1XIfEvent(3)                     XLIB FUNCTIONS                     XIfEvent(3)
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NAME

6       XIfEvent,  XCheckIfEvent,  XPeekIfEvent  - check the event queue with a
7       predicate procedure
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SYNTAX

10       int XIfEvent(Display  *display,  XEvent  *event_return,  Bool  (*predi‐
11              cate)(), XPointer arg);
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13       Bool XCheckIfEvent(Display *display, XEvent *event_return, Bool (*pred‐
14              icate)(), XPointer arg);
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16       int XPeekIfEvent(Display *display, XEvent *event_return, Bool  (*predi‐
17              cate)(), XPointer arg);
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ARGUMENTS

20       arg       Specifies  the  user-supplied argument that will be passed to
21                 the predicate procedure.
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23       display   Specifies the connection to the X server.
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25       event_return
26                 Returns either a copy of or the  matched  event's  associated
27                 structure.
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29       predicate Specifies  the procedure that is to be called to determine if
30                 the next event in the queue matches what you want.
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DESCRIPTION

33       The XIfEvent function completes only when the specified predicate  pro‐
34       cedure returns True for an event, which indicates an event in the queue
35       matches.  XIfEvent flushes the output buffer if it blocks  waiting  for
36       additional  events.  XIfEvent removes the matching event from the queue
37       and copies the structure into the client-supplied XEvent structure.
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39       When the predicate procedure finds a match,  XCheckIfEvent  copies  the
40       matched  event  into  the  client-supplied XEvent structure and returns
41       True.  (This event is removed from the queue.)  If the predicate proce‐
42       dure finds no match, XCheckIfEvent returns False, and the output buffer
43       will have been flushed.  All earlier events stored in the queue are not
44       discarded.
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46       The  XPeekIfEvent  function  returns  only when the specified predicate
47       procedure returns True for an event.   After  the  predicate  procedure
48       finds  a  match, XPeekIfEvent copies the matched event into the client-
49       supplied XEvent structure without removing the event  from  the  queue.
50       XPeekIfEvent  flushes  the output buffer if it blocks waiting for addi‐
51       tional events.
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53       Each of these functions requires you to pass a predicate procedure that
54       determines if an event matches what you want.  Your predicate procedure
55       must decide if the event is useful without calling any Xlib  functions.
56       If  the  predicate directly or indirectly causes the state of the event
57       queue to change, the result is not defined.  If Xlib has been  initial‐
58       ized  for  threads, the predicate is called with the display locked and
59       the result of a call by the predicate to any Xlib function  that  locks
60       the display is not defined unless the caller has first called XLockDis‐
61       play.
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63       The predicate procedure and its associated arguments are:
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65       Bool (*predicate)(Display *display, XEvent *event, XPointer arg)
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67       display   Specifies the connection to the X server.
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69       event     Specifies the XEvent structure.
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71       arg       Specifies the argument passed in from the  XIfEvent,  XCheck‐
72                 IfEvent, or XPeekIfEvent function.
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74       The  predicate procedure is called once for each event in the queue un‐
75       til it finds a match.  After finding a match, the  predicate  procedure
76       must return True.  If it did not find a match, it must return False.
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SEE ALSO

79       XAnyEvent(3), XNextEvent(3), XPutBackEvent(3) XSendEvent(3)
80       Xlib - C Language X Interface
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84X Version 11                     libX11 1.7.2                      XIfEvent(3)
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