class DEBUGGER__

This library provides debugging functionality to Ruby.

To add a debugger to your code, start by requiring debug in your program:

def say(word)
  require 'debug'
  puts word
end

This will cause Ruby to interrupt execution and show a prompt when the say method is run.

Once you're inside the prompt, you can start debugging your program.

(rdb:1) p word
"hello"

Getting help¶ ↑

You can get help at any time by pressing h.

(rdb:1) h
Debugger help v.-0.002b
Commands
  b[reak] [file:|class:]<line|method>
  b[reak] [class.]<line|method>
                             set breakpoint to some position
  wat[ch] <expression>       set watchpoint to some expression
  cat[ch] (<exception>|off)  set catchpoint to an exception
  b[reak]                    list breakpoints
  cat[ch]                    show catchpoint
  del[ete][ nnn]             delete some or all breakpoints
  disp[lay] <expression>     add expression into display expression list
  undisp[lay][ nnn]          delete one particular or all display expressions
  c[ont]                     run until program ends or hit breakpoint
  s[tep][ nnn]               step (into methods) one line or till line nnn
  n[ext][ nnn]               go over one line or till line nnn
  w[here]                    display frames
  f[rame]                    alias for where
  l[ist][ (-|nn-mm)]         list program, - lists backwards
                             nn-mm lists given lines
  up[ nn]                    move to higher frame
  down[ nn]                  move to lower frame
  fin[ish]                   return to outer frame
  tr[ace] (on|off)           set trace mode of current thread
  tr[ace] (on|off) all       set trace mode of all threads
  q[uit]                     exit from debugger
  v[ar] g[lobal]             show global variables
  v[ar] l[ocal]              show local variables
  v[ar] i[nstance] <object>  show instance variables of object
  v[ar] c[onst] <object>     show constants of object
  m[ethod] i[nstance] <obj>  show methods of object
  m[ethod] <class|module>    show instance methods of class or module
  th[read] l[ist]            list all threads
  th[read] c[ur[rent]]       show current thread
  th[read] [sw[itch]] <nnn>  switch thread context to nnn
  th[read] stop <nnn>        stop thread nnn
  th[read] resume <nnn>      resume thread nnn
  p expression               evaluate expression and print its value
  h[elp]                     print this help
  <everything else>          evaluate

Usage¶ ↑

The following is a list of common functionalities that the debugger provides.

Navigating through your code¶ ↑

In general, a debugger is used to find bugs in your program, which often means pausing execution and inspecting variables at some point in time.

Let's look at an example:

def my_method(foo)
  require 'debug'
  foo = get_foo if foo.nil?
  raise if foo.nil?
end

When you run this program, the debugger will kick in just before the foo assignment.

(rdb:1) p foo
nil

In this example, it'd be interesting to move to the next line and inspect the value of foo again. You can do that by pressing n:

(rdb:1) n # goes to next line
(rdb:1) p foo
nil

You now know that the original value of foo was nil, and that it still was nil after calling get_foo.

Other useful commands for navigating through your code are:

c: Runs the program until it either exists or encounters another breakpoint. You usually press c when you are finished debugging your program and want to resume its execution.
s: Steps into method definition. In the previous example, s would take you inside the method definition of get_foo.
r: Restart the program.
q: Quit the program.

Inspecting variables¶ ↑

You can use the debugger to easily inspect both local and global variables. We've seen how to inspect local variables before:

(rdb:1) p my_arg
42

You can also pretty print the result of variables or expressions:

(rdb:1) pp %w{a very long long array containing many words}
["a",
 "very",
 "long",
 ...
]

You can list all local variables with +v l+:

(rdb:1) v l
  foo => "hello"

Similarly, you can show all global variables with +v g+:

(rdb:1) v g
  all global variables

Finally, you can omit p if you simply want to evaluate a variable or expression

(rdb:1) 5**2
25

Going beyond basics¶ ↑

Ruby Debug provides more advanced functionalities like switching between threads, setting breakpoints and watch expressions, and more. The full list of commands is available at any time by pressing h.

Staying out of trouble¶ ↑

Make sure you remove every instance of +require 'debug'+ before shipping your code. Failing to do so may result in your program hanging unpredictably.

Debug is not available in safe mode.

Public Class Methods

break_points() click to toggle source

Returns the list of break points where execution will be stopped.

See DEBUGGER__ for more usage

# File lib/debug.rb, line 920
def break_points
  @break_points
end

context(thread=Thread.current) click to toggle source

# File lib/debug.rb, line 974
def context(thread=Thread.current)
  c = thread[:__debugger_data__]
  unless c
    thread[:__debugger_data__] = c = Context.new
  end
  c
end

debug_thread_info(input, binding) click to toggle source

# File lib/debug.rb, line 1030
def debug_thread_info(input, binding)
  case input
  when /^l(?:ist)?/
    make_thread_list
    thread_list_all

  when /^c(?:ur(?:rent)?)?$/
    make_thread_list
    thread_list(@thread_list[Thread.current])

  when /^(?:sw(?:itch)?\s+)?(\d+)/
    make_thread_list
    th = get_thread($1.to_i)
    if th == Thread.current
      @stdout.print "It's the current thread.\n"
    else
      thread_list(@thread_list[th])
      context(th).stop_next
      th.run
      return :cont
    end

  when /^stop\s+(\d+)/
    make_thread_list
    th = get_thread($1.to_i)
    if th == Thread.current
      @stdout.print "It's the current thread.\n"
    elsif th.stop?
      @stdout.print "Already stopped.\n"
    else
      thread_list(@thread_list[th])
      context(th).suspend
    end

  when /^resume\s+(\d+)/
    make_thread_list
    th = get_thread($1.to_i)
    if th == Thread.current
      @stdout.print "It's the current thread.\n"
    elsif !th.stop?
      @stdout.print "Already running."
    else
      thread_list(@thread_list[th])
      th.run
    end
  end
end

display() click to toggle source

Returns the display expression list

See DEBUGGER__ for more usage

# File lib/debug.rb, line 913
def display
  @display
end

get_thread(num) click to toggle source

# File lib/debug.rb, line 986
def get_thread(num)
  th = @thread_list.key(num)
  unless th
    @stdout.print "No thread ##{num}\n"
    throw :debug_error
  end
  th
end

interrupt() click to toggle source

# File lib/debug.rb, line 982
def interrupt
  context(@last_thread).stop_next
end

make_thread_list() click to toggle source

# File lib/debug.rb, line 1017
def make_thread_list
  hash = {}
  for th in Thread::list
    if @thread_list.key? th
      hash[th] = @thread_list[th]
    else
      @max_thread += 1
      hash[th] = @max_thread
    end
  end
  @thread_list = hash
end

resume() click to toggle source

# File lib/debug.rb, line 958
def resume
  MUTEX.synchronize do
    make_thread_list
    @thread_list.each do |th,|
      next if th == Thread.current
      context(th).clear_suspend
    end
    waiting.each do |th|
      th.run
    end
    waiting.clear
  end
  # Schedule other threads to restart as soon as possible.
  Thread.pass
end

set_last_thread(th) click to toggle source

# File lib/debug.rb, line 942
def set_last_thread(th)
  @last_thread = th
end

set_trace( arg ) click to toggle source

# File lib/debug.rb, line 932
def set_trace( arg )
  MUTEX.synchronize do
    make_thread_list
    for th, in @thread_list
      context(th).set_trace arg
    end
  end
  arg
end

stdout() click to toggle source

Returns the IO used as stdout. Defaults to STDOUT

# File lib/debug.rb, line 901
def stdout
  @stdout
end

stdout=(s) click to toggle source

Sets the IO used as stdout. Defaults to STDOUT

# File lib/debug.rb, line 906
def stdout=(s)
  @stdout = s
end

suspend() click to toggle source

# File lib/debug.rb, line 946
def suspend
  MUTEX.synchronize do
    make_thread_list
    for th, in @thread_list
      next if th == Thread.current
      context(th).set_suspend
    end
  end
  # Schedule other threads to suspend as soon as possible.
  Thread.pass
end

thread_list(num) click to toggle source

# File lib/debug.rb, line 995
def thread_list(num)
  th = get_thread(num)
  if th == Thread.current
    @stdout.print "+"
  else
    @stdout.print " "
  end
  @stdout.printf "%d ", num
  @stdout.print th.inspect, "\t"
  file = context(th).instance_eval{@file}
  if file
    @stdout.print file,":",context(th).instance_eval{@line}
  end
  @stdout.print "\n"
end

thread_list_all() click to toggle source

# File lib/debug.rb, line 1011
def thread_list_all
  for th in @thread_list.values.sort
    thread_list(th)
  end
end

waiting() click to toggle source

Returns the list of waiting threads.

When stepping through the traces of a function, thread gets suspended, to be resumed later.

# File lib/debug.rb, line 928
def waiting
  @waiting
end