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.

 # File debug.rb, line 914 def break_points @break_points end 

 # File debug.rb, line 968 def context(thread=Thread.current) c = thread[:__debugger_data__] unless c thread[:__debugger_data__] = c = Context.new end c end 

 # File debug.rb, line 1024 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 

 # File debug.rb, line 907 def display @display end 

 # File debug.rb, line 980 def get_thread(num) th = @thread_list.key(num) unless th @stdout.print "No thread ##{num}\n" throw :debug_error end th end 

 # File debug.rb, line 976 def interrupt context(@last_thread).stop_next end 

 # File debug.rb, line 1011 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 

 # File debug.rb, line 952 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 

 # File debug.rb, line 936 def set_last_thread(th) @last_thread = th end 

 # File debug.rb, line 926 def set_trace( arg ) MUTEX.synchronize do make_thread_list for th, in @thread_list context(th).set_trace arg end end arg end 

 # File debug.rb, line 895 def stdout @stdout end 

 # File debug.rb, line 900 def stdout=(s) @stdout = s end 

 # File debug.rb, line 940 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 

 # File debug.rb, line 989 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 

 # File debug.rb, line 1005 def thread_list_all for th in @thread_list.values.sort thread_list(th) end end 

 # File debug.rb, line 922 def waiting @waiting end