Re: [Caml-list] The OCaml garbage collector, finalisers, and the right way of disposing native pointers in C bindings

["Richard W.M. Jones" <rich@annexia.org>]

The other replies have covered some of the problems.  You may also be
interested in example code, and we've got lots :-)  Most of the bugs
have now been fixed, after several iterations.  Here are some things
to get you started.

(1) Simple example of a finalizer attached to a custom block:

https://github.com/libguestfs/libguestfs/blob/master/mllib/progress-c.c
https://github.com/libguestfs/libguestfs/blob/master/mllib/progress.ml
https://github.com/libguestfs/libguestfs/blob/master/mllib/progress.mli

(2) A more complex example using generational roots to deal with
callbacks from OCaml back to C:

https://github.com/libguestfs/libguestfs/blob/master/ocaml/guestfs-c.c

This one had a major bug, when we discovered that the root caused the
handle to be always reachable, so the finalizer was not called until
the program exited (the fact that we also have a #close method, which
we always called, made this less obvious than you might think at
first).  That was fixed in:

https://github.com/libguestfs/libguestfs/commit/8bbc5e73cb5b56b5cfbe979ac0e1c14d1701a0d8

(3) A tricky binding to libxml2.

Because libxml2 has objects containing pointers to other objects (at
the C level) we need to shadow these with OCaml structs, to ensure
that an OCaml object doesn't become unreachable when it is still
pointed to from another object.

https://github.com/libguestfs/libguestfs/blob/master/v2v/xml-c.c
https://github.com/libguestfs/libguestfs/blob/master/v2v/xml.ml
https://github.com/libguestfs/libguestfs/blob/master/v2v/xml.mli

If you look at the history of these files, you'll see we discovered
and fixed major bugs, eg this one concerned with the order in which we
freed objects:

https://github.com/libguestfs/libguestfs/commit/3888582da89c757d0740d11c3a62433d748c85aa

Note that (3) is a counter-example to the idea that you should use
custom blocks.  Custom block finalizers in OCaml have no ordering
guarantee - if you need an ordering guarantee you must use an OCaml
finalizer.

You can probably see this stuff gets complicated quickly.  Thank
goodness for valgrind!

Rich.