About the dll search algorithm of dlopen (patch-r3)

Corinna Vinschen corinna-cygwin@cygwin.com
Fri Aug 26 11:18:00 GMT 2016


On Aug 26 12:59, Corinna Vinschen wrote:
> Hi Michael,
> 
> On Aug 25 19:48, Michael Haubenwallner wrote:
> > Using tmp_pathbuf now, wrapped behind some trivial allocator - which
> > might fit better somewhere else than to dlfcn.cc?
> > 
> > BTW: Is it really intended for tmp_pathbuf to have a single active
> > instance (per thread) at a time?
> 
> Well, yes.  tmp_pathbuf is meant to be initialized on function entry
> (more or less, depends).  It's supposed to exist only once per frame.
> When the frame goes out of scope, the tmp_pathbuf usage counter is
> restored to the values of the parent frame.
> 
> > +   ATTENTION: Requesting memory from an instance of tmp_pathbuf breaks
> > +   when another instance on a newer stack frame has provided memory. */
> 
> I don't understand this comment, though.  tmp_pathbuf can be used
> multiple times in the same thread stackm see other Cygwin functions.
> What you can't do is to call a function, instantiate tmp_pathbuf,
> allocate memory in the called function, and then use it in the caller.
> Well, you *can* do that, but if you do this more than once, the same
> memory region is reused.
> That's the whole idea of tmp_pathbuf.
> Temporary per-thread memory for the current frame and it's child frames
> is allocated.  If a deeper frame using tmp_pathbuf goes out of scope,
> the memory is not free'd, but recycled next time temporary memory is
> needed.  The buffers are either 32K or 64K to matches the maximum long
> path length.  They are now used for any purpose where larger temporary
> per-thread memory is needed, but providing temporary long path buffers
> without killing the stack was their original purposes.

So I also don't quite understand splitting off tmp_pathbuf_allocator.

Why didn't you just include a tmp_pathbuf member into class pathfinder,
kind of like this:

  /* pathfinder.h */
  class pathfinder
  {
    tmp_pathbuf tp;
    [...]
  };

  /* dlfcn.cc */
  get_full_path_of_dll()
  {
    pathfinder finder ();	/* Has a tmp_pathbuf member */

    [...]
    finder.add_basename (basename);
    [...]
    finder.add_searchdir (dir, ...);


    return true;		/* finder goes out of scope, so
				   its tmp_pathbuf goes out of scope
				   so tmp_pathbuf::~tmp_pathbuf is
				   called and all is well. */
  }

?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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