package offheap

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Copies OCaml objects out of the OCaml heap


Dune Dependency






A copy of OCaml objects is moved to memory managed by malloc and free, out of the reach of the GC. If the objects are very large, performance can be improved as the traversal of large live objects does not slow the GC down.

Published: 21 Sep 2018



offheap moves copies of OCaml values off the heap into memory managed by malloc and free, allowing them to be manually managed and reclaimed. Moving large objects off the heap can potentially improve performance as they are not reachable by the garbage collector, avoiding expensive traversals.


  • val copy : ?alloc:alloc -> 'a -> 'a t

Creates a copy of an object, returning a handle to it. The handle is required to free the object. The old value is still usable.

  • val get : 'a t -> 'a

Returns a reference to the copied object from the handle.

  • val free : 'a t -> unit

Frees the off-heap memory. All references to the copied object are invalid from this point onwards.

Custom allocators

By default, a malloc-based allocator is used. Custom allocators can be provided, however they must be implemented in C: copying cannot call back into OCaml code. Callbacks could potentially access the copied object or trigger a heap sweep which would fail since at the point of the call the headers of blocks in the old object are left in an inconsistent state. The custom allocator must implement the following two functions:

  • void *malloc(value allocator, size_t size)

  • void free(value allocator)

They must be stored in the first two fields of an Abstract_tag block. Additional data may be stored in other fields of the same block, which is passed to the allocator functions.


The object to be copied cannot reference non-Ocaml values (Custom_tag or Abstract_tag).

The builtin polymorphic comparison and hash functions do not work with off-heap objects anymore since they do not traverse off-heap objects.


The implementation was inspired by the iterative traversal implemented in the Obj module of the OCaml runtime. This library was inspired by the existing ancient library, which is unable to handle large objects because of recursive traversals.


The code is published under the MIT License.

Dependencies (2)

  1. dune
  2. ocaml >= "4.07" & < "5.0.0"

Dev Dependencies


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