# Module Dynlink

    module Dynlink: sig .. end

Dynamic loading of bytecode object files.

###### Initialization

    val init : unit -> unit

Initialize the `Dynlink` library. Must be called before any other function in this module.

###### Dynamic loading of compiled bytecode files

    val loadfile : string -> unit

Load the given bytecode object file (`.cmo` file) or bytecode library file (`.cma` file), and link it with the running program. All toplevel expressions in the loaded compilation units 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 compilation units just loaded are hidden (cannot be referenced) from other modules dynamically loaded afterwards.

###### Access control

    val allow_only : string list -> unit

`allow_only units` restricts the compilation units that dynamically-linked units can reference: it only allows references to the units named in list `units`. References to any other compilation unit will cause a `Unavailable_unit` error during `loadfile` or `loadfile_private`.

Initially (just after calling `init`), all compilation units composing the program currently running are available for reference from dynamically-linked units. `allow_only` can be used to grant access to some of them only, e.g. to the units that compose the API for dynamically-linked code, and prevent access to all other units, e.g. private, internal modules of the running program.

    val prohibit : string list -> unit

`prohibit units` prohibits dynamically-linked units from referencing the units named in list `units`. This can be used to prevent access to selected units, e.g. private, internal modules of the running program.

    val default_available_units : unit -> unit

Reset the set of units that can be referenced from dynamically-linked code to its default value, that is, all units composing the currently running program.

    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.

###### Deprecated, low-level API for access control

    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).

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

Same as `Dynlink.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

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

###### Error reporting

`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.