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
(*
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 InterfaceX Version 11 libX11 1.8.10 XIfEvent(3)
