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