From: Alex Fenton <alex@...>
Date: 2008-08-22T01:31:19+09:00
Subject: Re: FILE * friom C to ruby with swig

Lyes Amazouz wrote:

> I have written a function which prototype is:
> 
>   FILE * getContent (void  * objetc, char  * code);
> 
> This function a FILE * to a temporary file it fill with user data, and copy
> in "code" (with strcpy) some string
> 
> I warpedd this function with Swig, but when I call it from a Ruby script, I
> have two problems:
> 
> 1- The class of the function output in Ruby is <SWIG::TYPE_p_FILE>,  what is
> this class? I looked for it in the net by without results, Hows can I
> recover the data from my temporary file with this?

<SWIG::TYPE_p_XXX> is what SWIG returns when it doesn't know how to deal 
with the type returned by a C function. It's an opaque and useless 
pointer. You need to deal with FILE* - typically by using a 
%typemap(out) - in some way.

I don't know exactly how to deal with FILE* - there may be a standard 
SWIG typemap to deal with this, or you might search for examples to 
convert it to a Ruby File object.

> 2- when I call this function from a C program, the variable code will
> contain a string value that the function copy to it from an other variable
> in internal. and with a C program, the content of the variable code is
> updated very well. But when I call the wrapped function from a Ruby script,
> the argument code stays empty (empty string "")? How have to give this
> argument in such way that it will be modified?

Passing in a char* buffer which is modified by a function is a very 
common idiom in C, but it's un-rubyish. Again, you'll need a 
%typemap(in) or write a wrapper function. Have a look at the *INOUT 
standard typemaps in SWIG.

Usually the most natural ruby way to deal with this is to use 
%typemap(in,numinputs=0) and return two VALUEs to ruby, the file object 
and a new string.

alex