Minor nitpick. Or confusion on my part. In the filc_malloc function the call to calloc doesn't seem to allocate enough memory to store an AllocationRecord for each location in visible_bytes. Should it be:

    ar->invisible_bytes = calloc(length, sizeof(AllocationRecord));

Note, I'm not the author of the OP.

I am the author of Fil-C

If you want to see my write-ups of how it works, start here: https://fil-c.org/how