General clipboard issue on cygwin (mintty, putclip, getclip, /dev/clipboard)

[Takashi Yano]

Hi Mark,

On Wed, 25 Feb 2026 01:47:04 -0800
Mark Geisert via Cygwin <cygwin@cygwin.com> wrote:
> On 2/23/2026 5:36 PM, Takashi Yano via Cygwin wrote:
> > Hi,
> > 
> > I encountered a clipboard issue common to cygwin.
> > 
> > Affected components:
> > /dev/clipboard, putclip/getclip, mintty
> > 
> > The issue:
> > If the clipbard is opened by `OpenClipboard(NULL)`, occasionally
> > SetClipboardData() and GetClipboardData() fails with
> > ERROR_CLIPBOARD_NOT_OPEN.
> > 
> > When UltraVNC Viewer (https://uvnc.com/) is opened, this issue
> > happens with a very high probability (> ~90%).
> > 
> > $ echo AAAA > /dev/clipboard
> > echo: write error: Permission denied
> > 
> > On mintty, text selection by mouse and clicking center button
> > fails at the first attempt.
> > 
> > For putlclip/getclip, the following command outputs nothing
> > in a few attempts. ("ABCD" is lost.)
> > $ echo ABCD |putclip; for a in `seq 100`; do getclip |putclip; done; getclip
> > 
> > $
> > 
> > 
> > Solutions:
> > This can be solved by passing non-zero HWND to OpenClipboard().
> > Therefore, as for mintty, the following patch is ok I think.
> > diff --git a/src/winclip.c b/src/winclip.c
> > index 12c54525..a7cbe365 100644
> > --- a/src/winclip.c
> > +++ b/src/winclip.c
> > @@ -1324,7 +1324,7 @@ static void
> >   do_win_paste(bool do_path)
> >   {
> >     //printf("OpenClipboard win_paste\n");
> > -  if (!OpenClipboard(null))
> > +  if (!OpenClipboard(wnd))
> >       return;
> > 
> >     if (cfg.input_clears_selection)
> > 
> > However, for /dev/clipbard and {put|get}clip, there is no
> > suitable HWND to pass to OpenClipboard(). Fortunately, I
> > found the patch attached solves the /dev/clipboard issue.
> > 
> > This is not smart, however it works.
> > 
> > I think similar patch is ok for putclip/getclip.
> > 
> > Any suggestions and comments will be appreciated.
> 
> I did not try your mintty or UltraVNC tests but the others worked fine 
> for me, even when in a loop of 10,000 attempts.

In my environment, I also confirmed that a loop of 1,000 attempts
worked without the issue if UltraVNC Viewer is not opened. I guess
UltraVNC Viewer accesses clipboard very frequently.

> IIUC you are finding OpenClipboard() returning as if it's opened the 
> clipboard, but a subsequent operation claims the clipboard is not open.
> I'm curious if GetLastError() reports anything from this phantom open.

Yeah, indeed.

> As for the "not smart" patch :-) I'm leery of cpu-bound loops that might 
> not end for some unforeseen situation.  I'd rather you make use of the 
> existing max_retry mechanism.  Maybe with shorter Sleep()s, say 100ms, 
> and possibly larger retry count.
> HTH,

Thanks. That might be better. Current Sleep(1) is 1ms, so just increase
max_retry is enough I think.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>