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neovim/src/nvim/README.md

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Nvim core

Module-specific details are documented at the top of each module (terminal.c, screen.c, …).

See :help dev for guidelines.

Filename conventions

The source files use extensions to hint about their purpose.

  • *.c, *.generated.c - full C files, with all includes, etc.
  • *.c.h - parametrized C files, contain all necessary includes, but require defining macros before actually using. Example: typval_encode.c.h
  • *.h - full headers, with all includes. Does not apply to *.generated.h.
  • *.h.generated.h - exported functions’ declarations.
  • *.c.generated.h - static functions’ declarations.

Logs

Low-level log messages sink to $NVIM_LOG_FILE.

Use LOG_CALLSTACK() (Linux only) to log the current stacktrace. To log to an alternate file (e.g. stderr) use LOG_CALLSTACK_TO_FILE(FILE*).

UI events are logged at DEBUG level (DEBUG_LOG_LEVEL).

rm -rf build/
make CMAKE_EXTRA_FLAGS="-DMIN_LOG_LEVEL=0"

Many log messages have a shared prefix, such as "UI" or "RPC". Use the shell to filter the log, e.g. at DEBUG level you might want to exclude UI messages:

tail -F ~/.local/share/nvim/log | cat -v | stdbuf -o0 grep -v UI | stdbuf -o0 tee -a log

Build with ASAN

Building Nvim with Clang sanitizers (Address Sanitizer: ASan, Undefined Behavior Sanitizer: UBSan, Memory Sanitizer: MSan, Thread Sanitizer: TSan) is a good way to catch undefined behavior, leaks and other errors as soon as they happen. It's significantly faster than Valgrind.

Requires clang 3.4 or later, and llvm-symbolizer must be in $PATH:

clang --version

Build Nvim with sanitizer instrumentation (choose one):

CC=clang make CMAKE_EXTRA_FLAGS="-DCLANG_ASAN_UBSAN=ON"
CC=clang make CMAKE_EXTRA_FLAGS="-DCLANG_MSAN=ON"
CC=clang make CMAKE_EXTRA_FLAGS="-DCLANG_TSAN=ON"

Create a directory to store logs:

mkdir -p "$HOME/logs"

Configure the sanitizer(s) via these environment variables:

# Change to detect_leaks=1 to detect memory leaks (slower).
export ASAN_OPTIONS="detect_leaks=0:log_path=$HOME/logs/asan"
# Show backtraces in the logs.
export UBSAN_OPTIONS=print_stacktrace=1
export MSAN_OPTIONS="log_path=${HOME}/logs/tsan"
export TSAN_OPTIONS="log_path=${HOME}/logs/tsan"

Logs will be written to ${HOME}/logs/*san.PID then.

For more information: https://github.com/google/sanitizers/wiki/SanitizerCommonFlags

TUI debugging

TUI troubleshoot

Nvim logs its internal terminfo state at 'verbose' level 3. This makes it possible to see exactly what terminfo values Nvim is using on any system.

nvim -V3log

TUI trace

The ancient script command is still the "state of the art" for tracing terminal behavior. The libvterm vterm-dump utility formats the result for human-readability.

Record a Nvim terminal session and format it with vterm-dump:

script foo
./build/bin/nvim -u NONE
# Exit the script session with CTRL-d

# Use `vterm-dump` utility to format the result.
./.deps/usr/bin/vterm-dump foo > bar

Then you can compare bar with another session, to debug TUI behavior.

TUI redraw

Set the 'writedelay' option to see where and when the UI is painted.

:set writedelay=1

Terminal reference

Nvim lifecycle

Following describes how Nvim processes input.

Consider a typical Vim-like editing session:

  1. Vim displays the welcome screen
  2. User types: :
  3. Vim enters command-line mode
  4. User types: edit README.txt<CR>
  5. Vim opens the file and returns to normal mode
  6. User types: G
  7. Vim navigates to the end of the file
  8. User types: 5
  9. Vim enters count-pending mode
  10. User types: d
  11. Vim enters operator-pending mode
  12. User types: w
  13. Vim deletes 5 words
  14. User types: g
  15. Vim enters the "g command mode"
  16. User types: g
  17. Vim goes to the beginning of the file
  18. User types: i
  19. Vim enters insert mode
  20. User types: word<ESC>
  21. 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:

  • command-line mode: command_line_{enter,check,execute}()(ex_getln.c)
  • insert mode: insert_{enter,check,execute}()(edit.c)
  • terminal mode: terminal_{enter,execute}()(terminal.c)

Async event support

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

  • RPC requests
  • job control callbacks
  • timers

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

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 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;