firefox addon - Build a DLL to be used by Mozilla js-ctypes

Question

In reference to my first post: Mozilla use a C DLL with js-ctypes

I'm trying to build a DLL to be used from a Mozilla Firefox extension. I created a little C code and compiled it with GCC.

Here is the C code :

#include<stdio.h>
int add(int a,int b)
{
    return(a+b);
}

Here are the compilation lines:

gcc -c library.c
gcc -shared -o library.dll library.o -Wl

The DLL is well compiled, I can open it with dllexp and can see the add() method exposed.

The problem is, when I try to use it from my extension, I always get the message: Error: couldn't open library

Here is my Javascript call:

var libc = ctypes.open("C:\WINDOWS\system32\user32.dll"); //OK
var libc2 = ctypes.open("C:\WINDOWS\system32\library.dll"); //KO

It seems the DLL cannot be opened by Firefox, but I wonder why. I don't see anything about building DLL for Firefox extension, it seems we can use every classic DLL library.

Any idea? Thanks

Answer 1

If you compile the library like that you get a dependency on msvcrt.dll which probably cannot be resolved on your system (redistributable package required), on mine it works fine. You can compile your library without the dependency on the CRT, you just have to define DllMain yourself:

#include<windows.h>

BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
  return TRUE;
}

int add(int a,int b)
{
    return(a+b);
}

And the link step looks like this:

gcc -shared -nostdlib -o library.dll library.o -Wl,-e_DllMain@12

You cannot use CRT functionality then - I couldn't find a way to compile the runtime statically with GCC on Windows (Visual C++ on the other hand does it just fine).