FreeTDS Development Group

["James K. Lowden" <jklowden AT freetds.org>]

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.
 

The C++ approach would be to put the FreeTDS functions in their own
namespace, converting them to C++ linkage at the same time.  Then you
could overload the functions in the primary namespace by, say, redefining
the connection handle type e.g.:

CS_RETCODE ct_connect(  TDS_CONNECTION * con, 
                        CS_CHAR * servername, 
                        CS_INT snamelen)
        {
                return freetds::ct_connect( con, 
                                        servername, 
                                        snamelen );
        }

As I said, no easy way.  But there are ways.  Maybe the best one is to
keep improving FreeTDS to the point that other implementations aren't
worth the bother.  

HTH.

--jkl