FreeTDS Development Group

[Michael Peppler <mpeppler AT peppler.org>]

James K. Lowden wrote:
> Fu, Steve wrote:
> > I need to access Sybase and MSSQL servers from the same C++ program and
> > I Prefer to use Syabse CT lib to access Sybase and FreeTDS lib to access
> > MSSQL serever.
> >  
> > I cannot link libdb.a from both Sybase and FreeTDS? Are there any
> > suggestions or pointers?
>
> I don't see any easy way to do this, Steve.  It's not a problem unique to
> you; there are people who'd like to use DBD::Sybase (a C ct-lib module)
> with the same discriminate-by-server feature.  
>
>
> The first problem, as you know, is that FreeTDS *intentionally* produces
> symbols that match Sybase's.  The second problem is that the header files,
> while similar, aren't interchangeable.  
>
> The C approach would be to change all the symbols at the source. 
> Something like:
>
>         #if FREETDS_IS_FREETDS
>         #define ct_connect ct_connect_via_freetds
>         #endif
>
> There are 90 functions that would need that treatment.  Then you could
> obviously link to both libraries (because you'd have new symbols).  Your
> application would then need a giant switch statement to choose the right
> version at runtime.  
>
> An admin approach is to build FreeTDS as is, and use dlopen(3) to load
> only one library at a time.  But you could use only one library at a time.

Actually that's not quite the case.

I have a perl module calld DBD::FreeTDS which links the freetds version 
of the open client libs, and I can load it in perl at the same time as I 
have DBD::Sybase (with Sybase open client) loaded.

The trick is to build a shared object that includes the functionality 
that is needed, and which links to the appropriate version of the API 
libraries (freetds or sybase), and then load this shared object to the 
main program via dlopen(). You need to make sure that you load without 
sharing the symbols between the two shared object modules (I forget the 
exact flag, the man page for dlopen() should mention it).

It's not really pretty, but it works quite well.

Michael
--
Michael Peppler           -                 Peppler Consulting SaRL
mpeppler AT peppler.org      -                  http://www.peppler.org
Sybase DBA/Developer      -   TeamSybase: http://www.teamsybase.com
Sybase on Linux FAQ       -   http://www.peppler.org/FAQ/linux.html