====== Interface declarations ======
  * Interface declarations define interface objects of a precisely defined type.
  * Interface objects are interface constants, interface signals, interface variables and interface files.

===== interface_list =====
''interface_element { ; interface_element }''

===== interface_element =====
  * interface_declaration

===== interface_declaration =====
  * interface_constant_declaration
  * interface_signal_declaration
  * interface_variable_declaration
  * interface_file_declaration

===== interface_constant_declaration =====
''[ [[.:constant_declarations|constant]] ] [[.:bnf#identifier_list]] : [ **in** ] [[.:subtype_declarations|subtype_indication]] [ := static _[[.:bnf#expression]] ]''

===== interface_signal_declaration =====
''[ [[.:signal_declarations|signal]] ] [[.:bnf#identifier_list]] : [ [[.:bnf#mode]] ] [[.:subtype_declarations|subtype_indication]] [ [[.:signal_declarations|bus]] ] [ := static _[[.:bnf#expression]] ]''

-- subtype_indication must not be a file or pointer type.

===== interface_variable_declaration =====
''[ [[.:variable_declarations|variable]] ] [[.:bnf#identifier_list]] : [ [[.:bnf#mode]] ] [[.:subtype_declarations|subtype_indication]] [ := static _[[.:bnf#expression]] ]''

===== interface_file_declaration =====
''file [[.:bnf#identifier_list]] : [[.:bnf#subtype_indication]]''

-- subtype_indication have to indicate a subtype of a file type.

===== mode =====
  * in -- Ports cannot be written; default mode
  * out -- Ports cannot be read
  * inout -- Ports can be read and written
  * buffer -- Ports can be read but only be written by one source
  * linkage -- Ports can be read and written, but only as "actuals".