a ptr to the value and not
> the stored value.  This is a read only pointer and is why xmllint gets a
> segv when built against libxml2_so.dsp and you use the --valid flag or
using
> --version.
>
> So, regardless the libxml2 headers need have some win32 references and
hence
> we need to keep LIBXML_DLL_IMPORTS.  So here's my proposed solution:
>
> 1.  All functions are exported through the .DEF file as currently done.
> 2.  All global variables that should be made available externally needed
to
> be declared in their implementation (.c) as LIBXML_DLL_IMPORT type name.
> 3.  In the headers, they are declared as LIBXML_DLL_IMPORT extern type
> variable.
> 4.  We need to change the definition of LIBXML_DLL_IMPORT.  When built as
a
> dynamic library (libxml2_so.dsp), an additional preprocessor defition of
> LIBXML_EXPORTS is defined.  This is used to identify that we are building
> libxml2, and not just linking against it.  When building the static
library
> (libxml2_a.dsp) it does not matter if LIBXML_EXPORTS is set or not.
>
> So, in win32xmlversion.h et al we have
> #ifndef LIBXML_DLL_IMPORT
> #ifdef _WIN32
> #ifdef LIBXML_EXPORTS
> #define LIBXML_DLL_IMPORT __declspec(dllexport)
> #else
> #define LIBXML_DLL_IMPORT __declspec(dllimport)
> #endif
> #else
> #define LIBXML_DLL_IMPORT
> #endif
> #endif
>
> This cleans up a lot of linker warnings which people are always
complaining
> about and allows programs, such as xmllint to be built against libxml2_so
> (which currently it cannot -- it will only work properly when linked
against
> libxml2_a.dsp).
>
> I don't know about cgywin so any input there?

Thats fine for cygwin and mingw. Obviously the Makefile.am's need to be
tweaked to define LIBXML_EXPORTS when building libxml.

Rob.