GHCi and C++.

Sven Panne Sven.Panne at
Fri Feb 25 16:50:36 EST 2005

Wolfgang Thaller wrote:
> [...] *) static constructors/destructors
> Sometimes there are pieces of code that expect to be called before 
> main() or at program termination. The GHCi Linker doesn't support this, 
> so some code may crash because things haven't been initialised properly.

This will definitely be a source of trouble if you use any C++ library whatsoever.
I don't know the story for Linux, but at least one other platform I know has the
addresses of those constructors/destructors in special ELF sections. GHCi could
easily iterate through these, but this is really highly platform-specific. There
used to be a description of a widespread C++ ABI on, but I can't
find it anymore... :-(

> *) multiple definitions
> There is often some code in C++ header files (e.g. templates, inline 
> functions, class members declared in the class declaration). These 
> pieces of code are sometimes inlined, but sometimes they are not; in 
> that case, gcc generates code for them in every .o file that uses them 
> and counts on the linker to just include one version in the final 
> executable. The GHCi linker will probably just abort and complain about 
> multiply-defined symbols.

IIRC, g++ uses weak symbols for this, at least on some platforms. Handling these
doesn't sound too hard (easy sentence when one doesn't intend to implement it :-).

> The above is just theory, there might be even more problems in practice 
> :-( .

Exception handling comes to my mind. Nowadays it's often done via stack unwinding,
but I don't know about the details anymore. This might cause some trouble when
C++ code is called from Haskell.

The only golden rule I know about this all: Use only a single C++ compiler for the
compilation *and* linking of all your libraries and programs, otherwise be prepared
for long hours in front of gdb... :-(


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