WM_CLIPBOARDUPDATE issue

Question

WM_CLIPBOARDUPDATE issue

drjackool 956

hi I registered my window to listen clipboard by AddClipboardFormatListener and when my window recieve WM_CLIPBOARDUPDATE I tried accessing clipboard data by isclipboardformatavailable and openclipboard data for cf_unicodetext by they fails! to solve this issue I use Sleep function with delay about 100ms before accessing clipboard and when I using sleep they works fine! this issue happens when I run my app on Win11 (some times its works but most of times not works) but when running it on Win7 (virtualbox) it works fine without sleep function. What is correct way to solve this issue? thanks

David Lowndes 2,745 Reputation points MVP

2023-07-09T20:44:10+00:00

When the API fails, what does GetLastError tell you the problem is?
I'd guess whatever application you're copying data to the clipboard with is a little slow in closing the clipboard, so that would be why the delay works. I'd recommend that you play safe and have your code loop a few times with a short delay before giving up if the error indicates the clipboard is still open elsewhere.
RLWA32 51,456 Reputation points

2023-07-10T11:23:17.4833333+00:00

I reproduced the issue in a Win10 22H2 VM. The process placing data on the clipboard used an edit control. The process that received the WM_CLIPBOARDUPDATE message and attempted to open the clipboard in this message handler received an "Access Denied" error.

Starting a timer from the clipboard update notification and accessing the clipboard while handling the WM_TIMER message worked just fine.
David Lowndes 2,745 Reputation points MVP

2023-07-10T11:27:22.4933333+00:00

How long a timer? Or is just posting a message to yourself sufficient delay?
RLWA32 51,456 Reputation points

2023-07-10T11:44:37.77+00:00

I tried re-posting a message but the delay was insufficient.

A 100ms timer seemed to be fairly reliable. Although at first blush this doesn't seem much different from Sleep() I think a timer has the benefit of not interacting with the scheduler.
David Lowndes 2,745 Reputation points MVP

2023-07-10T13:55:15.0733333+00:00

It does seem odd that the WM_CLIPBOARDUPDATE message gets sent before you can effectively use it. I wonder if it's sent when SetClipboardData is called, rather than being delayed until the process closes the clipboard.
RLWA32 51,456 Reputation points

2023-07-10T14:06:03.3033333+00:00

In my test the WM_CLIPBOARDUPDATE message was only received after the clipboard owner called CloseClipboard().
David Lowndes 2,745 Reputation points MVP

2023-07-10T14:08:41.9466667+00:00

Even more curious. I wonder what's preventing access for a short time?
drjackool 956 Reputation points

2023-07-10T17:18:58.3833333+00:00

GetLastError returns 5, Access Denied.
RLWA32 51,456 Reputation points

2023-07-10T23:45:01.87+00:00

@drjackool Are you using a Virtualbox VM when OpenClipboard fails?
RLWA32 51,456 Reputation points

2023-07-11T09:22:11.38+00:00

@David Lowndes, Maybe another listening process has also received WM_CLIPBOARDUPDATE and opened the clipboard first.
David Lowndes 2,745 Reputation points MVP

2023-07-11T10:05:44.9933333+00:00

@RLWA32 I guess that must be possible since the documentation for AddClipboardFormatListener says the WM_CLIPBOARDUPDATE messages are posted (rather than a sequential send message loop).
RLWA32 51,456 Reputation points

2023-07-11T10:42:50.03+00:00

@David Lowndes, Yes, they are posted, not sent. I checked by calling InSendMessageEx when handling WM_CLIPBOARDUPDATE.

Answer accepted by question author

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David Lowndes 2,745 Reputation points MVP

2023-07-09T20:44:10+00:00

When the API fails, what does GetLastError tell you the problem is?
I'd guess whatever application you're copying data to the clipboard with is a little slow in closing the clipboard, so that would be why the delay works. I'd recommend that you play safe and have your code loop a few times with a short delay before giving up if the error indicates the clipboard is still open elsewhere.
RLWA32 51,456 Reputation points

2023-07-10T11:23:17.4833333+00:00

I reproduced the issue in a Win10 22H2 VM. The process placing data on the clipboard used an edit control. The process that received the WM_CLIPBOARDUPDATE message and attempted to open the clipboard in this message handler received an "Access Denied" error.

Starting a timer from the clipboard update notification and accessing the clipboard while handling the WM_TIMER message worked just fine.
David Lowndes 2,745 Reputation points MVP

2023-07-10T11:27:22.4933333+00:00

How long a timer? Or is just posting a message to yourself sufficient delay?
RLWA32 51,456 Reputation points

2023-07-10T11:44:37.77+00:00

I tried re-posting a message but the delay was insufficient.

A 100ms timer seemed to be fairly reliable. Although at first blush this doesn't seem much different from Sleep() I think a timer has the benefit of not interacting with the scheduler.
David Lowndes 2,745 Reputation points MVP

2023-07-10T13:55:15.0733333+00:00

It does seem odd that the WM_CLIPBOARDUPDATE message gets sent before you can effectively use it. I wonder if it's sent when SetClipboardData is called, rather than being delayed until the process closes the clipboard.
RLWA32 51,456 Reputation points

2023-07-10T14:06:03.3033333+00:00

In my test the WM_CLIPBOARDUPDATE message was only received after the clipboard owner called CloseClipboard().
David Lowndes 2,745 Reputation points MVP

2023-07-10T14:08:41.9466667+00:00

Even more curious. I wonder what's preventing access for a short time?
drjackool 956 Reputation points

2023-07-10T17:18:58.3833333+00:00

GetLastError returns 5, Access Denied.
RLWA32 51,456 Reputation points

2023-07-10T23:45:01.87+00:00

@drjackool Are you using a Virtualbox VM when OpenClipboard fails?
RLWA32 51,456 Reputation points

2023-07-11T09:22:11.38+00:00

@David Lowndes, Maybe another listening process has also received WM_CLIPBOARDUPDATE and opened the clipboard first.
David Lowndes 2,745 Reputation points MVP

2023-07-11T10:05:44.9933333+00:00

@RLWA32 I guess that must be possible since the documentation for AddClipboardFormatListener says the WM_CLIPBOARDUPDATE messages are posted (rather than a sequential send message loop).
RLWA32 51,456 Reputation points

2023-07-11T10:42:50.03+00:00

@David Lowndes, Yes, they are posted, not sent. I checked by calling InSendMessageEx when handling WM_CLIPBOARDUPDATE.

Answer 1

Following demonstrates using a timer to handle OpenClipboard failures. Multiple instances of the clipboard format listener are running, and the DebugView output shows the number of times each instance tried to open the clipboard before it succeeded. In this example a 50ms interval was used for the timer. The image also demonstrates that the WM_CLIPBOARDUPDATE message is not posted until the clipboard is closed.

ClipboardUpdate

The next image shows results without using a timer or looping. The error message indicates which process has the clipboard open which causes the OpenClipboard function failure.

ClipboardNoTimer

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WM_CLIPBOARDUPDATE issue

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