Question

I’m new to Windows programming and I’ve just “lost” two hours hunting a bug which everyone seems aware of: you cannot create an object on the heap in a DLL and destroy it in another DLL (or in the main program).

I’m almost sure that on Linux/Unix this is NOT the case (if it is, please say it, but I’m pretty sure I did that thousands of times without problems…).

At this point I have a couple of questions:

1) Do statically linked DLLs use a different heap than the main program?

2) Is the statically linked DLL mapped in the same process space of the main program? (I’m quite sure the answer here is a big YES otherwise it wouldn’t make sense passing pointers from a function in the main program to a function in a DLL).

I’m talking about plain/regular DLL, not COM/ATL services

EDIT: By “statically linked” I mean that I don’t use LoadLibrary to load the DLL but I link with the stub library

Answer 1

DLLs / exes will need to link to an implementation of C run time libraries.

In case of C Windows Runtime libraries, you have the option to specify, if you wish to link to the following:

1. Single-threaded C Run time library (Support for single threaded libraries have been discontinued now)
2. Multi-threaded DLL / Multi-threaded Debug DLL
3. Static Run time libraries.
4. Few More (You can check the link)

Each one of them will be referring to a different heap, so you are not allowed pass address obtained from heap of one runtime library to other.

Now, it depends on which C run time library the DLL which you are talking about has been linked to. Suppose let’s say, the DLL which you are using has been linked to static C run time library and your application code (containing the main function) has linked to multi-threaded C Runtime DLL, then if you pass a pointer to memory allocated in the DLL to your main program and try to free it there or vice-versa, it can lead to undefined behaviour. So, the basic root cause are the C runtime libraries. Please choose them carefully.

Please find more info on the C run time libraries supported here & here

A quote from MSDN:

Caution Do not mix static and dynamic versions of the run-time libraries. Having more than one copy of the run-time libraries in a process can cause problems, because static data in one copy is not shared with the other copy. The linker prevents you from linking with both static and dynamic versions within one .exe file, but you can still end up with two (or more) copies of the run-time libraries. For example, a dynamic-link library linked with the static (non-DLL) versions of the run-time libraries can cause problems when used with an .exe file that was linked with the dynamic (DLL) version of the run-time libraries. (You should also avoid mixing the debug and non-debug versions of the libraries in one process.)