[cffi-devel] Mutable Pointers

[Stuart Sierra]

Hello,

I'm working on a CFFI interface to the Perl API (I thought
it would be fun - ha!) and I need to be able to do C-style
pointer arithmetic.  That is, I have a foreign pointer
declared with DEFCVAR and I need to alter the address stored
there.  I couldn't see a way to do this with CFFI, so I
created a new foreign type called :ADDRESS that would treat
the foreign pointer as an integer value that I could modify.

Here's my code:

  (defvar *pointer-size*
    (foreign-type-size :pointer)
    "The size of a pointer on the current platform, in bytes.")

  (ecase *pointer-size*
    (1 (defctype :address :uint8))   ; unlikely
    (2 (defctype :address :uint16))  ; possible
    (4 (defctype :address :uint32))  ; most common
    (8 (defctype :address :uint64))) ; possible

  (defmacro address-incf (address &optional (n 1))
    "Increment ADDRESS by N pointers, like ++i on a C pointer."
    `(incf ,address (* ,n *pointer-size*)))

  (defmacro address-decf (address &optional (n 1))
    "Decrement ADDRESS by N pointers, like --i on a C pointer."
    `(decf ,address (* ,n *pointer-size*)))

  (defmacro address-ref (address type)
    "Dereference ADDRESS to get the TYPE it points to, like MEM-REF."
    `(mem-ref (make-pointer ,address) ,type))

This does work for my purposes, but is there a better way
using CFFI internals?

Thanks,
-Stuart