file_search_path(demo, '/usr/lib/prolog/demo').

the file specification demo(myfile) will be expanded to /usr/lib/prolog/demo/myfile. The second argument of file_search_path/2 may be another alias.

Below is the initial definition of the file search path. This path implies swi(<Path>) and refers to a file in the SWI-Prolog home directory. The alias foreign(<Path>) is intended for storing shared libraries (.so or .DLL files). See also use_foreign_library/1.

user:file_search_path(library, X) :-
    library_directory(X).
user:file_search_path(swi, Home) :-
    current_prolog_flag(home, Home).
user:file_search_path(foreign, swi(ArchLib)) :-
    current_prolog_flag(arch, Arch),
    atom_concat('lib/', Arch, ArchLib).
user:file_search_path(foreign, swi(lib)).
user:file_search_path(path, Dir) :-
    getenv('PATH', Path),
    (   current_prolog_flag(windows, true)
    ->  atomic_list_concat(Dirs, (;), Path)
    ;   atomic_list_concat(Dirs, :, Path)
    ),
    member(Dir, Dirs).
user:file_search_path(user_app_data, Dir) :-
    '$xdg_prolog_directory'(data, Dir).
user:file_search_path(common_app_data, Dir) :-
    '$xdg_prolog_directory'(common_data, Dir).
user:file_search_path(user_app_config, Dir) :-
    '$xdg_prolog_directory'(config, Dir).
user:file_search_path(common_app_config, Dir) :-
    '$xdg_prolog_directory'(common_config, Dir).
user:file_search_path(app_data, user_app_data('.')).
user:file_search_path(app_data, common_app_data('.')).
user:file_search_path(app_config, user_app_config('.')).
user:file_search_path(app_config, common_app_config('.')).

The '$xdg_prolog_directory'/2 uses either the XDG Base Directory or win_folder/2 on Windows. On Windows, user config is mapped to roaming appdata (CSIDL_APPDATA), user data to the non-roaming (CSIDL_LOCAL_APPDATA) and common data to (CSIDL_COMMON_APPDATA).

The file_search_path/2 expansion is used by all loading predicates as well as by absolute_file_name/[2,3].

The Prolog flag verbose_file_search can be set to true to help debugging Prolog's search for files.