Re: what is the difference of binding under gcc and M$ VC 6?

[llewelly at xmission dot com]

"Kun Wei" <weikun@caltech.edu> writes:

> Hi, I have a project developed on openh323, because it is a more
> general C++ questions, I would like some C++ experts give me some
> hints.

If your question is really just about c++, you will get better results
    from comp.lang.c++.moderated, or comp.std.c++ .

However it appears you are trying to make a MSVC++ emulate a gcc on
    linux behavior - unfortunately for you I suspect there are not
    many MSVC++ wizards on gcc-help. :-( Certainly I am not one.

> 
> We customize some functions to comply our need, the details as follow:
> 
> on open323 lib, we have a h323rtp.h header file declare
> 
> virtual BOOL OnReceivedPDU( H323_RTPChannel & channel, /// Channel
> using this session.
> 
> const H245_H2250LogicalChannelParameters & param, /// Acknowledgement
> PDU
> 
> unsigned & errorCode /// Error on failure
> 
> );
> 
> and in source h323rtp.c defines
> 
> BOOL H323_RTP_UDP::OnReceivedPDU(H323_RTPChannel & channel,
> 
> const H245_H2250LogicalChannelParameters & param,
> 
> unsigned & errorCode)
> 
> {
> 
> //some code here
> 
> }
> 
> 
> To comply our need, we override the function in our own application
> code with exactly same signature
> 
> BOOL H323_RTP_UDP::OnReceivedPDU(H323_RTPChannel & channel,
> 
> const H245_H2250LogicalChannelParameters & param,
> 
> unsigned & errorCode)
> 
> {
> 
> //some customized code here
> 
> }
> 
> 
> When I build it with gcc under Linux, the funtion in our applicatoin
> gets called, which is what we want. But when I compile/build it with
> M$ Visual Studio C++ 6.0, it seems only the original function in
> h323rtp.c is called, out customized function is never called.

The C++ standard says your code is ill-formed and no diagnostic is
    required, because your code violates the One Definition Rule -
    that is, you have two definitions of
    H323_RTP_UDP::OnReceivedPDU(H323_RTPChannel & channel, const
    H245_H2250LogicalChannelParameters & param,unsigned & errorCode )
    in seperate translation units. Since no diagnostic is required
    (and ISO C++ never requires code be rejected), your C++ compilers
    are free to generate no warnings, no errors, and do whatever they
    like. 

The linux behavior is due to the old unix linker tradition - when a
    new object file is linked in, symbols defined in the new object
    file are used only to resolve symbols not already resolved. So
    when your application provides a definition, the uses of that
    symbol get resolved (to use your definition) before the library is
    linked in, and the library's definition is never used.

I don't use MSVC++, so I can't explain its behavior, except to say
    that windows generally has no respect for unix traditions. :-)

> So what is the difference between these 2 environments? How can my
> customzied function also get called in Windows?
[snip]

Perhaps you could experiment with the cygwin or mingw ports of gcc -
    see cygwin.com and mingw.org respectively.

Also, try asking in a forum specific to Visual C++, codewarrior, etc -
    a Visual C++ or codewarrior wizard might be present and know how
    to trick said compiler into giving you what you want.