XSynchronize, XSetAfterFunction - enable or disable synchronization

display   Specifies the connection to the X server.

       procedure Specifies the procedure to be called.

       onoff     Specifies a Boolean value that indicates whether to enable or disable synchronization.

       The  XSynchronize  function returns the previous after function.  If onoff is True, XSynchronize turns on
       synchronous behavior.  If onoff is False, XSynchronize turns off synchronous behavior.

       The specified procedure is called with only a display pointer.  XSetAfterFunction  returns  the  previous
       after function.

       XSynchronize, XSetAfterFunction - enable or disable synchronization

XSetErrorHandler(3)
       Xlib-CLanguageXInterface

X Version 11                                      libX11 1.8.12                                  XSynchronize(3)

       int (*XSynchronize(Display *display, Bool onoff))();

       int (*XSetAfterFunction(Display *display, int (*procedure)()))();