X Version 11 (Release 6.1)

XIfEvent(X3xlib)

XIfEvent -- check the event queue with a predicate procedure

Synopsis

   XIfEvent(display, event_return, predicate, arg)
         Display *display;
         XEvent *event_return;
         Bool (*predicate)();
         XPointer arg;
   
   Bool XCheckIfEvent(display, event_return, predicate, arg)
         Display *display;
         XEvent *event_return;
         Bool (*predicate)();
         XPointer arg;
   

   XPeekIfEvent(display, event_return, predicate, arg)
         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.

References

XAnyEvent(X3xlib), XNextEvent(X3xlib), XPutBackEvent(X3xlib), XSendEvent(X3xlib)

Xlib - C Language X Interface