Re: [PATCH v2] Cygwin: clipboard: Fix a bug in read().

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

On Dec  8 01:43, Mark Geisert wrote:
> Takashi Yano wrote:
> [...]
> > I think the following patch makes the intent clearer.
> > What do you think?
> > 
> > 
> >  From d0aee9af225384a24ac6301f987ce2e94f262500 Mon Sep 17 00:00:00 2001
> > From: Takashi Yano <takashi.yano@nifty.ne.jp>
> > Date: Wed, 8 Dec 2021 17:06:03 +0900
> > Subject: [PATCH] Cygwin: clipboard: Make intent of the code clearer.
> > 
> > ---
> >   winsup/cygwin/fhandler_clipboard.cc   | 4 ++--
> >   winsup/cygwin/include/sys/clipboard.h | 1 +
> >   2 files changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/winsup/cygwin/fhandler_clipboard.cc b/winsup/cygwin/fhandler_clipboard.cc
> > index 05f54ffb3..65a3cad97 100644
> > --- a/winsup/cygwin/fhandler_clipboard.cc
> > +++ b/winsup/cygwin/fhandler_clipboard.cc
> > @@ -76,7 +76,7 @@ fhandler_dev_clipboard::set_clipboard (const void *buf, size_t len)
> >         clipbuf->cb_sec  = clipbuf->ts.tv_sec;
> >   #endif
> >         clipbuf->cb_size = len;
> > -      memcpy (&clipbuf[1], buf, len); // append user-supplied data
> > +      memcpy (clipbuf->data, buf, len); // append user-supplied data
> >         GlobalUnlock (hmem);
> >         EmptyClipboard ();
> > @@ -229,7 +229,7 @@ fhandler_dev_clipboard::read (void *ptr, size_t& len)
> >         if (pos < (off_t) clipbuf->cb_size)
> >   	{
> >   	  ret = (len > (clipbuf->cb_size - pos)) ? clipbuf->cb_size - pos : len;
> > -	  memcpy (ptr, (char *) (clipbuf + 1) + pos, ret);
> > +	  memcpy (ptr, clipbuf->data + pos, ret);
> >   	  pos += ret;
> >   	}
> >       }
> > diff --git a/winsup/cygwin/include/sys/clipboard.h b/winsup/cygwin/include/sys/clipboard.h
> > index 4c00c8ea1..b2544be85 100644
> > --- a/winsup/cygwin/include/sys/clipboard.h
> > +++ b/winsup/cygwin/include/sys/clipboard.h
> > @@ -44,6 +44,7 @@ typedef struct
> >       };
> >     };
> >     uint64_t      cb_size; // 8 bytes everywhere
> > +  char          data[];
> >   } cygcb_t;
> >   #endif
> 
> Sigh.  I guess it's not possible to keep rid of a data item like I'd hoped.
> At least "data[]" is cleaner than the historical "data[1]" here.  If you
> call the item cb_data I can live with it.
> Thanks all for the discussion.

  sometype *ptr;

  ptr = (sometype *) somebuffer;
  do_something (ptr + 1);

is a perfectly valid and perfectly readable thing, and used a lot if
"sometype" is either a header in a buffer followed by arbitrary data, or
if the buffer consists of multiple packed blocks of type "sometype".

Takashi's suggestion adds the information that "sometype" is a header
followed by arbitrary data, so that's a good thing..


Corinna