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Debugger

Rubinius includes a reference Ruby source-level debugger.

Invoking in Code

The Rubinius debugger can be invoked from code by embedding a call to start the debugger

Consider the following code in app.rb

class Toast
  attr_accessor :setting
  def initialize
    require 'rubinius/debugger'
    Rubinius::Debugger.start
    @setting = :brown
  end
end

p Toast.new.setting

Running the code in Rubinius would produce the following:

$ rbx app.rb

| Breakpoint: Toast#initialize at app.rb:5 (15)
| 5:     Rubinius::Debugger.start
debug> help
                help: Show information about debugger commands
       b, break, brk: Set a breakpoint at a point in a method
    tb, tbreak, tbrk: Set a temporary breakpoint
           d, delete: Delete a breakpoint
             n, next: Move to the next line or conditional branch
             s, step: Step into next method call or to next line
           ni, nexti: Move to the next bytecode instruction
            f, frame: Make a specific frame in the call stack the current frame
   c, cont, continue: Continue running the target thread
       bt, backtrace: Show the current call stack
             p, eval: Run code in the current context
    dis, disassemble: Show the bytecode for the current method
             i, info: Show information about things
                 set: Set a debugger config variable
                show: Display the value of a variable or variables
debug> bt
| Backtrace:
|    0 Toast#initialize at app.rb:5 (15)
|    1 main.__script__ at app.rb:11 (46)
|    2 Rubinius::CodeLoader#load_script(debug) at kernel/delta/codeloader.rb:67 (44)
|    3 Rubinius::CodeLoader.load_script(name) at kernel/delta/codeloader.rb:91 (40)
|    4 Rubinius::Loader#script at kernel/loader.rb:460 (79)
|    5 Rubinius::Loader#main at kernel/loader.rb:571 (64)
|    6 Rubinius::Loader.main at kernel/loader.rb:609 (33)
|    7 Object#__script__ at kernel/loader.rb:621 (60)
debug> n

| Breakpoint: Toast#initialize at app.rb:6 (16)
| 6:     @setting = :brown
debug> n

| Breakpoint: Toast#initialize at app.rb:7 (21)
| 7:     @kind = :rye
debug> p @setting = :light_brown
$d0 = :light_brown
debug> c
:light_brown

As shown, help for the debugger commands is available by typing help when in the debugger.

Invoking from the Command Line

The Rubinius debugger can be invoked from the command line and will present the debugger prompt just before loading the script specified on the command line.

Consider the following Ruby code in bug.rb:

def problem_code
  puts "I have a problem"
  a = 1 + 2
  puts "a math problem" unless a == 4
end

problem_code

To debug this code, start the debugger from the command line with the -Xdebug option.

$ rbx -Xdebug bug.rb

| Breakpoint: Rubinius::Loader#debugger at kernel/loader.rb:424 (34)
| 424:         Rubinius::Debugger.start
debug> b Object#problem_code:2
* Unable to find method 'problem_code' in Object
| Would you like to defer this breakpoint to later? [y/n] y
| Defered breakpoint created.
debug> c
| Resolved breakpoint for Object#problem_code
| Set breakpoint 2: bug.rb:2 (+0)

| Breakpoint: Object#problem_code at bug.rb:2 (0)
| 2:   puts "I have a problem"
debug> n
I have a problem

| Breakpoint: Object#problem_code at bug.rb:3 (9)
| 3:   a = 1 + 2
debug> n

| Breakpoint: Object#problem_code at bug.rb:4 (16)
| 4:   puts "a math problem" unless a == 4
debug> p a = 4
$d0 = 4
debug> n

| Breakpoint: main.__script__ at bug.rb:7 (17)
| 7: problem_code
debug> c

As you can see, executing p a = 4 in the debugger changes the value of the local variable a and affects the execution of the Ruby code.

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