Why do libgit2 methods using kernel32.dll's GetProcAddress always return 0?

Question

I have the need to manually handle the loading / unloading of the actual git2.dll, instead of using [DllImport("git2")] in C#. I seem to have issues with creating an IntPtr for reference to the address of methods stored in libgit2.

Here're the good bits from my PluginManager class which are supposed to help facilitate manually loading, marshal/delegate (whenever I get this kink fixed), and unloading libraries.

public class PluginManager {
    public const string LIB = "Assets\\Plugins\\git2.dll";

    [DllImport( "kernel32.dll", CharSet = CharSet.Ansi )]
    public static extern IntPtr LoadLibrary( [In, MarshalAs( UnmanagedType.LPStr )] string lib );

    [DllImport( "kernel32.dll", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true )]
    public static extern IntPtr GetProcAddress( [In] IntPtr reference, [In, MarshalAs( UnmanagedType.LPStr )] string method );

    [DllImport( "kernel32.dll" )]
    public static extern bool FreeLibrary( [In] IntPtr reference );
}

Here's where I attempt to use them:

//# Working; always non-zero.
IntPtr reference = PluginManager.LoadLibrary( PluginManager.LIB );

//# Should be valid -- LibGit2Sharp.Core.NativeMethods.cs calls this method, too.
//# Always returns IntPtr.Zero.
IntPtr methodReference = PluginManager.GetProcAddress( reference, "git_repository_open" );

Is the library not exposed to this type of loading? I've tried all kinds of methods present in the LibGit2Sharp native hooks, but they always return zero.

score 0 · Accepted Answer · answered Feb 19 '13 at 17:17

It seems that git2.dll has mangled export names, so git_repository_open is actually _git_repository_open@4. After researching some more on why this is happening, it seems that, according to This Post, and several others that I've run into while looking for a fix, that the functions may not be exported using extern "C" or may not be using a .def file.

I did look through some of the libgit2 code, and I saw extern used on a couple functions, but on others I saw no export keyword at all, but they're still being accessed externally from LigGit2Sharp's NativeMethods.cs class.

While all of that is foreign to me, I did find a very useful post about a tool shipped with Visual Studio that allows you to see all exported items, and their names: Dumpbin. With this, I was able to determine the exported names, and used the /OUT:filename flag to save them to a file: PasteBin

With all of the exported names at my disposal, I just have to follow function calls, that I'm actually using, and replace the the methods in NativeMethods.cs to look something like this: LibGit2Sharp Issue #341.

If you need your dll to be built with `cdecl` instead of `stdcall`, you should build it that way. — Carlos Martín Nieto, Feb 23 '13 at 21:19

Why do libgit2 methods using kernel32.dll's GetProcAddress always return 0?

1 Answers1