TIL - X11 Selection Protocol Internals

28 Nov, 2024

Protocol Flow

1. On Copy:

   Owner sends SelectionClear to previous owner
   Owner sends SelectionNotify claiming ownership
   Sets XA_PRIMARY or CLIPBOARD atom
   

2. On Paste Request:

   Requestor -> Owner: SelectionRequest
     - target atom (desired format)
     - property atom (where to put data)
     - requestor window ID
   
   Owner -> Requestor: SelectionNotify  
     - sets property on requestor's window with data
     - or None if format unavailable
   

3. Format Discovery:

   Request with target=TARGETS
   Owner responds with array of atom IDs
   Each atom ID = supported format
   

All data transfer happens through X server window properties - no direct IPC between apps. Max property size = server's max request size (usually ~256KB).

Large transfers split into multiple INCR protocol chunks.