README for the Vim source code Here are a few hints for finding your way around the source code.  Thisdoesn't make it less complex than it is, but it gets you started. You might also want to read ":help development".  JUMPING AROUND First of all, use ":make tags" to generate a tags file, so that you can usethe ":tag" command to jump around the source code. To jump to a function or variable definition, move the cursor on the name anduse the CTRL-] command.  Use CTRL-T or CTRL-O to jump back. To jump to a file, move the cursor on its name and use the "gf" command. Most code can be found in a file with an obvious name (incomplete list):	buffer.c	manipulating buffers (loaded files)	diff.c		diff mode (vimdiff)	eval.c		expression evaluation	fileio.c	reading and writing files	fold.c		folding	getchar.c	getting characters and key mapping	mark.c		marks	mbyte.c		multy-byte character handling	memfile.c	storing lines for buffers in a swapfile	memline.c	storing lines for buffers in memory	menu.c		menus	message.c	(error) messages	ops.c		handling operators ("d", "y", "p")	option.c	options	quickfix.c	quickfix commands (":make", ":cn")	regexp.c	pattern matching	screen.c	updating the windows	search.c	pattern searching	spell.c		spell checking	syntax.c	syntax and other highlighting	tag.c		tags	term.c		terminal handling, termcap codes	undo.c		undo and redo	window.c	handling split windows  IMPORTANT VARIABLES The current mode is stored in "State".  The values it can have are NORMAL,INSERT, CMDLINE, and a few others. The current window is "curwin".  The current buffer is "curbuf".  These pointto structures with the cursor position in the window, option values, the filename, etc.  These are defined in structs.h. All the global variables are declared in globals.h.  THE MAIN LOOP This is conveniently called main_loop().  It updates a few things and thencalls normal_cmd() to process a command.  This returns when the command isfinished. The basic idea is that Vim waits for the user to type a character andprocesses it until another character is needed.  Thus there are several placeswhere Vim waits for a character to be typed.  The vgetc() function is used forthis.  It also handles mapping. Updating the screen is mostly postponed until a command or a sequence ofcommands has finished.  The work is done by update_screen(), which callswin_update() for every window, which calls win_line() for every line.See the start of screen.c for more explanations.  COMMAND-LINE MODE When typing a ":", normal_cmd() will call getcmdline() to obtain a line withan Ex command.  getcmdline() contains a loop that will handle each typedcharacter.  It returns when hitting <CR> or <Esc> or some other character thatends the command line mode.  EX COMMANDS Ex commands are handled by the function do_cmdline().  It does the genericparsing of the ":" command line and calls do_one_cmd() for each separatecommand.  It also takes care of while loops. do_one_cmd() parses the range and generic arguments and puts them in theexarg_t and passes it to the function that handles the command. The ":" commands are listed in ex_cmds.h.  The third entry of each item is thename of the function that handles the command.  The last entry are the flagsthat are used for the command.  NORMAL MODE COMMANDS The Normal mode commands are handled by the normal_cmd() function.  It alsohandles the optional count and an extra character for some commands.  Theseare passed in a cmdarg_t to the function that handles the command. There is a table nv_cmds in normal.c which lists the first character of everycommand.  The second entry of each item is the name of the function thathandles the command.  INSERT MODE COMMANDS When doing an "i" or "a" command, normal_cmd() will call the edit() function.It contains a loop that waits for the next character and handles it.  Itreturns when leaving Insert mode.  OPTIONS There is a list with all option names in option.c, called options[].  THE GUI Most of the GUI code is implemented like it was a clever terminal.  Typing acharacter, moving a scrollbar, clicking the mouse, etc. are all translatedinto events which are written in the input buffer.  These are read by themain code, just like reading from a terminal.  The code for this is scatteredthrough gui.c.  For example: gui_send_mouse_event() for a mouse click andgui_menu_cb() for a menu action.  Key hits are handled by the system-specificGUI code, which calls add_to_input_buf() to send the key code. Updating the GUI window is done by writing codes in the output buffer, justlike writing to a terminal.  When the buffer gets full or is flushed,gui_write() will parse the codes and draw the appropriate items.  Finally thesystem-specific GUI code will be called to do the work.  DEBUGGING THE GUI Remember to prevent that gvim forks and the debugger thinks Vim has exited,add the "-f" argument.  In gdb: "run -f -g". When stepping through display updating code, the focus event is triggeredwhen going from the debugger to Vim and back.  To avoid this, recompile withsome code in gui_focus_change() disabled.