C++/CLI class wrapper for c library - callbacks - callback

Question

I am wrapping a C library using C++/CLI. The C library was designed to be used from an unmanaged C++ class. This means that the library functions accept a C++ object pointer and then provide that pointer back in callbacks. This enables the callback code to redirect requests to an appropriate event function in the calling C++ object.
The actual functions are quite involved, so I have simplified the problem space to just a few basic items:
// C library function signature
void CLibFunc(CLIBCALLBACK *callback, void *caller);
// C callback signature
// Second parameter is meant to point to the calling C++ object
typedef int (__stdcall CLIBCALLBACK) (int param1, void *caller);
// C callback implementation
int CallBackImpl(int param1, void* caller)
{
// Need to call the ManagedCaller's EventFunction from here
// ???
}
// C++/CLI caller class
public ref class ManagedCaller
{
public:
void CallerFunction(void)
{
// Call the C library function
// Need to pass some kind of this class pointer that refers to this object
CLibFunc(CallBackImpl, ????);
}
void EventFunction(param1)
{
}
}
Now the C library functions need to be called from a managed C++ class. Under C++/CLI, the garbage collector moves objects around in memory, so passing a simple fixed pointer to the class does not work anymore. I can solve the problem by pinning the object, but that is not recommended because it leads to memory fragmentation. It seems that another option would be to use auto_gcroot pointers, but I am fairly new to managed C++ an I am not sure how to make this work.
Does anyone know how to make this work? What kind of pointer should be passed to the C function? How should the callback implementation redirect to the calling object's event function?

Answer 1

This just happens to be similar to something I'm in the middle of working on right now.
Here is an blog post on providing native callbacks using C++ classes: http://blogs.microsoft.co.il/blogs/alon/archive/2007/05/29/Native-Callback.aspx
I'm not familiar with calling C++ member functions from C, but I have done an interface (abstract base) class to another C++ class for callbacks (similar to the article). Here is a basic example of what I am providing a bridge for:
// Interface (abstract base) class providing the callback
class IProvider {
public:
virtual ~IProvider() {}
virtual void Callback() = 0;
};
// User class of the callback
class CUser {
IProvider * m_pProvider;
public:
CUser(IProvider * pProvider) {
m_pProvider = pProvider;
}
void DoSomething() {
m_pProvider->Callback();
}
};
// Implementation of the interface class
class CHelloWorldProvider : public IProvider {
void Callback() {
printf("Hello World!");
}
};
// Usage of the callback provider in a pure native setting
void PureNativeUsage() {
CHelloWorldProvider oProvider;
CUser oUser(&oProvider);
oUser.DoSomething();
}
Now in order to make this available for managed implementations of the provider, we have to create a series of classes that provide the bridge.
// Where gcroot is defined
#include <vcclr.h>
// Managed provider interface class
public interface class IManagedProvider {
void Callback();
};
// Native bridge class that can be passed to the user
class CProviderBridge : public IProvider {
// Give the managed class full access
friend ref class ManagedProviderBase;
// Store a reference to the managed object for callback redirects
gcroot<IManagedProvider ^> m_rManaged;
public:
void Callback(){
m_rManaged->Callback();
}
};
// Managed provider base class, this provides a managed base class for extending
public ref class ManagedProviderBase abstract : public IManagedProvider {
// Pointer to the native bridge object
CProviderBridge * m_pNative;
protected:
ManagedProviderBase() {
// Create the native bridge object and set the managed reference
m_pNative = new CProviderBridge();
m_pNative->m_rManaged = this;
}
public:
~ManagedProviderBase() {
delete m_pNative;
}
// Returns a pointer to the native provider object
IProvider * GetProvider() {
return m_pNative;
}
// Makes the deriving class implement the function
virtual void Callback() = 0;
};
// Pure managed provider implementation (this could also be declared in another library and/or in C#/VB.net)
public ref class ManagedHelloWorldProvider : public ManagedProviderBase {
public:
virtual void Callback() override {
Console::Write("Hello World");
}
};
// Usage of the managed provider from the native user
void MixedUsage() {
ManagedHelloWorldProvider ^ rManagedProvider = gcnew ManagedHelloWorldProvider;
CUser oUser(rManagedProvider->GetProvider());
oUser.DoSomething();
}
Edit: Added code to show w/o the managed interface class example I use.
Here is a modified version of my example that can be used given your CLibFunc above. This is assuming how the C function performs the callback is accurate.
Also this might be able to be slimmed down a bit depending on how involved your callback classes are and how much freedom for extension you need.
// Where gcroot is defined
#include <vcclr.h>
// C callback signature
// Second parameter is meant to point to the calling C++ object
typedef int (__stdcall CLIBCALLBACK) (int param1, void *caller);
// C library function
void CLibFunc(CLIBCALLBACK *callback, void *caller) {
// Do some work
(*callback)(1234, caller);
// Do more work
}
// Managed caller interface class
public interface class IManagedCaller {
void EventFunction(int param1);
};
// C++ native bridge struct
struct CCallerBridge {
// Give the managed class full access
friend ref class ManagedCaller;
// Store a reference to the managed object for callback redirects
gcroot<IManagedCaller ^> m_rManaged;
public:
// Cast the caller to the native bridge and call managed event function
// Note: This must be __stdcall to prevent function call stack corruption
static int __stdcall CallBackImpl(int param1, void * caller) {
CCallerBridge * pCaller = (CCallerBridge *) caller;
pCaller->m_rManaged->EventFunction(param1);
return 0;
}
};
// C++/CLI caller class
public ref class ManagedCaller : public IManagedCaller {
// Pointer to the native bridge object
CCallerBridge * m_pNative;
public:
ManagedCaller() {
// Create the native bridge object and set the managed reference
m_pNative = new CCallerBridge();
m_pNative->m_rManaged = this;
}
~ManagedCaller() {
delete m_pNative;
}
// Calls the C library function
void CallerFunction() {
CLibFunc(CCallerBridge::CallBackImpl, m_pNative);
}
// Managed callback function
virtual void EventFunction(int param1) {
Console::WriteLine(param1);
}
};
// Usage
int main(array<System::String ^> ^args) {
ManagedCaller ^ oCaller = gcnew ManagedCaller();
oCaller->CallerFunction();
return 0;
}