Hi Marios,
I also now realise that trying to build a GUI application has been your problem all along.
Your initial output also indicated you have been building a GUI application:
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..Linking Load Module stub BDS02
link /NOLOGO /SUBSYSTEM:WINDOWS -entry:WinMainCRTStartup -out:"C:\Models\BDS.ief\c\BDS02.EXE" "C:\Models\BDS.ief\c\OBJC\STUB.OBJ" "C:\Models\BDS.ief\c\OBJC\STUBMAIN.OBJ" "C:\Models\BDS.ief\c\STUB.RES" @"C:\Models\BDS.ief\c\STUB.LNK"
STUB.OBJ : error LNK2019: unresolved external symbol _sqlcxt referenced in function _C4CONNECT
C:\Models\BDS.ief\c\BDS02.EXE : fatal error LNK1120: 1 unresolved externals
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Ms VS 11.0\VC\BIN\link.EXE"' : return code '0x460'
===
The STUB concept is only used for GUI applications and token OPT.BUILD_SRC is also relevant to stubs for GUI applications. Sorry, I should have spotted that earlier!
Per my previous post, even if you set token OPT.BITS=64, that value is ignored for GUI applications which can only be 32-bit. Therefore a GUI application is incompatible with a 64-bit Oracle 12c client. That is also why your build was successful when you changed to use the 32-bit Oracle 12c client.
When I tried to build Gen 8.5 sample model Window Packaged GUI module GUIMENU with an Oracle 12c 64-bit client I see the exact same "STUB.OBJ : error LNK2019: unresolved external symbol _sqlcxt referenced in function _C4CONNECT" error.
If I change TP Monitor to IEFAE and regenerate then the build is successful when using OPT.BITS=64 and Oracle 12c 64-bit client.
EDIT:
If I change to use Cooperative Packaged server P900 with TP Monitor set to IEFAE and regenerate then the build is successful when using OPT.BITS=64 and Oracle 12c 64-bit client.
So in summary if you want to build a Windows Gen 64-bit application with a 64-bit Oracle client then a Cooperative Packaged Server Manager load module can be built by setting set the "TP Monitor" generation option to IEFAE. Then build a 32-bit GUI client (with "TP Monitor" set to Windows and no database access) to connect to that 64-bit server e.g. in the Gen Sample model "Generation > Cooperative Code", Server Manager P900 and Windows Manager W900.
Hope that helps
Lynn