# Module Stdlib.Obj

    module Obj: Obj

    type t

    val repr : 'a -> t

    val obj : t -> 'a

    val magic : 'a -> 'b

    val is_block : t -> bool

    val is_int : t -> bool

    val tag : t -> int

    val size : t -> int

    val reachable_words : t -> int

Computes the total size (in words, including the headers) of all heap blocks accessible from the argument. Statically allocated blocks are excluded.

@Since 4.04

    val field : t -> int -> t

    val set_field : t -> int -> t -> unit

When using flambda:

`set_field` MUST NOT be called on immutable blocks. (Blocks allocated in C stubs, or with `new_block` below, are always considered mutable.)

The same goes for `set_double_field` and `set_tag`. However, for `set_tag`, in the case of immutable blocks where the middle-end optimizers never see code that discriminates on their tag (for example records), the operation should be safe. Such uses are nonetheless discouraged.

For experts only: `set_field` et al can be made safe by first wrapping the block in `Sys.opaque_identity`, so any information about its contents will not be propagated.

    val set_tag : t -> int -> unit

    val double_field : t -> int -> float

    val set_double_field : t -> int -> float -> unit

    val new_block : int -> int -> t

    val dup : t -> t

    val truncate : t -> int -> unit

    val add_offset : t -> Int32.t -> t

    val first_non_constant_constructor_tag : int

    val last_non_constant_constructor_tag : int

    val lazy_tag : int

    val closure_tag : int

    val object_tag : int

    val infix_tag : int

    val forward_tag : int

    val no_scan_tag : int

    val abstract_tag : int

    val string_tag : int

    val double_tag : int

    val double_array_tag : int

    val custom_tag : int

    val final_tag : int

    val int_tag : int

    val out_of_heap_tag : int

    val unaligned_tag : int

    module Extension_constructor: sig .. end

    val extension_constructor : 'a -> extension_constructor

    val extension_name : extension_constructor -> string

    val extension_id : extension_constructor -> int

The following two functions are deprecated. Use module `Marshal` instead.

    val marshal : t -> bytes

    val unmarshal : bytes -> int -> t * int

    module Ephemeron: sig .. end