An Adventure in Function Pointers, Delegates, and Calling Conventions

March 12, 2008

I’ve been interfacing a third-party library for a project at work. The third party library is a DLL with a C interface. Now most libraries for Windows usually have  a __stdcall calling conventions and for some reason I assumed that would be case with the C library. I guess it was because the demo application had  not done anything to give a hint otherwise. Since the client component was going to be in .net component, I wrote a nice little C++/CLI wrapper around the unmanaged native library, so that I could easily call it from C#. I decided against using P/Invoke because I don’t like using meta-information in source code to identify implementation of interfaces. In my opinion using P/Invoke client code too dependent on a specific library. That’s a topic for another post.

One of the functions of the library needed a function pointer to a callback function. The prototype of the function was something like this, wrapped with linkage.

extern "C"
{
    typedef void (*SomeCallbackType)(int, char*, float);
    void SetCallBackInNativeFunction(SomeCallbackType p){/*...*/}
}

Since I wanted the callback mechanism to hook up to .Net delegates I did something as follows,

using namespace System;
using namespace System::Runtime::InteropServices;

delegate void FooDelegate(int arg1, char* arg2, float arg3);

ref class AClass
{
    ///...
public:
    void FooCallback(int arg1, char *arg2, float arg3){/*...*/}
    ///..
};


static GCHandle globalGCHandle;

void SetCallback()
{
    AClass ^theObj = gcnew AClass();
    FooDelegate ^del = gcnew FooDelegate(theObj, &AClass::FooCallback);
    //We want to do this because the callback will be called many times. 
    //and may live through many garbage collection sprees.
    //This would be freed at the end of the program. 
    globalGCHandle = GCHandle::Alloc(del); 

    IntPtr ptr2Func = Marshal::GetFunctionPointerForDelegate(del);

    SetCallBackInNativeFunction((SomeCallbackType)ptr2Func.ToPointer());
}

 

Now every time time I ran my program, it would start up normally, but then mysteriously crash with really exceptions like 0xC00005, 0xC0000013, etc. I recognized these exceptions that indicated access violation and stack corruption exceptions. I turned on exception handlers in the debugger but the debugger refused to catch those exceptions. I was mystified. I thought that the exceptions were happening because the garbage collector was moving around my delegate, which is why  I used the globalGCHandle = GCHandle::Alloc(del). That didn’t help. Turned out the program was crashing because Marshal::GetFunctionPointerForDelegate returns a function pointer with __stdcall calling convention. Now as everyone knows __stdcall and __cdecl (the default in C compilers) are incompatible, and it causes really confusing errors because the somehow or the other the stack always ends up corrupted. The compiler (at least VC compiler) would have helped had I been passing a native function pointer. With managed code, the compiler happily let me shoot myself in the foot. The solution was to add the following attribute to delegate declaration

[UnmanagedFunctionPointer(CallingConvention::Cdecl)]
delegate void FooDelegate(int arg1, char* arg2, float arg3);

which I got from here

The program worked flawlessly after that (I mean it did what I wanted it to do).

 


IronPython and/or F#

March 11, 2008

I’ve been working with a large project that has a lot of unmanaged MFC and C++ code, and new managed code using C#, C++/CLI with a many third-party libraries written in unmanaged code. Because of the complexity of the code, and sheer number of projects (>50 at the last count), I’ve been debating on using some scripting language to prototype and test code. I know Python, so I’ve started off with that. Unfortunately, Visual Studio 2005 does not have a good integration with the IronPython console. I tried getting the add-in sample to integrate with Visual Studio, but after wasting a few hours decided to use it without intellisense.

F# on the other hand has pretty good integration wit a REPL console, compiler and interpreter. Although, I know some functional programming,  (doing some programming in Lisp, Haskell, and Boost.(Lambda|Bind|Spirit) in C++), F# seems to have an alien syntax. I’m pretty sure that after a few hours, I should get the hang of it. Probably, next weekend. Although its looking more and more likely that I’ll be at work this weekend again.

I do like this T-shirt though.

image

Brought to you via here.

 

sidenote:

I wish plain old C enums played nicely with CLS enums, or that I was smart enough to remember the nuances of how they differ.


2008 Tipperary Hill Shamrock Run « Race Time Results

March 10, 2008

 2008 Tipperary Hill Shamrock Run « Bit By Bit, Byte by Byte, Word by Word

Gun Place 588
Gun Time 37:59
Gun Pace 9:30
Net Place 593
Net Time 37:18
Net Pace 9:20

2008 Tipperary Hill Shamrock Run

March 7, 2008

Running this race tomorrow morning. Its pretty nasty outside today. In fact there is a winter snow storm warning till Sunday evening. Wish me luck.

2008 Tipperary Hill Shamrock Run


Need a crash course in probability

March 1, 2008

… (again)