With most installations of X11 / Xlib, there is also an extension library: libXFixes. With this, it is possible to receive an event whenever the clipboard (or any other selection) changes (even if our program is not the owner). [0]

I used this in the past when porting a Windows text editor to Linux that needed to know when the clipboard had changed.

In the C bindings, it corresponds to XFixesSetSelectionOwnerNotifyMask / XFixesSelectionNotifyEvent. There is also a SelectionNotify event in the core X11 protocol, but I'm pretty sure it is different (occurs in response to XConvertSelection - asking for the clipboard contents). Though it has been some time since I wrote the code I'm looking at.

[0]: https://www.x.org/releases/current/doc/fixesproto/fixesproto... (6. Selection Tracking)