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