Module Std.Gc

module Gc: Core_gc

include Gc

val add_finalizer : 'a Core_kernel.Std.Heap_block.t ->
       ('a Core_kernel.Std.Heap_block.t -> unit) -> unit

add_finalizer b f ensures that f runs after b becomes unreachable. The OCaml runtime only supports finalizers on heap blocks, hence add_finalizer requires b : _ Heap_block.t. The runtime essentially maintains a set of finalizer pairs:

'a Heap_block.t * ('a Heap_block.t -> unit)

Each call to add_finalizer adds a new pair to the set. It is allowed for many pairs to have the same heap block, the same function, or both. Each pair is a distinct element of the set.

After a garbage collection determines that a heap block b is unreachable, it removes from the set of finalizers all finalizer pairs (b, f) whose block is b, and then and runs f b for all such pairs. Thus, a finalizer registered with add_finalizer will run at most once.

The GC will call the finalisation functions in the order of deallocation. When several values become unreachable at the same time (i.e. during the same GC cycle), the finalisation functions will be called in the reverse order of the corresponding calls to add_finalizer. If add_finalizer is called in the same order as the values are allocated, that means each value is finalised before the values it depends upon. Of course, this becomes false if additional dependencies are introduced by assignments.

In a finalizer pair (b, f), it is a mistake for the closure of f to reference (directly or indirectly) b -- f should only access b via its argument. Referring to b in any other way will cause b to be kept alive forever, since f itself is a root of garbage collection, and can itself only be collected after the pair (b, f) is removed from the set of finalizers.

The f function can use all features of O'Caml, including assignments that make the value reachable again. It can also loop forever (in this case, the other finalisation functions will be called during the execution of f). It can call add_finalizer on v or other values to register other functions or even itself. It can raise an exception; in this case the exception will interrupt whatever the program was doing when the function was called.

add_finalizer_exn b f is like add_finalizer, but will raise if b is not a heap block.

val add_finalizer_exn : 'a -> ('a -> unit) -> unit

val finalise_release : unit -> unit

A finalisation function may call finalise_release to tell the GC that it can launch the next finalisation function without waiting for the current one to return.