Access violation when calling C DLL from D

[AnoHito via Digitalmars-d-learn]

Hi, I am trying to write a simple interface to the MRuby Ruby 
interpreter so I can use ruby scripts in my D program. I was able 
to get MRuby compiled as a DLL without too much difficulty, but 
the headers are very long and complicated, and porting them 
entirely to D would be a huge project in and of itself. I am 
trying to get by with only what I need, so here is what I have so 
far:

module script.mruby;

alias mrb_bool = bool;
alias mrb_int = int;
alias mrb_float = double;
alias mrb_sym = uint;

alias mrb_aspec = uint;

struct mrb_value
{

}
struct RObject
{

}

struct RClass
{

}

struct mrb_value
{

}

struct mrb_state
{

}

extern(C) char *mrb_string_value_cstr(mrb_state *mrb, mrb_value 
*ptr);

extern (C) mrb_value mrb_load_string(mrb_state *mrb, const char 
*s);
extern (C) mrb_value mrb_load_nstring(mrb_state *mrb, const char 
*s, int len);

extern (C) mrb_state *mrb_open();
extern (C) void mrb_close(mrb_state *mrb);

In theory, this should be enough to test that MRuby is working, 
so I tried to run the following code:

mrb = mrb_open();
mrb_value result = mrb_load_string(mrb, 
toStringz("String('test')"));
Log.info(to!string(mrb_string_value_cstr(mrb, &result)));

But the result was:

object.Error@(0): Access Violation

I wasn't able to get the Visual D debugger to trace into the 
code, but after some investigation, I figured out that the error 
was occurring in the strlen runtime function. I don't think I did 
anything wrong with the way I passed a string into the 
mrb_load_string function, so I am kind of at a loss as to what 
the problem might be.