Swig and Lua: how to map Lua file to FILE*

Question

I have a C function that takes FILE* as an argument and I'd like to use this function in Lua, passing Lua file. I guess I need a %typemap for this. How to write it? (I just started learning Lua).

Answer 1

You're using SWIG to generate Lua bindings for your C code? Why not use the Lua C API directly, or if you can use C++, Luabind? I think either of those would be better than trying to make it work with SWIG, unless you've got a strong attachment to SWIG already.

Answer 2

There's no easy way to do what you're asking.

The Lua File class interface abstracts the underlying implementation. You cannot simply typemap it. You can, however, create a C proxy that wraps FILE operations you need and create an instance of this proxy in Lua using SWIG. You can then generate a typemap to convert a FILE* to a wrapper proxy instance.

Something like:

class MyFileProxy {
    private:
        FILE* fp;
    public:
        MyFileProxy(FILE* fp);
        MyFileProxy(const char* path); 

        FILE* GetFilePointer();

        Seek(...

In SWIG the binding is simply:

%module "MyFile"

%{
#include "MyFileProxy.h"
%}

// Tell SWIG how to use a proxy for functions that take a FILE*
%typemap(in) FILE*
{
    void* tmp = 0;
    SWIG_ConvertPtr(L,$argnum,(void**)&tmp,$1_descriptor,1);

    if (tmp)
    {
        MyFileProxy* proxy = (MyFileProxy)tmp;
        arg$argnum = proxy->GetFilePointer();
    }
}

// Tell SWIG how to create a proxy when returning FILE*
%typemap(out) FILE*
{
    MyFileProxy* pResult = new MyFileProxy($arg);
    SWIG_NewPointerObj(L, pResult, $1_descriptor, 1);
}

%include "MyFileProxy.h 

}

You won't be able to use io:File directly, however.

Answer 3

Here is the solution that I finally came up with.

In Lua source, in liolib.c, there is a function FILE *tofile (lua_State *L), which converts Lua file to C FILE*, but it's not a part of the API. I modified it a bit to make a typemap:

%typemap(in) FILE * {
    FILE **f;
    if (lua_isnil(L, $input))
        $1=NULL;
    else {
        f = (FILE **)luaL_checkudata(L, $input, "FILE*");
        if (*f == NULL)
            luaL_error(L, "attempt to use a closed file");
        $1=*f;
    }
}

This typemap accepts also nil, because I needed a way to pass NULL to the C function.