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 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
       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 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
       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 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
                 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.

       Note  that  the  predicate  procedure  is  called with the display locked, and thus is not
       allowed to call any function that would take the display lock again.

SEE ALSO

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