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

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.

-- 
Takashi Yano <[email protected]>


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From 1e1f3aca99a9309c7521980d364c32cba02f038e Mon Sep 17 00:00:00 2001
From: Takashi Yano <[email protected]>
Date: Thu, 19 Feb 2026 16:04:15 +0900
Subject: [PATCH] Cygwin: clipboard: Add workaround for
 ERROR_CLIPBOARD_NOT_OPEN

---
 winsup/cygwin/fhandler/clipboard.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/fhandler/clipboard.cc b/winsup/cygwin/fhandler/clipboard.cc
index 12691c7c1..221aaac62 100644
--- a/winsup/cygwin/fhandler/clipboard.cc
+++ b/winsup/cygwin/fhandler/clipboard.cc
@@ -26,10 +26,16 @@ static inline bool
 open_clipboard ()
 {
   const int max_retry = 10;
+retry:
   for (int i = 0; i < max_retry; i++)
     {
       if (OpenClipboard (NULL))
-	return true;
+	{
+	  if (!GetClipboardData (CF_UNICODETEXT)
+	      && GetLastError () == ERROR_CLIPBOARD_NOT_OPEN)
+	    goto retry;
+	  return true;
+	}
       Sleep (1);
     }
   return false;
-- 
2.51.0