Yesterday I posted an appeal for help with suggestions on using the new Firebird 3 API from a Free Pascal program, mainly to see if I had missed something in my analysis of the new interface. I did get one pointer to "UIB". However, that did nothing more than confirm my suspicions that there are some serious problems with the API as presented in the alpha 2 release.
In summary, I have the following issues with the new Firebird 3 API: 1. It is a C++ Object Oriented API and needs to be understood as such. I have no problem with it as a C++ API, but it is not a general purpose API and does not provide platform/compiler/language independence. As presented in 'firebird/interface.h', the API provides pointers to C++ Classes and is always dependent on the vtable layout internal to each C++ compiler. 2. There is no such thing as a standard ABI or even a standard C++ ABI and you cannot rely on the output of one compiler being able to navigate the vtable produced by another, or even by a different version of the same compiler. 3. A regular feature on the Free Pascal wish lists is the ability to use C++ shared objects from Pascal. If it were simply a matter of defining a canonical class structure in Pascal and passing pointers to classes then this would be easy. Unfortunately it is not. UIB contains such a canonical set of classes, translating the C++ Firebird 3 API classes into Pascal, getting a pointer to IMaster and taking it from there. This may work if the C++ and Pascal compilers have the same ABI and this may even be the case with some Windows compilers (the UIB code was specific to Windows). However, I tried it out with FPC 2.6.4 on 64-bit Linux and, as expected, it didn't work. The ABIs are not compatible enough. (Note: For C++Builder to work, it has to come with Delphi and Borland C++ compilers with compatible ABI - this is maybe why the UIB code appeared to work on Windows). The current status of calling C++ from FPC is summarised here http://free-pascal-lazarus.989080.n3.nabble.com/Lazarus-How-to-access-C-and-C-libraries-td4036118.html. 4. 'fb_get_master_interface' should be exported as a normal C++ function and not extern 'C'. By exporting it as extern 'C' you lose the protection offered by Name Mangling and which alerts the user to potential ABI incompatibility - and which avoids lots of hard to find "bugs". 5. If a C++ interface is provided by the Firebird Client then the number of binaries available will need to multiply exponentially as different versions of the client library should be provided for each platform/compiler variation available including different versions of the same compiler with a different ABI. 6. The Firebird 3 API is a low level C++ API in the sense that the database data is passed in an unstructured buffer. It begs the question: if a C++ API is offered then why not offer a higher level API at the level of say, Oracle OCCI or Firebird's very own IBPP. Indeed, given that IBPP is effectively the standard C++ interface to Firebird then why not integrate it into the Firebird library? Note: the IBPP maintainers recommend compiling IBPP into each project rather than making it available as a standard library - this might have something to do with ABI compatibility issues. Note: Oracle OCCI is only available/supported on a strictly limited set of platforms. 7. The release notes present the current API as a "legacy API". If this is true then reliable use of new Firebird features is restricted to C++ - which is undesirable. 8. The release notes also criticise COM interfaces for performance reasons. I will not necessary argue with this but do observe that COM is an open, reliable and compiler independent API (for Windows) and there may well be performance costs associated with this. 9. All APIs will have some ABI dependencies and to meet the criteria for "open, reliable and compiler independent" these need to be minimised by keeping to a very known and widely supported calling convention and limited well defined data types. This is what COM does (on Windows). This is what the existing 'C' interface to Firebird does. Firebird 3 needs such an API. Unfortunately, an API based on pointers to C++ classes does not meet that requirement. Tony Whyman MWA ------------------------------------------------------------------------------ Want fast and easy access to all the code in your enterprise? Index and search up to 200,000 lines of code with a free copy of Black Duck® Code Sight™ - the same software that powers the world's largest code search on Ohloh, the Black Duck Open Hub! Try it now. http://p.sf.net/sfu/bds Firebird-Devel mailing list, web interface at https://lists.sourceforge.net/lists/listinfo/firebird-devel
