linux clipboard read/write in C

Maybe you can look at xclip and see how they have done it.

It provides an interface to X selections ("the clipboard") from the command line. It can read data from standard in or a file and place it in an X selection for pasting into other X applications. xclip can also print an X selection to standard out, which can then be redirected to a file or another program.

I might be shooting myself in the foot, but this could give you a hint on how to do the clipboard for kde, not sure about Gnome myself but try it, the script is in python and demonstrates how to get/set stuff on the clipboard, via using dcop and klipper, it is on this site here.

Hope this helps, Best regards, Tom.