                    Binary Compatability and wxWidgets
                    ==================================
0. Purpose
----------

This is broad technote covering all aspects of binary compatability with
wxWidgets.

1. Releases
-----------

General overview of releases can be found in tn0012.txt, but for
completeness the wxWidgets release version number is as follows:

2.6.2

Where

  2      6      2
Major  Minor Release

(I.E. Major.Minor.Release).

All Release versions where the Minor is EVEN (2.4.x,2.6.x
etc.  ODD minors are development versions)  are expected to be binary
compatable.  Note that this means FORWARD binary compatability only -
new methods to classes are ok as long as they arn't virtual, etc.

2. What kind of changes are NOT binary compatable
-------------------------------------------------

If its still up, the KDE guide is a good reference:
http://developer.kde.org/documentation/other/binarycompatibility.html

The changes that are NOT binary compatable:
- Adding a virtual function
- Changing the name of a any function or variable
- Changing the signature of a virtual function (adding a parameter,
even a default one)
- Changing the order of the virtual functions in a class
["switching" them, etc.]
- Changing access privalages to a function (protected to private etc.)
[unlike KDE we need to support windows so this is not allowed]
- Adding a member variable
- Changing the order of non-static member variables


3. wxABI_VERSION and BACKWARD binary compatability
--------------------------------------------------

As mentioned we do not support BACKWARD binary compatability.

However, for this purpose we have the macro wxABI_VERSION.  All
new symbols added to binary compatable releases are to be ifed
with wxABI_VERSION: this allows the user code to avoid using them
by defining wxABI_VERSION to the value corresponding to an older
release.

The layout of wxABI_VERSION is as follows:

20602

where

 2     06     02
Major Minor Release

I.E. it corresponds to the wxWidgets release in {1}.

An example of using wxABI_VERSION is as follows for symbols
only in a 2.6.2 release:

#if wxABI_VERSION >= 20602 /* 2.6.2+ only */
bool Load(const wxURI& location, const wxURI& proxy);

wxFileOffset GetDownloadProgress();
wxFileOffset GetDownloadTotal();

bool    ShowPlayerControls(
  wxMediaCtrlPlayerControls flags =
    wxMEDIACTRLPLAYERCONTROLS_DEFAULT);

//helpers for the wxPython people
bool LoadURI(const wxString& fileName)
{   return Load(wxURI(fileName));       }
bool LoadURIWithProxy(const wxString& fileName, const wxString& proxy)
{   return Load(wxURI(fileName), wxURI(proxy));       }
#endif

Note that it only makes sense to use wxABI_VERSION in the headers as the
new function should always be compiled into the library, wxABI_VERSION
only controls its visibility to the user code.


4. Workarounds for adding virtual functions
-------------------------------------------

Originally the idea for adding virtual functions to binary compatable
releases was to pad out some empty "reserved" functions and then
rename those later when someone needed to add a virtual function.

However, after there was some actual testing of the idea a lot of
controversy erupted.  Eventually we decided against the idea, and
instead devised a new method for doing so called wxShadowObject.

wxShadowObject is a class derived from wxObject that provides a means
of adding functions and/or member variables to a class internally
to wxWidgets.  It does so by storing these in a hash map inside of
it, looking it up when the function etc. is called. wxShadowObject
is generally stored inside a reserved member variable.

wxShadowObject resides in include/wx/clntdata.h.

To use wxShadowObject, you first call AddMethod or AddField with
the first parameter being the name of the field and/or method
you want, and the second parameter being the value of the
field and/or method.

In the case of fields this is a void*, and in the case of method
is a wxShadowObjectMethod which is a typedef:
typedef int (*wxShadowObjectMethod)(void*, void*);

After you add a field, you can set it via SetField with the same
params as AddField, the second param being the value to set
the field to.  You can get the field after you call AddField
via GetField, with the parameters as the other two field functions,
only in the case the second parameter is the fallback
value for the field in the case of it not being found in the
hash map.

You can call a method after you add it via InvokeMethod, which
returns a bool indicating whether or not the method was found
in the hash map, and has 4 parameters.  The first parameter is
the name of the method you wish to call, the second is the first
parameter passed to the wxShadowObjectMethod, the third is the
second parameter passed to that wxShadowObjectMethod, and the
fourth is the return value of the wxShadowObjectMethod.

5. version-script.in
--------------------

For ld/libtool we use sun-style version scripts.  Basically
anything which fits the conditions of being ifed via wxABI_VERSION
needs to go here also.

See 'info ld scripts version' on a GNU system, it's online here:
http://www.gnu.org/software/binutils/manual/ld-2.9.1/html_node/ld_25.html

Or see chapter 5 of the 'Linker and Libraries Guide' for Solaris, available
online here:
http://docsun.cites.uiuc.edu/sun_docs/C/solaris_9/SUNWdev/LLM/p1.html

The file has the layout as follows:

@WX_VERSION_TAG@.X

Where X is the current Release as mentioned earlier, i.e. 2.  This
is following by an opening bracket "{", followed by "global:",
followed by patterns matching added symbols, then followed by "}", and then
the file is either followed by earlier Releases or ended by
a @WX_VERSION_TAG@ block without the period or Release.

The patterns used to specify added symbols are globbing patters and can
contain wildcards such as '*'.

For example for a new class member such as:
    wxFont wxGenericListCtrl::GetItemFont( long item ) const;

the mangled symbol might be:
    _ZNK17wxGenericListCtrl11GetItemFontEl

so a line like this could be added to version-script.in:
    *wxGenericListCtrl*GetItemFont*;

Allow for the fact that the name mangling is going to vary from compiler to
complier.

When adding a class you can match all the symbols it adds with a single
pattern, so long as that pattern is not likely to also match other symbols.
For example for wxLogBuffer a line like this:
    *wxLogBuffer*;


5.5. Checking the version information in libraries and programs
---------------------------------------------------------------

On Sun there is a tool for this, see pvs(1). On GNU you can use objdump, below
are some examples.

To see what versions of each library a program (or library) depends on:

$ objdump -p widgets | sed -ne '/Version References/,/^$/p'
Version References:
  required from libgcc_s.so.1:
    0x0b792650 0x00 10 GCC_3.0
  required from libwx_based-2.6.so.0:
    0x0cca2546 0x00 07 WXD_2.6
  required from libstdc++.so.6:
    0x056bafd3 0x00 09 CXXABI_1.3
    0x08922974 0x00 06 GLIBCXX_3.4
  required from libwx_gtk2d_core-2.6.so.0:
    0x0a2545d2 0x00 08 WXD_2.6.2
    0x0cca2546 0x00 05 WXD_2.6
  required from libc.so.6:
    0x09691a75 0x00 04 GLIBC_2.2.5

To see what WXD_2.6.2 symbols a program uses:

$ objdump -T widgets | grep 'WXD_2\.6\.2'
0000000000000000 g    DO *ABS*  0000000000000000  WXD_2.6.2   WXD_2.6.2
00000000004126d8      DF *UND*  0000000000000177  WXD_2.6.2   _ZN19wxTopLevelWindowGTK20RequestUserAttentionEi

To see what WXD_2.6.2 symbols a library defines:

$ objdump -T libwx_based-2.6.so | grep 'WXD_2\.6\.2' | grep -v 'UND\|ABS'
0000000000259a10  w   DO .data  0000000000000018  WXD_2.6.2   _ZTI19wxMessageOutputBest
00000000002599e0  w   DO .data  0000000000000028  WXD_2.6.2   _ZTV19wxMessageOutputBest
000000000010a98e  w   DF .text  000000000000003e  WXD_2.6.2   _ZN19wxMessageOutputBestD0Ev
0000000000114efb  w   DO .rodata        000000000000000e  WXD_2.6.2   _ZTS11wxLogBuffer
0000000000255590  w   DO .data  0000000000000018  WXD_2.6.2   _ZTI11wxLogBuffer
000000000011b550  w   DO .rodata        0000000000000016  WXD_2.6.2   _ZTS19wxMessageOutputBest
00000000000bfcc8 g    DF .text  00000000000000dd  WXD_2.6.2   _ZN11wxLogBuffer5DoLogEmPKcl
000000000010a3a6 g    DF .text  0000000000000153  WXD_2.6.2   _ZN19wxMessageOutputBest6PrintfEPKcz
00000000000c0b22  w   DF .text  000000000000004b  WXD_2.6.2   _ZN11wxLogBufferD0Ev
00000000000bfc3e g    DF .text  0000000000000089  WXD_2.6.2   _ZN11wxLogBuffer5FlushEv
00000000000c0ad6  w   DF .text  000000000000004b  WXD_2.6.2   _ZN11wxLogBufferD1Ev
00000000000b1130  w   DF .text  0000000000000036  WXD_2.6.2   _ZN11wxLogBufferC1Ev
00000000000c095c  w   DF .text  0000000000000029  WXD_2.6.2   _ZN19wxMessageOutputBestC1Ev
00000000000c08e8  w   DF .text  000000000000003e  WXD_2.6.2   _ZN19wxMessageOutputBestD1Ev
00000000002554c0  w   DO .data  0000000000000038  WXD_2.6.2   _ZTV11wxLogBuffer
00000000000bfda6 g    DF .text  0000000000000036  WXD_2.6.2   _ZN11wxLogBuffer11DoLogStringEPKcl
00000000000abe10 g    DF .text  0000000000000088  WXD_2.6.2   _ZN14wxZipFSHandler7CleanupEv


6. Testing binary compatability between releases
------------------------------------------------

An easy way of testing binary compatability is just to build wxWidgets
in dll/dynamic library mode and then switch out the current library
in question with an earlier stable version of the library, then running
the application in question again.  If it runs OK then there is usually
binary compatability between those releases.

You can also break into your debugger or whatever program you want
to use and check the memory layout of the class.  If it is the same
then it is binary compatable.

Also remember to look at http://www.wxwidgets.org/bincompat.html page which
summarizes the results of testing of all the samples built against old
libraries headers with the new library binaries under Unix.


=== EOF ===

Author:  RN
Version: $Id: tn0020.txt,v 1.3.2.1 2006/03/18 21:58:17 VZ Exp $
