From: Aaron Harsh <harsh@...>
Date: 2001-08-17T05:56:58+09:00
Subject: [ruby-talk:19918] Why the " * volatile " declarations in the C source?

Sorry in advance if this is a stupid question, but...

I noticed a lot of pointers in the C source being declared as '* 
volatile'.  For example, this declaration at the top of rb_eval in eval.c:

   NODE * volatile node = n;

I would have just ignored them if they'd been 'volatile NODE * node', 
but 'NODE * volatile' is pretty interesting.  No one on #ruby-lang at 
the time was quite sure, so I thought I'd bug c.l.r.

Here are my guesses so far, and my reasons for rejecting them:

1) The garbage collector could conceivably move an object, and would 
need to modify any pointers to the old location.  This one seemed 
promising, but I couldn't see any compaction in the GC code, and the 
#ruby-lang'ers confirmed that Ruby's GC doesn't compact

2) The 'volatile' thing might just be a trick to make sure the pointer 
is stored on the stack (rather than a register), so that the GC sees it 
during marking.  But it seems a lot more straightforward to just scan 
through the registers during mark (rather than put a 'volatile' 
definition in every place where a variable could be the only pointer to 
an object).

So I'm stumped, and I'm losing sleep over this.  What's the real reason?