XCheckIfEvent






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

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);



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.

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 (This event is removed from the
queue.)  If the predicate procedure finds no match,
XCheckIfEvent returns 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









                             ‐2‐


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

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 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 If it did not find a match,
it must return

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