Memory allocation and deallocation across dll boundaries

Question

I understand that memory allocations made in one dll then subsequently free\'d in another can cause all sort of problems, especially regarding the CRT. These sorts of probl

Answer 1

At the moment we're working on a dll which exposes C++ functionality via a C interface (for the sake of C# being capable of using the said dll).

for instance : the dll has a struct myStruct_s the interface exposes the following functions :

interface.h

#ifndef INTERFACE_TYPES_H
# error Please include interace_types.h
#endif
    myStruct_s * CreateTheStruct() { return new myStruct_s(); }
    void DestroyTheStruct(myStruct_s * the_struct) { delete the_struct; }
    void DoSomethingToTheStruct(myStruct_s * the_struct);

interface_types.h

#define INTERFACE_TYPES_H
struct myStruct_s; // fwd declaration
#endif

interface.cpp

#if defined(__CPPPLUS) || defined(__cplusplus) || defined (__CPLUSPLUS)
#include<TheRealFileContainingTheRealMyStruct_s.h>
// handle the .h's functions here
#endif

comeOutsideCppFile.cpp

#include "interface_types.h"
#include "interface.h"

void main()
{
    myStuct_s * x = CreateTheStruct;
    DoSomethingToTheStruct(x);
    DestroyTheStruct(x);
}

The above is a rough outline of how our stuff works, basically : Whatever the dll exposes needs to be : Created, Handled, Destroyed dll-side

This code is not 100% accurate!!!

Also, please keep in mind that if you're using a pure C++ dll, you probably need the same compiler with the same settings as the one used to build the dll.

Answer 2

You might try looking to see if there are any formal C++ rules for what happens when an exception is thrown in one DLL and caught in another and then goes out of scope -- it seems very similar. For exceptions, I think you are required to provide a copy constructor with a special signature, though I'm unsure now exactly what it is.

Answer 3

Ironically enough, the Adobe Source Libraries has a adobe::capture_allocator class that was written specifically with this kind of DLL safety in mind. The way it works is to capture the local new and delete at this point it is instantiated, and to carry them both around for the lifetime of the object. (See adobe::new_delete_t for details on how it does this, especially the implementation here.) Deallocations take place with the captured delete routine, guaranteeing that no matter where you are you are deleting with the proper delete.

You can see capture_allocator used throughout the version_1 types in the Adobe Source Libraries, such as adobe::any_regular_t and adobe::copy_on_write. capture_allocator should be compatible with all STL container types as well.

Update: capture_allocator is not standard-compliant because it retains state. This should not be a big hindrance to its usability, but it does mean its use is not guaranteed to work with standard-compliant containers.