DEBUGGER__

Class

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.

Constants

MUTEX

No documentation available

Class Methods

break_points

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 921
def break_points
  @break_points
end

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

See DEBUGGER__ for more usage

context(thread=Thread.current)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 975
def context(thread=Thread.current)
  c = thread[:__debugger_data__]
  unless c
    thread[:__debugger_data__] = c = Context.new
  end
  c
end

No documentation available

debug_thread_info(input, binding)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 1054
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

No documentation available

display

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 914
def display
  @display
end

Returns the display expression list

See DEBUGGER__ for more usage

get_thread(num)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 987
def get_thread(num)
  th = @thread_list.key(num)
  unless th
    @stdout.print "No thread ##{num}\n"
    throw :debug_error
  end
  th
end

No documentation available

interrupt

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 983
def interrupt
  context(@last_thread).stop_next
end

No documentation available

make_thread_list

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 1041
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

No documentation available

resume

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 959
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

No documentation available

set_last_thread(th)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 943
def set_last_thread(th)
  @last_thread = th
end

No documentation available

set_trace( arg )

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 933
def set_trace( arg )
  MUTEX.synchronize do
    make_thread_list
    for th, in @thread_list
      context(th).set_trace arg
    end
  end
  arg
end

No documentation available

stdout

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 902
def stdout
  @stdout
end

Returns the IO used as stdout. Defaults to STDOUT

stdout=(s)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 907
def stdout=(s)
  @stdout = s
end

Sets the IO used as stdout. Defaults to STDOUT

suspend

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 947
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

No documentation available

thread_list(num)

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 996
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

No documentation available

thread_list_all

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 1035
def thread_list_all
  for th in @thread_list.values.sort
    thread_list(th)
  end
end

Prints all threads in @thread_list to @stdout. Returns a sorted array of values from the @thread_list hash.

While in the debugger you can list all of the threads with: DEBUGGER__.thread_list_all

(rdb:1) DEBUGGER__.thread_list_all
+1 #<Thread:0x007fb2320c03f0 run> debug_me.rb.rb:3
 2 #<Thread:0x007fb23218a538@debug_me.rb.rb:3 sleep>
 3 #<Thread:0x007fb23218b0f0@debug_me.rb.rb:3 sleep>
[1, 2, 3]

Your current thread is indicated by a +

Additionally you can list all threads with th l

(rdb:1) th l
 +1 #<Thread:0x007f99328c0410 run>  debug_me.rb:3
  2 #<Thread:0x007f9932938230@debug_me.rb:3 sleep> debug_me.rb:3
  3 #<Thread:0x007f9932938e10@debug_me.rb:3 sleep> debug_me.rb:3

See DEBUGGER__ for more usage.

waiting

lib/debug.rb View on GitHub

          
            # File tmp/rubies/ruby-2.4.10/lib/debug.rb, line 929
def waiting
  @waiting
end

Returns the list of waiting threads.

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