Dev_arch

How to develop Nvim, explanation of modules and subsystems

Module-specific details are documented at the top of each module (terminal.c, undo.c, …). The top of each major module has (or should have) an overview in a comment at the top of its file.

The purpose of this document is to give:

1. an overview of how it all fits together 2. how-to guides for common tasks such as:

Filename conventions

The source filenames use extensions to hint about their purpose.

Data structures

Use kvec.h for most lists. When you absolutely need a linked list, use lib/queue_defs.h which defines an "intrusive" linked list.

Buffer text is stored as a tree of line segments, defined in src/nvim/memline.c. The central idea is found in ml_find_line.

Many of the editor concepts are defined as Lua data files:

Events dev-events

The events historically called "autocmds", referred to here as "editor events" or simply "events", are high-level events for use by plugins, user config, and the Nvim editor. (There is an unrelated, low-level concept defined by the event/defs.h#Event struct, which is just a bag of data passed along the internal event-loop.)

All new editor events must be implemented using aucmd_defer() (and where possible, old events should be migrated to this), so that they are processed in a predictable manner, which avoids crashes and race conditions. See do_markset_autocmd for an example.

UI events dev-ui-events

The long-term vision is that UI events are just another type of "editor event" (formerly known as "autocmds"). There is no real reason that we have separate types of user-facing or plugin-facing events. Events are events. Their "transport" is irrelevant and any event should be possible to emit over any transport (editor or RPC).

Meanwhile the current situation is that UI events are a particular RPC event packaged in a generic redraw notification. They also can be listened to in-process via vim.ui_attach().

UI events are deferred to UIs, which implies a deepcopy of the UI event data.

The source files most directly involved with UI events are: 1. src/nvim/ui.*: calls handler functions of registered UI structs (independent from msgpack-rpc) 2. src/nvim/api/ui.*: forwards messages over msgpack-rpc to remote UIs.

UI events are defined in src/nvim/api/ui_events.in.h , this file is not compiled directly, rather it parsed by src/nvim/generators/gen_api_ui_events.lua which autogenerates wrapper functions used by the source files above. It also generates metadata accessible as api_info().ui_events.

See commit d3a8e9217f39c59dd7762bd22a76b8bd03ca85ff for an example of adding a new UI event. Remember to bump NVIM_API_LEVEL if it wasn't already during this development cycle.

Other references:

API

dev-api-fast
API functions and Vimscript "eval" functions may be marked as api-fast which means they are safe to call in Lua callbacks and other scenarios. A functions CANNOT be marked as "fast" if could trigger os_breakcheck(), which may "yield" the current execution and start a new execution of code not expecting this:

In practice, this means any code that could trigger os_breakcheck() cannot be "fast". For example, commit 3940c435e405 fixed such a bug with nvim__get_runtime by explicitly disallowing os_breakcheck() via the EW_NOBREAK flag.

Common examples of non-fast code: regexp matching, wildcard expansion, expression evaluation.

The event-loop event-loop

The internal, low-level, libuv event-loop (luv-event-loop) is used to schedule arbitrary work in a predictable way. One such obvious use-case for scheduling is deferred editor-events (autocmds). Another example is job-control.

ASYNC EVENT SUPPORT

One of the features Nvim added is the support for handling arbitrary asynchronous events, which can include:

Nvim implements this functionality by entering another event loop while waiting for characters, so instead of:

def state_enter(state_callback, data):
  do
    key = readkey()                 # read a key from the user
  while state_callback(data, key)   # invoke the callback for the current state

The Nvim program loop is more like:

def state_enter(state_callback, data):
  do
    event = read_next_event()       # read an event from the operating system
  while state_callback(data, event) # invoke the callback for the current state

where event is something the operating system delivers to us, including (but not limited to) user input. The read_next_event() part is internally implemented by libuv, the platform layer used by Nvim.

Since Nvim inherited its code from Vim, the states are not prepared to receive "arbitrary events", so we use a special key to represent those (When a state receives an "arbitrary event", it normally doesn't do anything other than update the screen).

MAIN LOOP

The Loop structure (which describes main_loop) abstracts multiple queues into one loop:

uv_loop_t uv;
MultiQueue *events;
MultiQueue *thread_events;
MultiQueue *fast_events;

loop_poll_events checks Loop.uv and Loop.fast_events whenever Nvim is idle, and also at os_breakcheck intervals.

MultiQueue is cool because you can attach throw-away "child queues" trivially. For example do_os_system() does this (for every spawned process!) to automatically route events onto the main_loop:

Process *proc = &uvproc.process;
MultiQueue *events = multiqueue_new_child(main_loop.events);
proc->events = events;

NVIM LIFECYCLE

How Nvim processes input.

Consider a typical Vim-like editing session:

01. Vim displays the welcome screen 02. User types: : 03. Vim enters command-line mode 04. User types: edit README.txt<CR> 05. Vim opens the file and returns to normal mode 06. User types: G 07. Vim navigates to the end of the file 09. User types: 5 10. Vim enters count-pending mode 11. User types: d 12. Vim enters operator-pending mode 13. User types: w 14. Vim deletes 5 words 15. User types: g 16. Vim enters the "g command mode" 17. User types: g 18. Vim goes to the beginning of the file 19. User types: i 20. Vim enters insert mode 21. User types: word<ESC> 22. Vim inserts "word" at the beginning and returns to normal mode

Note that we split user actions into sequences of inputs that change the state of the editor. While there's no documentation about a "g command mode" (step 16), internally it is implemented similarly to "operator-pending mode".

From this we can see that Vim has the behavior of an input-driven state machine (more specifically, a pushdown automaton since it requires a stack for transitioning back from states). Assuming each state has a callback responsible for handling keys, this pseudocode represents the main program loop:

def state_enter(state_callback, data):
  do
    key = readkey()                 # read a key from the user
  while state_callback(data, key)   # invoke the callback for the current state

That is, each state is entered by calling state_enter and passing a state-specific callback and data. Here is a high-level pseudocode for a program that implements something like the workflow described above:

def main()
  state_enter(normal_state, {}):
def normal_state(data, key):
  if key == ':':
    state_enter(command_line_state, {})
  elif key == 'i':
    state_enter(insert_state, {})
  elif key == 'd':
    state_enter(delete_operator_state, {})
  elif key == 'g':
    state_enter(g_command_state, {})
  elif is_number(key):
    state_enter(get_operator_count_state, {'count': key})
  elif key == 'G'
    jump_to_eof()
  return true
def command_line_state(data, key):
  if key == '<cr>':
    if data['input']:
      execute_ex_command(data['input'])
    return false
  elif key == '<esc>'
    return false
  if not data['input']:
    data['input'] = ''
  data['input'] += key
  return true
def delete_operator_state(data, key):
  count = data['count'] or 1
  if key == 'w':
    delete_word(count)
  elif key == '$':
    delete_to_eol(count)
  return false  # return to normal mode
def g_command_state(data, key):
  if key == 'g':
    go_top()
  elif key == 'v':
    reselect()
  return false  # return to normal mode
def get_operator_count_state(data, key):
  if is_number(key):
    data['count'] += key
    return true
  unshift_key(key)  # return key to the input buffer
  state_enter(delete_operator_state, data)
  return false
def insert_state(data, key):
  if key == '<esc>':
    return false  # exit insert mode
  self_insert(key)
  return true

The above gives an idea of how Nvim is organized internally. Some states like the g_command_state or get_operator_count_state do not have a dedicated state_enter callback, but are implicitly embedded into other states (this will change later as we continue the refactoring effort). To start reading the actual code, here's the recommended order:

1. state_enter() function (state.c). This is the actual program loop, note that a VimState structure is used, which contains function pointers for the callback and state data. 2. main() function (main.c). After all startup, normal_enter is called at the end of function to enter normal mode. 3. normal_enter() function (normal.c) is a small wrapper for setting up the NormalState structure and calling state_enter. 4. normal_check() function (normal.c) is called before each iteration of normal mode. 5. normal_execute() function (normal.c) is called when a key is read in normal mode.

The basic structure described for normal mode in 3, 4 and 5 is used for other modes managed by the state_enter loop:

IMPORTANT VARIABLES

The current mode is stored in State. The values it can have are MODE_NORMAL, MODE_INSERT, MODE_CMDLINE, and a few others.

The current window is curwin. The current buffer is curbuf. These point to structures with the cursor position in the window, option values, the file name, etc.

All the global variables are declared in globals.h.

THE MAIN EVENT-LOOP

The main loop is implemented in state_enter. The basic idea is that Vim waits for the user to type a character and processes it until another character is needed. Thus there are several places where Vim waits for a character to be typed. The vgetc() function is used for this. It also handles mapping.

What we consider the "Nvim event loop" is actually a wrapper around uv_run to handle both the fast_events queue and possibly (a suitable subset of) deferred events. Therefore "raw" vim.uv.run() is often not enough to "yield" from Lua plugins; instead they can call vim.wait(0).

Updating the screen is mostly postponed until a command or a sequence of commands has finished. The work is done by update_screen(), which calls win_update() for every window, which calls win_line() for every line. See the start of [drawscreen.c](drawscreen.c) for more explanations.

COMMAND-LINE MODE

When typing a :, normal_cmd() will call getcmdline() to obtain a line with an Ex command. getcmdline() calls a loop that will handle each typed character. It returns when hitting <CR> or <Esc> or some other character that ends the command line mode.

EX COMMANDS

Ex commands are handled by the function do_cmdline(). It does the generic parsing of the : command line and calls do_one_cmd() for each separate command. It also takes care of while loops.

do_one_cmd() parses the range and generic arguments and puts them in the exarg_t and passes it to the function that handles the command.

The : commands are listed in [ex_cmds.lua](ex_cmds.lua).

NORMAL MODE COMMANDS

The Normal mode commands are handled by the normal_cmd() function. It also handles the optional count and an extra character for some commands. These are passed in a cmdarg_T to the function that handles the command.

There is a table nv_cmds in [normal.c](normal.c) which lists the first character of every command. The second entry of each item is the name of the function that handles 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. It returns when leaving Insert mode.