On 07/21/14 17:09, Dmitry Yemanov wrote:
> 21.07.2014 16:32, Alex Peshkoff wrote:
>
>> First of all - formal definition of API is certainly not a set of C++
>> classes. Each firebird interface is a single pointer, pointing to the
>> table of virtual functions that are contained in this interface. First
>> parameter of each function is always a pointer to an interface itself.
>>
>> This definition in plain english may be rewritten in C:
>> struct ISample
>> {
>> struct VTable
>> {
>> void functionOne(ISample* this, Type1 par1, Type2 par2, /*
>> other parameters */);
>> void functionTwo(ISample* this, TypeA parA, TypeB parB, /*
>> other parameters */);
>> /* other functions ... */
>> };
>>
>> VTable* vTable;
>> };
> >
>> Such definition of pure virtual interface should work and be correct for
>> any working C compiler.
> >
> > [snip]
> >
>> in C++ we get exactly same binary layout for any known to us C++ compiler.
> Is this really so? This is why I'm asking:
>
> https://gcc.gnu.org/ml/gcc-help/2003-11/msg00257.html
Please pay attention to the year when a question was asked - probably
this is what Jim mentions about vtable position in the end of an object?
Anyway such old versions make no practical interest for us.
> http://stackoverflow.com/questions/5712808/understanding-the-vtable-entries
>
> i.e. GCC has a different vtable layout and moreover it's version-dependent.
Ahh - it's related with multiple inheritance and rtti. Luckily we do not
use both in interfaces.
The test I've written some time ago to check how does interface upgrade
run at different systems does work with C++ classes as with pure virtual
interfaces - this is what makes possible to upgrade interface version.
http://web.firebirdsql.org/download/rabbits/alex/checkAbi.tgz
Run
sh build.sh
./ldr
Expected output is:
C1-f1
C1-f2
int 75
C2-f1
C2-f2
C2-f3
I've rechecked it right now starting with
gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7) // was used to
build FB 1.5,
definitely 32-bit std RH8 layout, and ending with
gcc version 4.8.2 (GCC) // was used to build FB3 Alpha2
64-bit compiler.
Both work as expected.
> As for Free Pascal, they also reserve 12 bytes at the beginning of the
> vtable, hence the compatibility issue.
12 byte? On 64-bit system?
They are crazy!
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