Learning Lua Step-By-Step (Part 19)

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Debugging: Exploring the Lua Debug Library

In Lua, while there’s no built-in debugger, we have a rich debug library that equips us to construct our debugger or leverage existing ones made by developers. This library furnishes a spectrum of primitive functions that are pivotal for debugging tasks. Let’s delve into these functions and their applications:

No.	Method & Purpose
1	debug()
	Enters interactive debugging mode until “cont” is entered.
	Allows variable inspection during this mode.
2	getfenv(object)
	Returns the environment of an object.
3	gethook(optional thread)
	Returns current hook settings of a thread.
4	getinfo(optional thread, function or stack level, optional flag)
	Returns info about a function.
5	getlocal(optional thread, stack level, local index)
	Returns the name and value of a local variable.
6	getmetatable(value)
	Returns the metatable of an object.
7	getregistry()
	Returns the registry table.
8	getupvalue(function, upvalue index)
	Returns the name and value of an upvalue.
9	setfenv(function or thread or userdata, environment table)
	Sets the environment of an object.
10	sethook(optional thread, hook function, hook mask, optional instruction count)
	Sets a function as a hook.
11	setlocal(optional thread, stack level, local index, value)
	Assigns a value to a local variable.
12	setmetatable(value, metatable)
	Sets the metatable for an object.
13	setupvalue(function, upvalue index, value)
	Assigns a value to an upvalue.
14	traceback(optional thread, optional message, optional level)
	Builds an extended error message with a traceback.

While these functions empower us to create custom debuggers, it’s often more practical to use existing libraries that simplify debugging. One such example is showcased below:

function myfunction ()print(debug.traceback("Stack trace"))
   print(debug.getinfo(1))
   print("Stack trace end")

   return10end

myfunction ()
print(debug.getinfo(1))

When executed, this program generates a stack trace and utilizes debug.getinfo to access function information.

Debugging Types

Debugging in Lua can be approached via command line or graphical interfaces. Let’s explore these two paradigms:

Command Line Debugging

1. RemDebug: A remote debugger for Lua 5.0 and 5.1, facilitating remote program control, breakpoints, and program state inspection.
2. clidebugger: A command-line interface debugger for Lua 5.1 written in pure Lua.
3. ctrace: A tool for tracing Lua API calls.
4. xdbLua: A Lua command-line debugger for Windows.
5. LuaInterface – Debugger: A debugger extension for LuaInterface, enhancing the built-in Lua debug interface.

Graphical Debugging

1. SciTE: The default Windows IDE for Lua, offering a range of debugging features.
2. Decoda: A graphical debugger with remote debugging support.
3. ZeroBrane Studio: Lua IDE with an integrated remote debugger, stack view, watch view, and more.
4. akdebugger: Debugger and editor Lua plugin for Eclipse.
5. luaedit: A feature-rich IDE with remote and local debugging capabilities.

Exercises

1. Write a Lua script that utilizes the debug.getinfo function to retrieve information about different functions.
2. Experiment with setting breakpoints using debug.sethook and observe the program’s behavior during debugging.
3. Create a custom Lua debugger using the debugging functions provided by the debug library.

Conclusion

Understanding and mastering the Lua debug library is crucial for efficient debugging in Lua. Whether you choose to build your debugger or use existing tools, the knowledge of these debugging techniques will significantly enhance your development workflow and help you create more robust Lua applications.

Resources

• Lua Debug Library Documentation
• RemDebug GitHub Repository
• Decoda Debugger
• ZeroBrane Studio
• akdebugger Eclipse Plugin
• luaedit Debugger

By leveraging these resources and practicing debugging techniques, you’ll become proficient in handling errors and improving the quality of your Lua code.

Series Navigation

<< Learning Lua Step-By-Step (Part 18)Learning Lua Step-By-Step: (Part 20) Memory Management >>