From: ahoward <ahoward@...>
Date: 2003-06-06T02:20:23+09:00
Subject: Re: Infinite Loop

On Thu, 5 Jun 2003, George Moschovitis wrote:

> I 've written a fairly big ruby programm that works mostly ok.
> Sometimes the programm seems to enter an infinite loop :( Does anyone
> have a neat idea, how to debug this problem? Perhaps there is a hook
> in the interpreter to count recursive method calls that I can exploit
> or something similar?

such a hook already exists:

/usr/home/howardat/eg/ruby > ruby -e 'def foo;foo();end; foo();'
-e:1:in `foo': stack level too deep (SystemStackError)
....
....
....

so you can rule out recursive method calls and narrow it down to loops for
which the exit condition never becomes false, perhaps one which returns '0'
(which is true in ruby) or something like that.  of course you could be blocked
on io or a mutex and not actually in a loop - but that's another subject all
together.  in many cases strace (or similar) is very valuable at finding such
problems, for example running:

/usr/home/howardat/eg/ruby > strace ruby -e 'STDIN.gets' 2>&1 | tee out

gives:

  execve("/usr/local/bin/ruby", ["ruby", "-e", "STDIN.gets"], [/* 39 vars */]) = 0
  brk(0)                                  = 0x80d9280
  old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40014000
  open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or directory)
  open("/etc/ld.so.cache", O_RDONLY)      = 3
  <snip much nonsense>
	more nonsense
  </snip much nonsense>
  old_mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40015000
  ioctl(0, TCGETS, {B9600 opost isig icanon echo ...}) = 0
  read(0,
         ^
         ^
         ^
	cursor...

so whether or not the output makes complete sense, this program clears blocks
on i/o.  ruby is not so far from c/system calls that it is difficult to make
good guesses at where in the ruby program a particular line in strace has
originated from...  if it's a gui program strace can attach to a pid to do the
same.


-a
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