XSynchronize, XSetAfterFunction - enable or disable synchronization
int (*XSynchronize(Display *display, Bool
    onoff))();
int (*XSetAfterFunction(Display *display, int
    (*procedure)()))();
  - 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.
XSetErrorHandler(3)
Xlib - C Language X Interface