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.ds xT X Toolkit Intrinsics \- C Language Interface
.ds xW Athena X Widgets \- C Language X Toolkit Interface
.ds xL Xlib \- C Language X Interface
.ds xC Inter-Client Communication Conventions Manual
.TH XIfEvent 3 "libX11 1.8.7" "X Version 11" "XLIB FUNCTIONS"
.SH NAME
XIfEvent, XCheckIfEvent, XPeekIfEvent \- check the event queue with a predicate procedure
.SH SYNTAX
.HP
int XIfEvent\^(\^Display *\fIdisplay\fP\^, XEvent *\fIevent_return\fP\^, Bool
(\^*\fIpredicate\fP\^)\^(\^)\^, XPointer \fIarg\fP\^);
.HP
Bool XCheckIfEvent\^(\^Display *\fIdisplay\fP\^, XEvent *\fIevent_return\fP\^,
Bool (\^*\fIpredicate\fP\^)\^(\^)\^, XPointer \fIarg\fP\^);
.HP
int XPeekIfEvent\^(\^Display *\fIdisplay\fP\^, XEvent *\fIevent_return\fP\^,
Bool (\^*\fIpredicate\fP\^)\^(\^)\^, XPointer \fIarg\fP\^);
.SH ARGUMENTS
.IP \fIarg\fP 1i
Specifies the user-supplied argument that will be passed to the predicate procedure.
.IP \fIdisplay\fP 1i
Specifies the connection to the X server.
.IP \fIevent_return\fP 1i
Returns either a copy of or the matched event's associated structure.
.IP \fIpredicate\fP 1i
Specifies the procedure that is to be called to determine
if the next event in the queue matches what you want.
.SH DESCRIPTION
The
.B XIfEvent
function completes only when the specified predicate
procedure returns
.B True
for an event,
which indicates an event in the queue matches.
.B XIfEvent
flushes the output buffer if it blocks waiting for additional events.
.B XIfEvent
removes the matching event from the queue
and copies the structure into the client-supplied
.B XEvent
structure.
.LP
When the predicate procedure finds a match,
.B XCheckIfEvent
copies the matched event into the client-supplied
.B XEvent
structure and returns
.BR True .
(This event is removed from the queue.)
If the predicate procedure finds no match,
.B XCheckIfEvent
returns
.BR False ,
and the output buffer will have been flushed.
All earlier events stored in the queue are not discarded.
.LP
The
.B XPeekIfEvent
function returns only when the specified predicate
procedure returns
.B True
for an event.
After the predicate procedure finds a match,
.B XPeekIfEvent
copies the matched event into the client-supplied
.B XEvent
structure without removing the event from the queue.
.B XPeekIfEvent
flushes the output buffer if it blocks waiting for additional events.
.LP
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
.BR XLockDisplay .
.LP
The predicate procedure and its associated arguments are:
.HP
Bool (\^*\fIpredicate\fP\^)\^(\^Display *\fIdisplay\fP, XEvent *\fIevent\fP, XPointer \fIarg\fP\^)
.IP \fIdisplay\fP 1i
Specifies the connection to the X server.
.IP \fIevent\fP 1i
Specifies the
.B XEvent
structure.
.IP \fIarg\fP 1i
Specifies the argument passed in from the
.BR XIfEvent ,
.BR XCheckIfEvent ,
or
.B XPeekIfEvent
function.
.LP
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
.BR True .
If it did not find a match, it must return
.BR False .
.LP
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.
.SH "SEE ALSO"
XAnyEvent(3),
XNextEvent(3),
XPutBackEvent(3)
XSendEvent(3)
.br
\fI\*(xL\fP