C++ and void __cxa_pure_virtual()

[bluecode]

hi,

I'm going to write my kernel in C++.
For now I'm testing which functions I need in order to support the basic C++ functionality (no exception handling, but perhaps rtti). Today I came across the function __cxa_pure_virtual() which gcc needs if have at least one pure virtual method in a class. But I can't figure out, when it is actually called. I wrote two small classes for testing purpose with one pure virtual function, but __cxa_pure_virtual() doesn't get called at all. Is it true that it's only called if the pure virtual function call fails? I'm not really sure. The only implementation I've came across is http://cvs.blackfin.uclinux.org/cgi-bin ... sroot=gcc3. Is this implementation complete?
thx

cu bluecode

[AR]

IIRC, __cxa_pure_virtual() is a crash function, it is only meant to be called when the linker made a broken binary, in user apps it prints to stderr and aborts, in the kernel you'll just want to print an error and panic.

(The actual mechanism is if a pure virtual function is called without being overridden in the class that inherited it)

[bluecode]

thanks for the very fast reply!
But how can a class that did not override the pure virtual function actually be instantiated? Is this possible?

[AR]

no it shouldn't be possible, you might be able to hack it with multiple header definitions of the same class though.

[fraserjgordon]

Just wondering, do you know of a comprehensive reference of what functions G++ requires for each C++ feature (pure virtual, multiple inheritance, RTTI, exceptions, etc)? I was thinking about adding some of these to my kernel, but I don't know what G++ needs.

Thanks in advance,
Fraser

[AR]

You can use: /usr/src/gcc-4.0.1/libstdc++-v3/libsupc++
The header files declare most of this stuff

[vurt]

thanks for the very fast reply!
But how can a class that did not override the pure virtual function actually be instantiated? Is this possible?

Here's one way: You have a parent class with pure virtual functions and a child class with the implementations. If during construction of a child object, the constructor takes .. oh.. for example ... 10 minutes to complete construction (maybe you have a sleep(600) in there).. and another thread uses the child object which is busy constructing, then the pure virtual methods on the parent object will actually be called.

The child object is actually an instantiated parent object until it's finished constructing - with pure virtual methods and everything else. Only once it's constructor returns, is it marked as an instantiated child class and only then can the implemented methods be used.

[nick8325]

Just wondering, do you know of a comprehensive reference of what functions G++ requires for each C++ feature (pure virtual, multiple inheritance, RTTI, exceptions, etc)? I was thinking about adding some of these to my kernel, but I don't know what G++ needs.

http://www.codesourcery.com/cxx-abi/abi.html is the official IA-64 ABI. Apparently G++ follows that.

[nick8325]

http://www.codesourcery.com/cxx-abi/abi.html is the official IA-64 ABI. Apparently G++ follows that.

I forgot to mention: much of that is done by the compiler. Searching for __cxa on that page should tell you most of the functions the compiler might need (though I've never seen most of them before...some of them are only called in quite weird circumstances).