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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.