[SDL] SDL Clipboard API: Recommendations based on research

[Bob Pendleton]

On Tue, Jul 20, 2010 at 3:09 AM, Sam Lantinga <slouken at libsdl.org> wrote:
> Interestingly enough, I just implemented simple text clipboard support
> in SDL 1.3 last week:
> http://hg.libsdl.org/SDL/file/f8c3870af5a2/include/SDL_clipboard.h
>
> I started playing around with images but that got complicated fast.
>
> There's also a cross-platform difference between X11 and Windows and
> Mac OS X: On Windows and Mac OS X you can inexpensively generate an
> event when the clipboard contents changes.  As far as I can tell on
> X11 you have to do a fairly expensive poll to achieve the same result.
>
> Anyway, FYI...

Yeah, it had to be that way. Windows and Mac OS were written with the
assumption that the graphics system and the window manager are part of
the OS and all applications run on the same machine that the graphics
and  window manager are running on. X didn't make that assumption. X
assumed that the graphics system (the X server) would run on one
machine, the window manager could run on another machine, and each
application might run on a different machine. So, when you cut and
past on Windows/Mac the data only has to move from one part of memory
to another on the same disk. On X if I cut from one application It has
to be sent to the server over the network. A paste has to reach out to
the server and ask for information. It would seem to be perfectly
reasonable for the server to send every application an event when the
clipboard changes. But, X has a subtext that was there from the
beginning that doesn't seem to be in either Windows or Mac OS.It was
always assumed that X would be used in high security environments. In
environments like that each application has a security rating and an
access list. (Am I allowed to say "Tempest" these days? Oh, just found
it in Wikipedia :-)  If my application does a screen dump every thing
on the screen that it isn't allowed to see (even icons) must be
removed from the screen image before it is sent along to my
application. If I cut from a low security app I may be able to paste
into a high security app, but not the other way around. The existence
of higher security applications must be hidden from low security
applications and everyone else. I can't even send an event saying that
something has been added to the clip board because if I do then
someone tracking the mouse and sniffing the network would be able to
detect that a high security application is running. That would tell
someone when to start looking for "interesting" traffic on the
network.

You can't send an event to let an application know that something has
been added to the clipboard because you might compromise national
security.

Whenever you run into something that makes no ()&)^&*_(^%)*_& sense in
X it is most likely that simple logical way of doing things would
create network traffic that could be correlated with other information
to reveal the existence of a high security application and maybe even
which pixels on the screen are occupied by that application. That
information would allow the right kind of system to know which key
strokes are going to the secure application. Security people are
scary, very scary.

Bob Pendleton

>
> On Tue, Jul 13, 2010 at 11:22 PM, Nathaniel J Fries <nfries88 at yahoo.com> wrote:
>> Okay, I haven't produced any code based on my research (other than text-only
>> copy/paste on Windows and OS X, which is used in a GPL-licensed project).
>>
>> But here's what I've found that really affects the API:
>> 1) Using the freedesktop.org (X11) clipboard method, copy/paste data is held
>> in the CLIPBOARD selection and drag'n'drop data in the PRIMARY selection.
>> 2) Drag'n'drop on Windows and Cocoa (OS X) requires a defined area.
>> 3) This should already be known by Sam from his SDL_scrap work, but on X11
>> you request clipboard data and then receive an X11 event with the requested
>> data.
>> 4) On all three systems, Images and Text are treated differently.
>>
>> Based on this, here's my suggestion on a Copy/Paste API:
>> /* structures and enums, self-explanatory I'd think */
>> typedef enum SDL_ClipType {
>> SDL_CLIPTYPE_TEXT,
>> SDL_CLIPTYPE_UTF8,
>> SDL_CLIPTYPE_IMAGE
>> } SDL_ClipType;
>> typedef struct SDL_ClipData {
>> SDL_ClipType type;
>> Uint32 length; /* note: ignored in the case of an image */
>> union {
>> char* text;
>> unsigned char* utf8;
>> SDL_Surface* image;
>> };
>> } SDL_ClipData;
>>
>> /* functions */
>> /* returns non-zero if clipboard data was available. The actual data will
>> appear in the SDL Event queue */
>> SDL_Bool SDL_RequestClipData(SDL_ClipType type);
>>
>> /* same as above, but you wait for the event (X11) instead. This will seem
>> more normal to an application developer, and maybe should be just done like
>> this anyway */
>> SDL_Bool SDL_WaitClipData(SDL_ClupType type, void* buffer, Uint32 buflen,
>> Uint32* size); /* alt: SDL_Bool SDL_WaitClipData(SDL_ClipType type,
>> SDL_ClipData* data); */
>>
>> /* this function is pretty self-explanatory */
>> void SDL_AddClipData(SDL_ClipType type, void* data, Uint32 length); /* alt:
>> void SDL_AddClipData(SDL_ClipType type, SDL_ClipData* data); */
>>
>> /* clears the clipboard of all contents */
>> void SDL_ClearClip();
>>
>> and we add a new event for receiving data from the clipboard:
>> typedef struct SDL_ClipEvent {
>> Uint8 type;
>> SDL_ClipData data; /* alt: SDL_ClipType type; Uint32 length; union { char*
>> text; unsigned char* utf8; SDL_Surface* image; }; */
>> } SDL_ClipEvent;
>>
>> As for Drag'n'Drop:
>> /* creates a new drag'n'drop region. returns an identifier for the region */
>> Uint32 SDL_CreateDNDArea(SDL_Rect* rect);
>>
>> /* destroys the region */
>> void SDL_DestroyDNDArea(Uint32 areaId);
>>
>> /* SDL application decided to start drag'n'drop-ing something */
>> void SDL_BeginDND(Uint32 areaId, SDL_ClipData data); /* alt: void
>> SDL_BeginDND{Text,Unicode,Image}(Uint32 areaId, SDL_ClipType type, Uint32
>> len, {char* text, unsigned char* utf8, SDL_Surface* image}); */
>>
>> /* SDL application decides to clear DND data */
>> void SDL_ClearDND();
>>
>> and again, new SDL events:
>> /* when an item is dragged over a DND area */
>> typedef struct SDL_DragOverEvent {
>> Uint32 areaId;
>> SDL_ClipData data; /* alt... you know, same as paste event */
>> };
>> /* when the user finally releases the mouse over a DND area */
>> typedef struct SDL_DragDropEvent {
>> Uint32 areaId;
>> SDL_ClipData data; /* alt... you know, same as paste event */
>> };
>>
>>
>>
>> Well, that's it. Hopefully whoever actually does this finds the information
>> useful, and hopefully I don't bore anyone or leave them confused.
>>
>> ________________________________
>> EM3 Nathaniel Fries, U.S. Navy
>>
>> http://natefries.net/
>> _______________________________________________
>> SDL mailing list
>> SDL at lists.libsdl.org
>> http://lists.libsdl.org/listinfo.cgi/sdl-libsdl.org
>>
>>
>
>
>
> --
>     -Sam Lantinga, Founder and President, Galaxy Gameworks LLC
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>



-- 
+-----------------------------------------------------------
+ Bob Pendleton: writer and programmer
+ email: Bob at Pendleton.com
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