[Chicken-users] A more detailed plea for help with FFI...

[Ed Watkeys]

Hi,

I've been looking through various eggs, trying to find, to no avail, an 
example of the following...

Just say I have a C struct, like this:

struct whatever {
        FILE *fp;
        ...
};

I want to create a procedure visible to Scheme code that allocates the 
struct, opens a file, and places the file pointer in fp. I want Scheme 
to assume control of the object, and call my procedure that closes the 
file associated with fp before Scheme frees the memory. Essentially, I 
want to be able to specify a destructor.

This is something, suprisingly, has been far easier to deal with in 
Gambit than in Chicken. In Gambit (4.0b11), I can do something like 
this:

(c-declare "
        static ___SCMOBJ release_whatever(void *w) {
                fclose(w.fp);
                return __FIX(__NO_ERR);
        }")

(c-define-type
        whatever
        (struct "whatever" whatever "release_whatever"))

(define whatever:make
        (c-lambda (char-string) whatever "
                struct whatever w;
                w.fp = fopen(___arg1, \"r\");
                ___result_voidstar = &w;"))

Now, of course, I'm running into problems with Gambit which have me 
pulling out my hair, so I'm running back to the warm embrace of 
Chicken.

What I do not want to do is require that user Scheme code do resource 
management and remember that it created an object and now needs to 
dispose of it. I consider that Pure Evil. Is there a way to accomplish 
what I'm trying to do in Chicken?

Regards,
Ed



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