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 procedure.

       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 indi-
       cates 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 discarded.

       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 addi-
       tional 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 initialized 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 XLockDis-
       play.

       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 XPeekIfEvent func-
                 tion.

       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 1.1.5               XIfEvent(3)