How does one create a stacked file interface without having to know about the internals of the ALLEGRO_FILE format?

It seems like there needs to be a function like:

ALLEGRO_FILE *f = al_fopen("blah.txt", "r");
al_set_new_file_interface(foo);
al_fopen_f(f); // calls foo.fi_fopen_f(f)

Now the foo file interface gets passed a reference to the ALLEGRO_FILE. So basically it acts as a filter or wrapper over top of an existing file interface.

The function could return NULL if there is no reasonable implementation.

Some applications:

• file slices - overrides seeking, then passes reads/writes through

• compression - compress data on the fly

• encryption - encrypt data on the fly

Edit: removed second question.

I know. That's not what I'm talking about...

Look at the memfile implementation. It has a NULL fopen entry because it makes no sense. memfile doesn't open a char* filename, it opens a file handle. So it must include the internal Allegro file header to set the vtable by hand. If there was an al_fopen_f function and related hook in the interface, it wouldn't have to do that.

Edit: Although this would also require something like al_set/get_file_data() function that would return a void* pointer to attach custom user data or something similar to accomplish that.

So it would look like:

struct ALLEGRO_FILE {
  ... vtable;
  void *data;
}

static size_t memfile_fwrite(ALLEGRO_FILE *fp, const void *ptr, size_t size)
{
   ALLEGRO_FILE_MEMFILE *mf = al_get_file_data(fp);
...
}

The al_open_memfile function would call al_fopen_f internally.

This is the principle concept: Hide the Allegro internals.