## A.26 library(registry): Manipulating the Windows registry

The `library(registry)` is only available on the MS-Windows version of SWI-Prolog. It loads the foreign extension `plregtry.dll`, providing the predicates described below. This library only makes the most common operations on the registry available through the Prolog user. The underlying DLL provides a more complete coverage of the Windows registry API. Please consult the sources in `pl/src/win32/foreign/plregtry.c` for further details.

In all these predicates, Path refers to a `/' separated path into the registry. This is not an atom containing `/'-characters as used for filenames, but a term using the functor ``/`/2`. Windows defines the following roots for the registry: `classes_root`, `current_user`, `local_machine` and `users`.

registry_get_key(+Path, -Value)
  Get the principal (default) value associated to this key. Fails silently if the key does not exist.
registry_get_key(+Path, +Name, -Value)
  Get a named value associated to this key.
registry_set_key(+Path, +Value)
  Set the principal (default) value of this key. Creates (a path to) the key if it does not already exist.
registry_set_key(+Path, +Name, +Value)
  Associate a named value to this key. Creates (a path to) the key if it does not already exist.
registry_delete_key(+Path)
  Delete the indicated key.
shell_register_file_type(+Ext, +Type, +Name, +OpenAction)
  Register a file-type. Ext is the extension to associate. Type is the type name, often something like `prolog.type`. Name is the name visible in the Windows file-type browser. Finally, OpenAction defines the action to execute when a file with this extension is opened in the Windows explorer.
shell_register_dde(+Type, +Action, +Service, +Topic, +Command, +IfNotRunning)
  Associate DDE actions to a type. Type is the same type as used for the 2nd argument of shell_register_file_type/4, Action is the action to perform, Service and Topic specify the DDE topic to address, and Command is the command to execute on this topic. Finally, IfNotRunning defines the command to execute if the required DDE server is not present.
shell_register_prolog(+Ext)
  Default registration of SWI-Prolog, which is invoked as part of the initialisation process on Windows systems. As the source also includes the above predicates, it is given as an example:

      shell_register_prolog(Ext) :-
              current_prolog_flag(argv, [Me|_]),
              atomic_list_concat(['"', Me, '" "%1"'], OpenCommand),
              shell_register_file_type(
                  Ext, 'prolog.type', 'Prolog Source', OpenCommand),
              shell_register_dde(
                  'prolog.type', consult,
                  prolog, control, 'consult(''%1'')', Me),
              shell_register_dde(
                  'prolog.type', edit,
                  prolog, control, 'edit(''%1'')', Me).