-- Run this file and follow along with the prompts.

-- You'll want to keep this file open to follow along.

local dbg = require("debugger")

print[[

Welcome to the interactive debugger.lua tutorial.

You'll want to open tutorial.lua in an editor to follow along.

First of all, just drop debugger.lua in your project. It's one file.

Load it the usual way using require. Ex:

local dbg = require("debugger")

debugger.lua doesn't support traditional breakpoints.

So to get into the debugger, call it like a function.

Real breakpoints would be better, but this

keeps debugger.lua simple and very fast.

At the end you'll find out how to open it automatically on a crash.

Notice how debugger.lua prints out your current file and line

as well as which function you are in.

Keep a close watch on this as you follow along.

It should be stopped a line after the dbg() call.

(Line 88 unless I forgot to double update it)

Sometimes functions don't have global names.

It might print the name of a method, local variable

that held the function, or file:line where it starts.

Type 'w' to show 5 lines of surrounding code directly in

the debugger. (w = Where) Type 'w 3' to show 3 lines, etc.

Alternatively, set dbg.auto_where to a number

to run it automatically every time the program advances.

Once you've tried the where command, type 's' to step to

the next line. (s = Step to the next executable line)

]]

-- Multi-line strings are executable statements apparently

-- need to put this in an local to make the tutorial flow nicely.

local str1 = [[

The 's' command steps to the next executable line.

This may step you into a function call.

In this case, then next line was a C function that printed this message.

You can't step into C functions, so it just steps over them.

If you hit <return>, the debugger will rerun your last command.

Hit <return> 5 times to step into and through func1().

Watch the line numbers.

]]

local str2 = [[

Stop!

You've now stepped through func1()

Notice how entering and exiting a function takes a step.

Now try the 'n' command.

(n = step to the Next line in the source code)

]]

local function func1()

print(" Stepping through func1()...")

print(" Almost there...")

end

local function func2()

print(" You used the 'n' command.")

print(" So it's skipping over the lines in func2().")

local function f()

print(" ... and anything it might call.")

end

f()

print()

print[[

The 'n' command steps to the next line in the source file.

Unlike the 's' command, it steps over function calls, and not into them.

Now try the 'c' command to continue on to the next breakpoint.

(c = Continue execution)

]]

end

dbg()

print(str1)

func1()

print(str2)

func2()

local function func3()

print[[

You are now sitting at a breakpoint inside of func3().

Let's say you got here by stepping into the function.

After poking around for a bit, you just want to step until the

function returns, but don't want to

run the next command over and over.

For this you would use the 'f' command. Try it now.

(f = Finish current function)

]]

dbg()

print[[

Now you are inside func4(), right after where it called func3().

func4() has some arguments, local variables and upvalues.

Let's assume you want to see them.

Try the 'l' command to list all the locally available variables.

(l = List local variables)

Type 'c' to continue on to the next section.

]]

end

local my_upvalue1 = "Wee an upvalue"

local my_upvalue2 = "Awww, can't see this one"

globalvar = "Weeee a global"

function func4(a, b, ...)

local c = "sea"

local varargs_copy = {...}

-- Functions only get upvalues if you reference them.

local d = my_upvalue1.." ... with stuff appended to it"

func3()

print[[

Some things to notice about the local variables list.

'...'

This is the list of varargs passed to the function.

(This only works with Lua 5.2+ or LuaJIT 2.0+)

Note: varargs are not an array, but debugger.lua stores them that way.

'my_upvalue1'

This is a local variable defined outside of, but

referenced by the function. Upvalues show up

*only* when you reference them within your

function. 'my_upvalue2' isn't in the list

because func4() doesn't reference it.

Listing the locals is nice, but sometimes there are too many to see at once.

Often times it's useful to print just a single variable,

evaluate an expression, or call a function to see what it returns.

For that you use the 'p' command.

(p = Print the result of an expression)

Try these commands:

p my_upvalue1

p 1 + 1

p print("foo")

p math.cos(0)

You can also interact with varargs. (Except on Lua 5.1)

For example:

p ...

p select(2, ...)

p {...}

Type 'c' to continue to the next section.

]]

dbg()

print[[

One thing you can't do with the print command is set variables.

Assignments are statements, and need to be run with the

'e' command. (e = Evaluate a statement)

Try these commands:

e my_upvalue1 = "foobar"

e print(my_upvalue1)

Type 'c' to continue to the next section.

]]

dbg()

end

func4(1, "two", "vararg1", "vararg2", "vararg3")

local function func5()

local my_var = "func5()"

print[[

You are now in func5() which was called from func6().

func6() was called from func7().

Try the 't' command to print out a backtrace and see for yourself.

(t = backTrace)

Type 'c' to continue to the next section

]]

dbg()

print[[

Notice that func5(), func6() and func7() all have a

'my_var' local. You can print the func5()'s my_var easily enough.

What if you wanted to see what local variables were in func6()

or func7() to see how you got where you were?

For that you use the 'u' and 'd' commands.

(u = Move up a stack frame)

(d = Move down a stack frame)

Try the 'u' and 'd' commands a few times.

Print out the value of my_var using the 'p' command each time.

Type 'c' to continue.

]]

dbg()

end

local function func6()

local my_var = "func6()"

func5()

end

local function func7()

local my_var = "func7()"

func6()

end

func7()

print[[

That leaves only one more command.

Wouldn't it be nice if there was a way to remember

all these one letter debugger commands?

Type 'h' to show the command list.

(h = Help)

Type 'c' to continue.

]]

dbg()

print[[

The following loop uses an assert-style breakpoint.

It will only engage when the conditional fails. (when i == 5)

Type 'c' to continue.

]]

for i=0, 10 do

print("i = "..tostring(i))

dbg(i ~= 5)

end

print[[

Last but not least, is the dbg.call() function.

It works sort of like Lua's xpcall() function,

but starts the debugger when an uncaught error occurs.

You can pretty much use it as a drop in replacement.

For example:

dbg.call(function()

-- Potentially buggy code goes here.

end)

Wrap it around your program's main loop or main entry point.

Then when your program crashes, you won't need to go back

and add breakpoints.

That pretty much wraps ups the basics.

Hopefully you find debugger.lua to be simple but useful.

]]

dbg.call(function()

local foo = "foo"

-- Try adding a string and integer

local bar = foo + 12

-- Program never makes it to here...

end)