Method returns: stopping, setting, and showing the return value

One problem often remarked on with ruby-debug is that is that if you have debugger as the last statement of a method, the debugger call occurs in the next statement which occurs after the method returns. This problem is a result of line-event orientation that ruby-debug has.

trepan stops on events other than line events. In particular c_return and return events are the important events to stop at here.

The next stopping event when you issue finish or step< is a “return” or “c_return” event. The icons for this in output are “<” or “<C” respectively. When the debugger is stopped at one of these events, the return value can be inspected. It can be changed it too!

Here’s a session:

trepan /tmp/foo.rb
-- (/tmp/foo.rb:1)
def foo
(trepan): list
  1  ->	def foo
  2    	  5
  3    	end
  4    	puts foo
(trepan): set events return
Trace events we may stop on:
	return
(trepan): step
<- (/tmp/foo.rb:3)
end
(trepan): info return
Return value: 5
(trepan): set return "I just changed the return value"
Return value was: 5
New value is: "I just changed the return value"
(trepan): c
I just changed the return value
$ 

You can also see and set return values from C functions as well. For variety, in the session below I’ll show this from an irb rather than starting from a file.

$ irb
>> require 'trepanning'
=> true
>> Trepan.debug_str("File.basename('a/b.c')")
-- (/tmp/eval-20100331-15170-5js2i5.rb:1)
File.basename('a/b.c')
(trepan): set events c_call, c_return  # make sure we catch the right events
Trace events we may stop on:
	c_call, c_return
(trepan): s-                          # we want to step staying on the same line
CFUNC File#basename("a/b.c")
C> (eval File.basename('...:1 via /tmp/eval-20100331-15170-5js2i5.rb:1)
File.basename('a/b.c')
  1    	File.basename('a/b.c')
(trepan): s- # we want to step staying on the same line   
<C (eval File.basename('...:1 via /tmp/eval-20100331-15170-5js2i5.rb:1)
r=> "b.c"                             # return value from the function
File.basename('a/b.c')
  1    	File.basename('a/b.c')
(trepan): set return 'Hello, world!'
Return value was: "b.c"
New value is: "Hello, world!"
(trepan): c
=> "Hello, world!"                     # return value from irb reflecting above modification
>> 

The “/tmp/eval-…” position messages are because the debugger wrote the string out as a temporary file in order to show what is happening as you debug.