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X11R7.4 - man page for xcheckifevent (x11r4 section 3)

XIfEvent(3)				  XLIB FUNCTIONS			      XIfEvent(3)

NAME
       XIfEvent, XCheckIfEvent, XPeekIfEvent - check the event queue with a predicate procedure

SYNTAX
       int XIfEvent(Display *display, XEvent *event_return, Bool (*predicate)(), XPointer arg);

       Bool XCheckIfEvent(Display *display, XEvent *event_return, Bool (*predicate)(), XPointer
	      arg);

       int XPeekIfEvent(Display *display, XEvent *event_return, Bool (*predicate)(), XPointer
	      arg);

ARGUMENTS
       arg	 Specifies the user-supplied argument that will be passed to the predicate proce-
		 dure.

       display	 Specifies the connection to the X server.

       event_return
		 Returns either a copy of or  the matched event's associated structure.

       predicate Specifies the procedure that is to be called to determine if the next event in
		 the queue matches what you want.

DESCRIPTION
       The XIfEvent function completes only when the specified predicate procedure returns True
       for an event, which indicates an event in the queue matches.  XIfEvent flushes the output
       buffer if it blocks waiting for additional events.  XIfEvent removes the matching event
       from the queue and copies the structure into the client-supplied XEvent structure.

       When the predicate procedure finds a match, XCheckIfEvent copies the matched event into
       the client-supplied XEvent structure and returns True.  (This event is removed from the
       queue.)	If the predicate procedure finds no match, XCheckIfEvent returns False, and the
       output buffer will have been flushed.  All earlier events stored in the queue are not dis-
       carded.

       The XPeekIfEvent function returns only when the specified predicate procedure returns True
       for an event.  After the predicate procedure finds a match, XPeekIfEvent copies the
       matched event into the client-supplied XEvent structure without removing the event from
       the queue.  XPeekIfEvent flushes the output buffer if it blocks waiting for additional
       events.

       Each of these functions requires you to pass a predicate procedure that determines if an
       event matches what you want.  Your predicate procedure must decide if the event is useful
       without calling any Xlib functions.  If the predicate directly or indirectly causes the
       state of the event queue to change, the result is not defined.  If Xlib has been initial-
       ized for threads, the predicate is called with the display locked and the result of a call
       by the predicate to any Xlib function that locks the display is not defined unless the
       caller has first called XLockDisplay.

       The predicate procedure and its associated arguments are:

       Bool (*predicate)(Display *display, XEvent *event, XPointer arg)

       display	 Specifies the connection to the X server.

       event	 Specifies the XEvent structure.

       arg	 Specifies the argument passed in from the XIfEvent, XCheckIfEvent, or XPeek-
		 IfEvent function.

       The predicate procedure is called once for each event in the queue until it finds a match.
       After finding a match, the predicate procedure must return True.  If it did not find a
       match, it must return False.

SEE ALSO
       XAnyEvent(3X11), XNextEvent(3X11), XPutBackEvent(3X11) XSendEvent(3X11)
       Xlib - C Language X Interface

X Version 11				   libX11 1.2.1 			      XIfEvent(3)


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