XIfEvent(3)

NAME

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

SYNTAX

       int XIfEvent(Display *display, XEvent *event_return, Bool (*predi-
              cate)(), XPointer arg);

       Bool XCheckIfEvent(Display *display, XEvent *event_return, Bool (*pred-
              icate)(), XPointer arg);

       int XPeekIfEvent(Display *display, XEvent *event_return, Bool (*predi-
              cate)(), 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 pro-
       cedure 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 proce-
       dure 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 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 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, XCheck-
                 IfEvent, or XPeekIfEvent 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 1.1.1                      XIfEvent(3)