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

[png]

Raymond Chen

June 8th, 20224

   

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.

[png]

Raymond Chen

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Tagged Code

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[png]Raymond Chen
June 6, 2022
9 comments
Converting between Windows FILETIME and Unix time_t without having to
type the magic number 116444736000000000
It's not exactly a number that you can rattle off the top of your
head.
[png]Raymond Chen
June 2, 2022
1 comment

4 comments

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  * [png]
    Kevin Spellman June 8, 2022 10:34 am
    collapse this comment  
   
    Interesting, thank you.

    Log in to Reply
  * [png]
    Chris Jackson June 8, 2022 12:17 pm
    collapse this comment  
   
    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.

    Log in to Reply
  * [png]
    Frank Zammetti June 8, 2022 2:00 pm
    collapse this comment  
   
    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 end of the timeout period that says something like
    "This operation is taking longer than expected, do you want to
    continue to wait?" If yes, wait another 30 seconds, and so on,
    for as long as the user wants or it takes to get the data.

    Given the proper wording in the dialog, I could see that being
    pretty user-friendly and would allow for arbitrarily large
    clipboard content to be delivered even if it took minutes, and
    this way you don't have to try and come up with the "right" wait
    time (I might even suggest lowering that 30 seconds to 10 or 15
    seconds, but that's a separate consideration).

    Log in to Reply
  * [png]
    Ian Yates June 8, 2022 2:07 pm
    collapse this comment  
   
    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 going on with
    clipboard history in the middle

    Log in to Reply

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