XIfEvent(3) XLIB FUNCTIONS 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 (*predi-
cate)(), 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 proce-
dure 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 pro-
cedure 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. XPeek-
IfEvent 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 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.
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
X Version 11 libX11 1.8.12 XIfEvent(3)
Generated by dwww version 1.16 on Sat Oct 3 04:26:04 CEST 2026.