C++/VB - Clipboard internals

Asked By Colin Peters

28-Jan-10 03:20 PM

Hi,

Hopefully this is the right newsgroup.

Can someone recommend any documentation that explains what process is
responsible for administering the clipboard.

When I put a richedit control on a form and call paste, an extra thread
is created (marked "rpc"). This thread seems to disappear after a few
minutes, and I do not understand why.

Apologies if this all sounds a bit vague.

29-Jan-10 01:13 PM

Possibly.  Given that you use the singular "process", what that
documentation will tell you will not be what you are expecting.  (-:

30-Jan-10 02:45 PM

Well, what I was after was some kind of description about what happens
when you call the API to put something on or off the clipboard. I assume
there is some mechanism at play to ensure that when two processes try to
copy at the same time (because the amount of data is large) that they
are somehow coordinated. Its really this kind of detail I am after.

I am not even sure I am on the right newsgroup.

30-Jan-10 03:21 PM

Some apps handle copy&paste in quite non trivial ways.

Example: copy from IE and paste into OneNote and notepad.
Notepad instantly pastes the plain text.
By contrast, onenote takes a *lot* of time to complete the paste, it
apparently talks to the browser.
--pa

30-Jan-10 05:22 PM

Indeed, the procedure can be complex. But who holds on to the data in
between? You can copy from notepad, close notepad then start another app
and paste. So the clipboard contents are persisted system wide. This is
what I want to find out about.

30-Jan-10 11:29 PM

Apps that set the clipboard can either copy the data into shared
memory, so that it is owned by the system and the app is free to exit,
or they can just put a flag into the clipboard which effectively says

Apps which do the latter often have a user-configurable option to
populate the clipboard with the actual data on exit, or clear the
clipboard on exit. (Assuming it is still the same thing in the
clipboard, of course.)

The "call me back" method is obviously a lot more complex so it is
usually only done when providing that data takes significant time/
memory and it is likely that the only thing requesting the data, if
anyone at all, will be the same app that put it there (allowing it to
short-circuit most of that work).

01-Feb-10 08:54 PM

IIRC the interactions are complex and the semantics date back to the pre-NT
shared memory & cooperative multi-tasking environment and are overlain with
COM in many cases.  For more details look at SetClipboardData in MSDN.
There are several system defied formats, but applications frequently
register other 'custom' formats using RegisterClipboadFormat.

01-Feb-10 09:01 PM

Another interesting example: Copy a large file from a VMware virtual
machine,
and paste it on the host (or v.v.). Obviously, it will not be copied via the
clipboard.

--pa