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      1 #	@(#)structures	5.4 (Berkeley) 10/4/95
      2 
      3 There are three major data structures in this package, plus a single data
      4 structure per screen type.  The first is a single global structure (GS)
      5 which contains information common to all files and screens.  It hold
      6 global things like the input key queues, and functions as a single place
      7 to hang things.  For example, interrupt routines have to be able to find
      8 screen structures, and they can only do this if they have a starting
      9 point.  The number of globals in nvi is dependent on the screen type, but
     10 every screen type will have at least one global, __global_list, which
     11 references the GS structure.
     12 
     13 The GS structure contains linked lists of screen (SCR) structures.
     14 Each SCR structure normally references a file (EXF) structure.
     15 
     16 The GS structure has a set of functions which update the screen and/or
     17 return information about the screen from the underlying screen package.
     18 The GS structure never goes away.  The SCR structure persists over
     19 instances of screens, and the EXF structure persists over references to
     20 files.
     21 
     22 File names have different properties than files themselves, so the name
     23 information for a file is held in an FREF structure which is chained from
     24 the SCR structure.
     25 
     26 In general, functions are always passed an SCR structure, which usually
     27 references an underlying EXF structure.  The SCR structure is necessary
     28 for any routine that wishes to talk to the screen, the EXF structure is
     29 necessary for any routine that wants to modify the file.  The relationship
     30 between an SCR structure and its underlying EXF structure is not fixed,
     31 and various ex commands will substitute a new EXF in place of the current
     32 one, and there's no way to detect this.
     33 
     34 The naming of the structures is consistent across the program.  (Macros
     35 even depend on it, so don't try and change it!)  The global structure is
     36 "gp", the screen structure is "sp", and the file structure is "ep".
     37 
     38 A few other data structures:
     39 
     40 TEXT	In nvi/cut.h.  This structure describes a portion of a line,
     41 	and is used by the input routines and as the "line" part of a
     42 	cut buffer.
     43 
     44 CB	In nvi/cut.h.	A cut buffer.  A cut buffer is a place to
     45 	hang a list of TEXT structures.
     46 
     47 CL	The curses screen private data structure.  Everything to
     48 	do standalone curses screens.
     49 
     50 MARK	In nvi/mark.h.  A cursor position, consisting of a line number
     51 	and a column number.
     52 
     53 MSG	In nvi/msg.h.  A chain of messages for the user.
     54 
     55 SEQ	In nvi/seq.h.  An abbreviation or a map entry.
     56 
     57 TK	The Tcl/Tk screen private data structure.  Everything to
     58 	do standalone Tcl/Tk screens.
     59 
     60 EXCMD   In nvi/ex/ex.h.  The structure that gets passed around to the
     61 	functions that implement the ex commands.  (The main ex command
     62 	loop (see nvi/ex/ex.c) builds this up and then passes it to the
     63 	ex functions.)
     64 
     65 VICMD	In nvi/vi/vi.h.  The structure that gets passed around to the
     66 	functions that implement the vi commands.  (The main vi command
     67 	loop (see nvi/vi/vi.c) builds this up and then passes it to the
     68 	vi functions.)
     69