Re: [Boost-users] Boost Regex in a DLL

From: Bronek Kozicki (brok_at_[hidden])
Date: 2007-04-20 14:05:28

Metasebia Moges wrote:
> Like I said in an earlier e-mail, the code is isolated. However, to make 
> things sure I added it as:
> #define BOOST_REGEX_DYN_LINK in the source code and there is no change.

wrong.

> Then I defined it in Properties->Preprocessor Definitions. Then it 

correct, this is how macros like this should be defined. BTW, I see you 
are using Visual C++ - it is relevant for the rest of this email. If I'm 
mistaken, just ignore it.

> complains as:
> "Mixing a Boost dll with a static runtime is a really bad idea ...". I 
> don't know why?

because it really is bad idea. I will explain why: C++ runtime needs 
access to the heap in order to be able to create objects (like strings). 
On Windows each process owns multiple heaps and the runtime holds handle 
to one. If you use static runtime, this handle is linked into executable 
(it's a static variable). On Windows you have two kinds of executables: 
DLL and EXE. Each copy of static C++ runtime that is statically linked 
into an executable will have own heap handle, thus heaps (of EXE and all 
DLLs linked with static runtime, running inside the process) will be 
separate. Now imagine what happens if function inside a DLL creates a 
string (be it 'std::string("foo bar")' or simply 'new char[20]') and 
returns it to EXE that called it - string would be created in the 
context of C++ runtime statically linked with the DLL (one heap) and 
then passed (and some time later destroyed) in the context of another 
C++ runtime, the one statically linked into the EXE (other heap). The 
trouble is that you cannot allocate memory on one heap and then free it 
on another, because they know nothing of one another - the attempt will 
likely simply corrupt the heap of the other runtime (thus crashing your 
process some time later). This is why whenever you want to use DLL and 
EXE and pass C++ objects of or pointers between them, it is crucial that 
they all (DLLs and EXE) use one copy of the C++ runtime - that is 
dynamically linked one (option /MD for release build and /MDd for debug).

I hope this answers your questions

B.