Dev_tools

Tools and techniques for developing Nvim

The following advice is helpful when working on or debugging issues with Nvim itself.

TODO: merge debug.txt into here.

Backtraces dev-tools-backtrace

LINUX

Core dumps are disabled by default on Ubuntu, CentOS and others. To enable core dumps:

ulimit -c unlimited

On systemd-based systems getting a backtrace is as easy as:

coredumpctl -1 gdb

coredumpctl is an optional tool, so you may need to install it:

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:

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:

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).

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:

sudo mkdir /cores
sudo chown root:admin /cores
sudo chmod 1775 /cores

Then set the core size limit to unlimited:

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:

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but note that some of the things on this page are out of date (such as enabling core dumps with /etc/launchd.conf).

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 ...", ...)):

GDB=1 TEST_TAG=foo make functionaltest

Then, in another terminal:

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:

gdb -tui -p 1234 build/bin/nvim

The gdb interactive prompt will appear. At any time you can:

GDB REVERSE DEBUGGING

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:

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:

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

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 typemake debug`.

.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