Chapter 26 | The dynlink library: dynamic loading and linking of object files

The `dynlink` library supports type-safe dynamic loading and linking of bytecode object files (`.cmo` and `.cma` files) in a running bytecode program. Type safety is ensured by limiting the set of modules from the running program that the loaded object file can access, and checking that the running program and the loaded object file have been compiled against the same interfaces for these modules.
Programs that use the `dynlink` library simply need to link `dynlink.cma` with their object files and other libraries. Dynamic linking is available only to bytecode programs compiled with `ocamlc`, not to native-code programs compiled with `ocamlopt`.

26.1 | Module `Dynlink`: dynamic loading of bytecode object files

    val init : unit -> unit

> Initialize the library. Must be called before `loadfile`.

    val loadfile : string -> unit

> Load the given bytecode object file and link it. All toplevel expressions in the loaded compilation unit are evaluated. No facilities are provided to access value names defined by the unit. Therefore, the unit must register itself its entry points with the main program, e.g. by modifying tables of functions.

    val loadfile_private : string -> unit

> Same as `loadfile`, except that the module loaded is not made available to other modules dynamically loaded afterwards.

    val add_interfaces : string list -> string list -> unit

> `add_interfaces units path` grants dynamically-linked object files access to the compilation units named in list `units`. The interfaces (`.cmi` files) for these units are searched in `path` (a list of directory names). Initially, dynamically-linked object files do not have access to any of the compilation units composing the running program, not even the standard library. `add_interfaces` or `add_available_units` (see below) must be called to grant access to some of the units.

    val add_available_units : (string * Digest.t) list -> unit

> Same as `add_interfaces`, but instead of searching `.cmi` files to find the unit interfaces, uses the interface digests given for each unit. This way, the `.cmi` interface files need not be available at run-time. The digests can be extracted from `.cmi` files using the `extract_crc` program installed in the Objective Caml standard library directory.

    val clear_available_units : unit -> unit

> Clear the list of compilation units accessible to dynamically-linked programs.

    val allow_unsafe_modules : bool -> unit

> Govern whether unsafe object files are allowed to be dynamically linked. A compilation unit is ``unsafe'' if it contains declarations of external functions, which can break type safety. By default, dynamic linking of unsafe object files is not allowed.

    type linking_error =
        Undefined_global of string
      | Unavailable_primitive of string
      | Uninitialized_global of string
    type error =
        Not_a_bytecode_file of string
      | Inconsistent_import of string
      | Unavailable_unit of string
      | Unsafe_file
      | Linking_error of string * linking_error
      | Corrupted_interface of string
      | File_not_found of string
      | Cannot_open_dll of string
    exception Error of error

> Errors in dynamic linking are reported by raising the `Error` exception with a description of the error.

    val error_message: error -> string

> Convert an error description to a printable message.