June 8th, 2022

Is there a maximum size for Windows clipboard data? Because I’m getting null for something I know should be there

Raymond Chen
Raymond Chen

A customer had a program that opened a very large spreadsheet in Excel. Very large, like over 300,000 rows. They then selected all of the rows in the very large spreadsheet, copied those rows to the clipboard, and then ran a program that tried to extract the data. The program used the Get­Clipboard­Data function to retrieve the data in Rich Text Format. What they found was that the call to Get­Clipboard­Data was returning NULL. Is there a maximum size for clipboard data?

No, there is no pre-set maximum size for clipboard data. You are limited only by available memory and address space.

However, that’s not the reason why the call to Get­Clipboard­Data is failing.

There are two ways to make clipboard data available. One is to put the data directly on the clipboard at the time you take ownership of it. Another option is to declare that you have data of a particular type, but instead of putting the data on the clipboard, you say, “If anybody asks, let me know, and I’ll produce the data.” This is known as clipboard delay-rendering, and it is a common optimization for data formats that are rarely used or expensive to produce. If somebody asks for the format, the clipboard owner gets a WM_RENDER­FORMAT message to say, “Somebody wants that data, can you produce it now?”

Excel does not operate natively in Rich Text Format. The Rich Text Format data that it places on the clipboard is delay-rendered. And then when another program asks for the Rich Text Format data, Excel says, “Oh yeah, just hang on a second, let me go get that for you.” Mind you, Rich Text Format is not the best format for a data table. As its name suggests, Rich Text Format is designed for text. It can represent basic tables, but that’s not its bread and butter.

When the system asks a clipboard owner to render clipboard data, it sends the WM_RENDER­FORMAT message and waits up to 30 seconds for it to return. If the clipboard owner fails to produce the data within 30 seconds, then system abandons the request, and Get­Clipboard­Data returns NULL.

What’s happening is that this table is so large that it’s taking Excel longer than 30 seconds to produce the Rich Text Format table. The Get­Clipboard­Data function times out and returns NULL.

Next time, we’ll see what we can do to extend this timeout.

Category
Old New Thing
Topics
Code

Author

Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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  • Mark Powell

    Working in the financial services industry, I got a chuckle out of a very large spreadsheet being only about 300,000 rows.

    I’m not saying it’s a good idea, I’m not defending the practice- just pointing out that “very large” means very different things to different people, in ways I sometimes find amusing.

  • Neil Rashbrook

    At this point it’s not entirely clear that the clipboard is even a reasonable way to transfer data from Excel to this custom application.

  • Ian Yates

    Hmmm... This gives me more theories now about why I sometimes have the snipping tool in Windows 11 freeze. I'll make a small annotation, ctrl+c, alt-tab to Teams, and ctrl+v. Freeze for 30s of the snip tool. Not every time though, which has made reproducing for the feedback tool annoying.

    Something to do with delay rendering, although I feel this freeze would affect the consumer of the clipboard rather than the producer, unless there's something interesting...

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  • Frank Zammetti

    Very interesting, thanks for that writeup! It makes sense that it works that way frankly.

    But, since the client app is waiting for the producer the whole time, and I presume the client will wait effectively forever until a response is received (vis a vis, the timeout is at the system level, not the client app level), whether that response is null or actual data, how about the idea of a system-generate dialog at the...

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  • Chris Jackson · Edited

    Thanks for the write-up, very informative. I think my immediate thoughts are:
    -why 30s?
    -how one might provide extra resources to excel or similar application to increase the amount of data that can be converted and beat the current 30s limit. I suppose it depends on the application and the format of the data copied.

    Edit (off topic): is the suggestions box now deprecated? i tried in vain to find an open one.

    • Paul Sanders

      Users get bored, yeah? And Raymond did promise a follow-up…

      (Off topic) Tried to authenticate using my Windows Live ID and got “error creating user ‘1’”. Facebook worked fine, lol.

  • Kevin Spellman

    Interesting, thank you.

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