If to run xev and press Alt, it generates following event among others:
KeyPress event, serial 37, synthetic NO, window 0xa600001, root 0x5bc, subw 0x0, time 823983987, (672,346), root:(672,411), state 0x0, keycode 64 (keysym 0xffe9, Alt_L), same_screen YES, XLookupString gives 0 bytes: XmbLookupString gives 0 bytes: XFilterEvent returns: False What exactly "Synthetic" means in this context?
If synthetic is YES instead of NO, it means that the event is a "fake" one, generated by another X11 client with XSendEvent(3), not by the server.
Many programs (e.g. xterm, firefox) ignore synthetic events (except for ClientMessage events). This is more of a feel-good measure than any kind of real security, because an X11 client can easily generate key and mouse events via the XTest extension (present and enabled by default).
