*dev_tools.txt*          Nvim


                            NVIM REFERENCE MANUAL


Tools and techniques for developing Nvim                        *dev-tools*

This is for developing or debugging Nvim itself.

                                  Type |gO| to see the table of contents.

==============================================================================
Quickstart guide to developing Nvim                           *dev-quickstart*

You can start hacking on Nvim in less than 5 minutes:

1. Ensure you have the build prerequisites from `BUILD.md`.
2. Clone the source repo and "cd" into it: >
        git clone https://github.com/neovim/neovim
        cd neovim
        # (Optional) Build and run Nvim:
        make
        VIMRUNTIME=./runtime ./build/bin/nvim --luamod-dev
3. Run a single test. We will start with "example_spec.lua", which is a real
   test that shows how tests are written: >
        make functionaltest TEST_FILE=test/functional/example_spec.lua
4. Notice the `before_each` block in the test file. Because it calls
   `clear()`, each `it()` test will start a new Nvim instance.
5. Tests will do stuff in the Nvim instance and make assertions using `eq()`.
   Tests that want to check the UI can also use `screen:expect()`.
6. Now make a code change in Nvim itself, then you can see the effects. The
   example test does `feed('iline1…')`, so let's make a change to the
   insert-mode code, which lives in `src/nvim/edit.c`. In the
   `insert_handle_key` function, just after the `normalchar` label, add this
   code: >
        s->c = 'x';
7. Then run the "example_spec.lua" test again, and it should fail with
   something like this: >
        test/functional/example_spec.lua:31: Row 1 did not match.
        Expected:
          |*line1               |
          |*line^2               |
          |{0:~                   }|
          |{0:~                   }|
          |                    |
        Actual:
          |*xine1               |
          |*xine^2               |
          |{0:~                   }|
          |{0:~                   }|
          |                    |

You now understand how to modify the codebase, write tests, and run tests. See
|dev-arch| for details about the internal architecture.

==============================================================================
Logs                                                          *dev-tools-logs*

Low-level log messages sink to `$NVIM_LOG_FILE`.

UI events are logged at DEBUG level. >

    rm -rf build/
    make CMAKE_BUILD_TYPE=Debug

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*)`. Requires
`-no-pie` ([ref](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860394#15)): >

    rm -rf build/
    make CMAKE_BUILD_TYPE=Debug CMAKE_EXTRA_FLAGS="-DCMAKE_C_FLAGS=-no-pie"

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/state/nvim/nvim.log | cat -v | stdbuf -o0 grep -v UI | stdbuf -o0 tee -a log


==============================================================================
Reproducible build

To make a reproducible build of Nvim, set cmake variable `LUA_GEN_PRG` to
a LuaJIT binary built with `LUAJIT_SECURITY_PRN=0`. See commit
cb757f2663e6950e655c6306d713338dfa66b18d.


==============================================================================
Debug TUI                                                      *dev-tools-tui*

TUI INSPECT

Use the Ghostty https://ghostty.org/ inspector tool to observe and query the
output and events from any terminal application such as Nvim.

From the Ghostty inspector you can click the "Terminal IO" tab to get a trace.

TERMINFO LOGGING

At 'verbose' level 3, Nvim logs its internal terminfo state, so you can see
exactly what terminfo values it is using on the current system. >

    nvim -V3log

TUI DEBUGGING WITH GDB LLDB

Launching the Nvim TUI involves two processes, one for main editor state and one
for rendering the TUI. Both of these processes use the nvim binary, so somewhat
confusingly setting a breakpoint in either will generally succeed but may not be
hit depending on which process the breakpoints were set in.

To debug the main process, you can debug the nvim binary with the `--headless`
flag which does not launch the TUI and will allow you to set breakpoints in code
not related to TUI rendering like so: >

    lldb -- ./build/bin/nvim --headless --listen ~/.cache/nvim/debug-server.pipe

While in lldb, enter `run`. You can then attach to the headless process in a
new terminal window to interact with the editor like so: >

    ./build/bin/nvim --remote-ui --server ~/.cache/nvim/debug-server.pipe

Conversely for debugging TUI rendering, you can start a headless process and
debug the remote-ui process multiple times without losing editor state.

For details on using nvim-dap and automatically debugging the child (main)
process, see [here](https://zignar.net/2023/02/17/debugging-neovim-with-neovim-and-nvim-dap/)

TUI REDRAW

For debugging Nvim TUI redraw behavior it is sometimes useful to slow down its
redraws. Set the 'writedelay' and 'redrawdebug' options to see where and when
the UI is painted. >

    :set writedelay=50 rdb=compositor

Note: Nvim uses an internal screenbuffer to only send minimal updates even if a large
region is repainted internally. To also highlight excess internal redraws, use >

    :set writedelay=50 rdb=compositor,nodelta

TUI TRACE

From the Ghostty inspector you can click the "Terminal IO" tab to get a trace.

Alternatively, the ancient `script` command is the "state of the art". The
libvterm `vterm-dump` utility formats the result for human-readability.

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

    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.

TERMINAL REFERENCE

- `man terminfo`
- https://github.com/vim/vim/blob/0124320c97b0fbbb44613f42fc1c34fee6181fc8/src/libvterm/doc/seqs.txt
- https://invisible-island.net/xterm/ctlseqs/ctlseqs.html

==============================================================================
Debug Performance                                             *dev-tools-perf*

PROFILING (EASY)

For debugging performance bottlenecks in any code, there is a simple (and very
effective) approach:

1. Run the slow code in a loop.
2. Break execution ~5 times and save the stacktrace.
3. The cause of the bottleneck will (almost always) appear in most of the stacktraces.


PROFILING (FANCY)

For more advanced profiling, consider `perf` + `flamegraph`.


USDT PROFILING (POWERFUL)

Or you can use USDT probes via `NVIM_PROBE` ([#12036](https://github.com/neovim/neovim/pull/12036)).

> USDT is basically a way to define stable probe points in userland binaries.
> The benefit of bcc is the ability to define logic to go along with the probe
> points.

Tools:
- bpftrace provides an awk-like language to the kernel bytecode, BPF.
- BCC provides a subset of C. Provides more complex logic than bpftrace, but takes a bit more effort.

Example using bpftrace to track slow vim functions, and print out any files
that were opened during the trace. At the end, it prints a histogram of
function timing: >

    #!/usr/bin/env bpftrace

    BEGIN {
      @depth = -1;
    }

    tracepoint:sched:sched_process_fork /@pidmap[args->parent_pid]/ {
      @pidmap[args->child_pid] = 1;
    }

    tracepoint:sched:sched_process_exit /@pidmap[args->pid]/ {
      delete(@pidmap[args->pid]);
    }

    usdt:build/bin/nvim:neovim:eval__call_func__entry {
        @pidmap[pid] = 1;
        @depth++;
        @funcentry[@depth] = nsecs;
    }

    usdt:build/bin/nvim:neovim:eval__call_func__return {
        $func = str(arg0);
        $msecs = (nsecs - @funcentry[@depth]) / 1000000;

        @time_histo = hist($msecs);

        if ($msecs >= 1000) {
          printf("%u ms for %s\n", $msecs, $func);
          print(@files);
        }

        clear(@files);
        delete(@funcentry[@depth]);
        @depth--;
    }

    tracepoint:syscalls:sys_enter_open,
    tracepoint:syscalls:sys_enter_openat {
      if (@pidmap[pid] == 1 && @depth >= 0) {
        @files[str(args->filename)] = count();
      }
    }

    END {
      clear(@depth);
    }

    $ sudo bpftrace funcslower.bt
    1527 ms for Slower
    @files[/usr/lib/libstdc++.so.6]: 2
    @files[/etc/fish/config.fish]: 2
    <snip>

    ^C
    @time_histo:
    [0]                71430 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
    [1]                  346 |                                                    |
    [2, 4)               208 |                                                    |
    [4, 8)                91 |                                                    |
    [8, 16)               22 |                                                    |
    [16, 32)              85 |                                                    |
    [32, 64)               7 |                                                    |
    [64, 128)              0 |                                                    |
    [128, 256)             0 |                                                    |
    [256, 512)             6 |                                                    |
    [512, 1K)              1 |                                                    |
    [1K, 2K)               5 |                                                    |
<

==============================================================================
Backtraces                                               *dev-tools-backtrace*

LINUX

Core dumps are disabled by default on Ubuntu, CentOS and others.
To enable core dumps:
>bash
    ulimit -c unlimited
<
On systemd-based systems getting a backtrace is as easy as:
>bash
    coredumpctl -1 gdb
<
`coredumpctl` is an optional tool, so you may need to install it:
>bash
    sudo apt install systemd-coredump
<

The full backtrace is most useful; please send us the `backtrace.txt` file
when reporting a bug related to a crash:
>bash
    2>&1 coredumpctl -1 gdb | tee -a backtrace.txt
    (gdb) thread apply all bt full
<

On systems without `coredumpctl`, you may find a `core` dump file appearing
in the current directory or in other locations. On Linux systems where
`apport` is installed (such as Ubuntu), the directory where core dump files
are saved can be `/var/lib/apport/coredump` or elsewhere, depending on the
system configuration (see `/proc/sys/kernel/core_pattern`). See also:
https://stackoverflow.com/a/18368068

To get a backtrace from the `./core` dump file:
>bash
    gdb build/bin/nvim ./core 2>&1 | tee backtrace.txt
    (gdb) thread apply all bt full
<

MACOS

If `nvim` crashes, you can see the backtrace in `Console.app` (under "Crash
Reports" or "User Diagnostic Reports" for older macOS versions).
>bash
    open -a Console
<
You may also want to enable core dumps on macOS. To do this, first make sure
the `/cores/` directory exists and is writable:
>bash
    sudo mkdir /cores
    sudo chown root:admin /cores
    sudo chmod 1775 /cores
<
Then set the core size limit to `unlimited`:
>bash
    ulimit -c unlimited
<
Note that this is done per shell process. If you want to make this the default
for all shells, add the above line to your shell's init file (e.g. `~/.bashrc`
or similar).

You can then open the core file in `lldb`:
>bash
    lldb -c /cores/core.12345
<
Apple's documentation archive has some other useful information
https://developer.apple.com/library/archive/technotes/tn2124/_index.html#//apple_ref/doc/uid/DTS10003391-CH1-SECCOREDUMPS,
but note that some of the things on this page are out of date (such as enabling
core dumps with `/etc/launchd.conf`).


WINDOWS

If the Windows version of Nvim crashes in a reproducible manner, you can take
some steps to provide a useful bug report.

You must obtain the debugger symbols (PDB) file for the Nvim executable: nvim.pdb.
The PDB should be available from the same place that you obtained the
executable (TODO: not currently provided by Nvim CI releases).  Be sure to use
the PDB that matches the EXE (same build).

If you built the executable yourself with the Microsoft Visual C++ compiler,
then the PDB was built with the EXE.

If you have Visual Studio, use that instead of the VC Toolkit and WinDbg.

For other compilers, always use the corresponding debugger: gdb or lldb.

Debugging Nvim crashes with Visual Studio 2005 ~

First launch nvim.exe and then launch Visual Studio.  (If you don't have
Visual Studio, get it from https://visualstudio.microsoft.com/downloads/).

On the Tools menu, click Attach to Process.  Choose the Nvim process.

In Nvim, reproduce the crash.  A dialog will appear in Visual Studio, telling
you about the unhandled exception in the Nvim process.  Click Break to break
into the process.

Visual Studio will pop up another dialog, telling you that no symbols are
loaded and that the source code cannot be displayed.  Click OK.

Several windows will open.  Right-click in the Call Stack window.  Choose Load
Symbols.  The Find Symbols dialog will open, looking for (g)vim.pdb.  Navigate
to the directory where you have the PDB file and click Open.

At this point, you should have a full call stack with vim function names and
line numbers.  Double-click one of the lines and the Find Source dialog will
navigate to the directory where the Nvim source is (if you have it.)

If you don't know how to debug this any further, follow the instructions
at ":help bug-report".  Paste the call stack into the bug report.

From Visual Studio you can also try saving a minidump via the Debug menu and
send it with the bug report.  A minidump is a small file (<100KB), which
contains information about the state of your process.

==============================================================================
Gdb                                                          *dev-tools-gdb*

USING GDB TO STEP THROUGH FUNCTIONAL TESTS

Use `TEST_TAG` to run tests matching busted tags (of the form `#foo` e.g.
`it("test #foo ...", ...)`):
>bash
    GDB=1 TEST_TAG=foo make functionaltest
<
Then, in another terminal:
>bash
    gdb build/bin/nvim
    (gdb) target remote localhost:7777

-- See `nvim_argv` in https://github.com/neovim/neovim/blob/master/test/functional/testnvim.lua.

USING LLDB TO STEP THROUGH UNIT TESTS

>
    lldb .deps/usr/bin/luajit -- .deps/usr/bin/busted --lpath="./build/?.lua" test/unit/
<
USING GDB

To attach to a running `nvim` process with a pid of 1234 (Tip: the pid of a
running Nvim instance can be obtained by calling |getpid()|), for instance:
>bash
    gdb -tui -p 1234 build/bin/nvim
<
The `gdb` interactive prompt will appear. At any time you can:

- `break foo` to set a breakpoint on the `foo()` function
- `n` to step over the next statement
    - `<Enter>` to repeat the last command
- `s` to step into the next statement
- `c` to continue
- `finish` to step out of the current function
- `p zub` to print the value of `zub`
- `bt` to see a backtrace (callstack) from the current location
- `CTRL-x CTRL-a` or `tui enable` to show a TUI view of the source file in the
  current debugging context. This can be extremely useful as it avoids the
  need for a gdb "frontend".
- `<up>` and `<down>` to scroll the source file view

GDB REVERSE DEBUGGING

- `set record full insn-number-max unlimited`
- `continue` for a bit (at least until `main()` is executed
- `record`
- provoke the bug, then use `revert-next`, `reverse-step`, etc. to rewind the
  debugger

USING GDBSERVER

You may want to connect multiple `gdb` clients to the same running `nvim`
process, or you may want to connect to a remote `nvim` process with a local
`gdb`. Using `gdbserver`, you can attach to a single process and control it
from multiple `gdb` clients.

Open a terminal and start `gdbserver` attached to `nvim` like this:
>bash
    gdbserver :6666 build/bin/nvim 2> gdbserver.log
<
`gdbserver` is now listening on port 6666. You then need to attach to this
debugging session in another terminal:
>bash
    gdb build/bin/nvim
<
Once you've entered `gdb`, you need to attach to the remote session:
>
    (gdb) target remote localhost:6666
<
In case gdbserver puts the TUI as a background process, the TUI can become
unable to read input from pty (and receives SIGTTIN signal) and/or output data
(SIGTTOU signal). To force the TUI as the foreground process, you can add
>c
    signal (SIGTTOU, SIG_IGN);
    if (!tcsetpgrp(data->input.in_fd, getpid())) {
        perror("tcsetpgrp failed");
    }
<
to `tui.c:terminfo_start`.

USING GDBSERVER IN TMUX

Consider using a custom makefile
https://github.com/neovim/neovim/blob/master/BUILD.md#custom-makefile to
quickly start debugging sessions using the `gdbserver` method mentioned above.
This example `local.mk` will create the debugging session when you type
`make debug`.
>make
    .PHONY: dbg-start dbg-attach debug build

    build:
        @$(MAKE) nvim

    dbg-start: build
        @tmux new-window -n 'dbg-neovim' 'gdbserver :6666 ./build/bin/nvim -D'

    dbg-attach:
        @tmux new-window -n 'dbg-cgdb' 'cgdb -x gdb_start.sh ./build/bin/nvim'

    debug: dbg-start dbg-attach
<
Here `gdb_start.sh` includes `gdb` commands to be called when the debugger
starts. It needs to attach to the server started by the `dbg-start` rule. For
example:
>
    (gdb) target remote localhost:6666
    (gdb) br main
<
==============================================================================
Debugging crashes or memory leaks                             *dev-tools-asan*

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="-DENABLE_ASAN_UBSAN=ON"
    CC=clang make CMAKE_EXTRA_FLAGS="-DENABLE_MSAN=ON"
    CC=clang make CMAKE_EXTRA_FLAGS="-DENABLE_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, noisier).
    export ASAN_OPTIONS="detect_leaks=0:log_path=$HOME/logs/asan"
    # Show backtraces in the logs.
    export MSAN_OPTIONS="log_path=${HOME}/logs/msan"
    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



 vim:tw=78:ts=8:sw=4:et:ft=help:norl:
