A simple, embedabble debugger for Lua 5.x, and LuaJIT 2.x.

debugger.lua is a simple, single file, pure Lua debugger that is easy to integrate with any project. The lua-users wiki lists a number of debuggers. clidebugger was closest to what I was looking for, but I ran into several compatibility issues, and the rest are pretty big libraries with a whole lot of dependencies. I just wanted something simple to integrate that would work through stdin/stdout. I also decided that it sounded fun to try and make my own!

You can find the new project page on Codeberg.

At least for now, I will be supporting the project on both sites, but eventually I plan to eventually drop GitHub entirely.

To migrate an existing checkout, you can use the following command, or make a new checkout of the project from the new location.

git remote set-url origin https://codeberg.org/slembcke/debugger.lua
git fetch origin master
git reset origin/master

You may additionally need to run git checkout . to cleanup the deleted files. This will also discard your local changes if you have made any.

To read more about why this project moved, you can read about it in PROJECT_HAS_MOVED.md.

• Trivial to "install". Can be integrated as a single .lua or .h file.
• The regular assortment of commands you'd expect from a debugger: continue, step, next, finish, print/eval expression, move up/down the stack, backtrace, print locals, inline help.
• Evaluate expressions, call functions interactively, and get/set variables.
• Pretty printed output so you see {1 = 3, "a" = 5} instead of table: 0x10010cfa0
• Speed! The debugger hooks are only set during the step/next/finish commands.
• Conditional, assert-style breakpoints.
• Colored output and line editing support when possible.
• Drop in replacements for Lua's assert(), error(), and pcall() functions that trigger the debugger.
• When using the C API, dbg_call() works as a drop-in replacement for lua_pcall().
• IO can easily be remapped to a socket or window by overwriting the dbg.write() and dbg.read() functions.
• Permissive MIT license.

debugger.lua can be easily integrated into an embedded project with just a .h file. First though, you'll need to run lua make_c_header.lua. This generates debugger_lua.h by inserting the lua code into a template .h file.

// You need to define this in one of your C files. (and only one!)
#define DEBUGGER_LUA_IMPLEMENTATION
#include "debugger_lua.h"

int main(int argc, char **argv){
	// Do normal Lua init stuff.
	lua_State *lua = luaL_newstate();
	luaL_openlibs(lua);
	
	// Load up the debugger module as "debugger".
	// Also stores it in a global variable "dbg".
	// Use dbg_setup() to change these or use custom I/O.
	dbg_setup_default(lua);
	
	// Load some buggy Lua code.
	luaL_loadstring(lua,
		"local num = 1 \n"
		"local str = 'one' \n"
		"local res = num + str \n"
	);
	
	// Run it in the debugger. This function works just like lua_pcall() otherwise.
	// Note that setting your own custom message handler disables the debugger.
	if(dbg_pcall(lua, 0, 0, 0)){
		fprintf(stderr, "Lua Error: %s\n", lua_tostring(lua, -1));
	}
}

Now in your Lua code you can access dbg or use require 'debugger'.

If you have used other CLI debuggers, debugger.lua shouldn't be surprising. I didn't make a fancy parser, so the commands are just single letters. Since the debugger is pretty simple there are only a small handful of commands anwyay.

[return] - re-run last command
c(ontinue) - contiue execution
s(tep) - step forward by one line (into functions)
n(ext) - step forward by one line (skipping over functions)
p(rint) [expression] - execute the expression and print the result
f(inish) - step forward until exiting the current function
u(p) - move up the stack by one frame
d(own) - move down the stack by one frame
w(here) [line count] - print source code around the current line
t(race) - print the stack trace
l(ocals) - print the function arguments, locals and upvalues.
h(elp) - print this message
q(uit) - halt execution

If you've never used a command line debugger before, start a nice warm cozy fire, run tutorial.lua, and open it up in your favorite editor so you can follow along.

There are several overloadable functions you can use to customize debugger.lua.

• dbg.read(prompt) - Show the prompt and block for user input. (Defaults to read from stdin)
• dbg.write(str) - Write a string to the output. (Defaults to write to stdout)
• dbg.shorten_path(path) - Return a shortened version of a path. (Defaults to simply return path)
• dbg.exit(err) - Stop debugging. (Defaults to os.exit(err))
• dbg.pretty(obj) - Output a pretty print string for an object. (Defaults to a reasonable version using __tostring metamethods and such)

Using these you can customize the debugger to work in your environment. For instance, you can divert the I/O over a network socket or to a GUI window.

There are also some goodies you can use to make debugging easier.

• dbg.writeln(format, ...) - Basically the same as dbg.write(string.format(format.."\n", ...))
• dbg.pretty_depth = int - Set how deep dbg.pretty() formats tables.
• dbg.pretty(obj) - Will return a pretty print string of an object.
• dbg.pp(obj) - Basically the same as dbg.writeln(dbg.pretty(obj))
• dbg.auto_where = int_or_false - Set the where command to run automatically when the active line changes. The value is the number of context lines.
• dbg.error(error, [level]) - Drop in replacement for error() that breaks in the debugger.
• dbg.assert(error, [message]) - Drop in replacement for assert() that breaks in the debugger.
• dbg.call(f, ...) - Drop in replacement for pcall() that breaks in the debugger.

Want to disable ANSI color support or disable GNU readline? Set the DBG_NOCOLOR and/or DBG_NOREADLINE environment variables.

• Lua 5.1 lacks the API to access varargs. The workaround is to do something like local args = {...} and then use unpack(args) when you want to access them. In Lua 5.2+ and LuaJIT, you can simply use ... in your expressions with the print command.
• You can't add breakpoints to a running program or remove them. Currently the only way to set them is by explicitly calling the dbg() function explicitly in your code. (This is sort of by design and sort of because it's difficult/slow otherwise.)
• Different interpreters (and versions) print out slightly different stack trace information.
• Tail calls are handled silghtly differently in different interpreters. You may find that 1.) stepping into a function that does nothing but a tail call steps you into the tail called function. 2.) The interpreter gives you the wrong name of a tail called function (watch the line numbers). 3.) Stepping out of a tail called function also steps out of the function that performed the tail call. Mostly this is never a problem, but it is a little confusing if you don't know what is going on.
• Coroutine support has not been tested extensively yet, and Lua vs. LuaJIT handle them differently anyway. -_-

Copyright (c) 2026 Scott Lembcke and Howling Moon Software

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