How to make C (P/invoke) code called from C# “Thread-safe”

Question

I have some simple C-code which uses a single global-variable. Obviously this is not thread-safe, so when I call it from multiple threads in C# using P/invoke, things screw up.

Answer 1

Personally if the C code was to be called elsewhere I would use a mutex there. If that doesn't float your boat you can lock in .Net quite easily:

static object SomeFunctionLock = new Object();

public static int SomeFunction(int parameter1, int parameter2){
  lock ( SomeFunctionLock ){
    return _SomeFunction( parameter1, parameter2 );
  }
}

[DllImport("MyDll", CallingConvention = CallingConvention.Cdecl)]
internal static extern int _SomeFunction(int parameter1, int parameter2);

[Edit..]

As pointed out, this serializes access to the function which you can't do yourself in this case. You have some C/C++ code that (wrongly IMO) uses a global for state during the call to the exposed function.

As you have observed that the __declspec(thread) trick doesn't work here then I would try to pass your state/context back and forth as an opaque pointer like so:-

extern "C" 
{
    int _SomeOtherFunction( void* pctx, int p1, int p2 )
    { 
        return stuff;
    }

    // publically exposed library function
    int __declspec(dllexport) SomeFunction(int parameter1, int parameter2)
    {
        StateContext ctx;
        return _SomeOtherFunction( &ctx, parameter1, parameter2 );
    }

    // another publically exposed library function that takes state
    int __declspec(dllexport) SomeFunctionWithState(StateContext * ctx, int parameter1, int parameter2)
    {
        return _SomeOtherFunction( ctx, parameter1, parameter2 );
    }

    // if you wanted to create/preserve/use the state directly
    StateContext * __declspec(dllexport) GetState(void) {
        ctx = (StateContext*) calloc( 1 , sizeof(StateContext) );
        return ctx;
    }

    // tidy up
    void __declspec(dllexport) FreeState(StateContext * ctx) {
        free (ctx);
    }
}

And the corresponding C# wrapper as before:

[DllImport("MyDll", CallingConvention = CallingConvention.Cdecl)]
internal static extern int SomeFunction(int parameter1, int parameter2);

[DllImport("MyDll", CallingConvention = CallingConvention.Cdecl)]
internal static extern int SomeFunctionWithState(IntPtr ctx, int parameter1, int parameter2);

[DllImport("MyDll", CallingConvention = CallingConvention.Cdecl)]
internal static extern IntPtr GetState();

[DllImport("MyDll", CallingConvention = CallingConvention.Cdecl)]
internal static extern void FreeState(IntPtr);