Ruby: debugging a live process

You can attach GDB to a Ruby process and get all sorts of information. Call with gdb /path/to/ruby <PID>, then running call rb_backtrace() from within gdb will get you a stack trace and you can raise exceptions with call rb_raise(rb_eException, "Testing...").

Neat trick, see here for the original source.

I should add that in Ruby 1.9.3 with Apple’s GDB 6.3.50 the cast to (int) isn’t necessary in the call to rb_raise shown on the page above.

Also, once in gdb, you can use the following functions to help out.

define redirect_stdout
  call rb_eval_string("$_old_stdout, $stdout = $stdout, File.open('/tmp/ruby-debug.' + Process.pid.to_s, 'a'); $stdout.sync = true")
end

define ruby_eval
  call(rb_p(rb_eval_string_protect($arg0,(int*)0)))
end

You can paste these into ~/.gdbinit in order to always have them available. Once defined, you can run redirect_stdout to have output redirected into a tmp file. Then you can run Ruby code with ruby_eval('Kernel.caller'), etc. to get access to objects, local variables, etc.

Part of this information comes from here.

Some more neat GDB tricks are here.