1fileevent(3) User Contributed Perl Documentation fileevent(3)
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6 Tk::fileevent - Execute a callback when a filehandle becomes readable
7 or writable
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10 $widget->fileevent(fileHandle,readable?,callback?)
11 $widget->fileevent(fileHandle,writable?,callback?)
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15 This command is used to create file event handlers. A file event
16 handler is a binding between a filehandle and a callback, such that the
17 callback is evaluated whenever the filehandle becomes readable or
18 writable. File event handlers are most commonly used to allow data to
19 be received from another process on an event-driven basis, so that the
20 receiver can continue to interact with the user while waiting for the
21 data to arrive. If an application invokes "<>", "sysread" or "read" on
22 a blocking filehandle when there is no input data available, the
23 process will block; until the input data arrives, it will not be able
24 to service other events, so it will appear to the user to ``freeze
25 up''. With fileevent, the process can tell when data is present and
26 only invoke gets or read when they won't block.
27 The fileHandle argument to fileevent refers to an open filehandle, such
29 as the return value from a previous open or socket command. If the
30 callback argument is specified, then fileevent creates a new event
31 handler: callback will be evaluated whenever the filehandle becomes
32 readable or writable (depending on the argument to fileevent). In this
33 case fileevent returns an empty string. The readable and writable
34 event handlers for a file are independent, and may be created and
35 deleted separately. However, there may be at most one readable and one
36 writable handler for a file at a given time in a given interpreter. If
37 fileevent is called when the specified handler already exists in the
38 invoking interpreter, the new callback replaces the old one.
39 If the callback argument is not specified, fileevent returns the
41 current callback for fileHandle, or an empty string if there is none.
42 If the callback argument is specified as an empty string then the event
43 handler is deleted, so that no callback will be invoked. A file event
44 handler is also deleted automatically whenever its filehandle is closed
45 or its interpreter is deleted.
46 A filehandle is considered to be readable if there is unread data
48 available on the underlying device. A filehandle is also considered to
49 be readable if an end of file or error condition is present on the
50 underlying file or device. It is important for callback to check for
51 these conditions and handle them appropriately; for example, if there
52 is no special check for end of file, an infinite loop may occur where
53 callback reads no data, returns, and is immediately invoked again.
54 A filehandle is considered to be writable if at least one byte of data
56 can be written to the underlying file or device without blocking, or if
57 an error condition is present on the underlying file or device.
58 Event-driven I/O works best for filehandles that have been placed into
60 nonblocking mode. In blocking mode, a "print" command may block if you
61 give it more data than the underlying file or device can accept, and a
62 "<>", "sysread" or "read" command will block if you attempt to read
63 more data than is ready; no events will be processed while the
64 commands block. In nonblocking mode "print", "<>", "sysread" and
65 "read" never block. See the documentation for the individual commands
66 for information on how they handle blocking and nonblocking
67 filehandles.
68 The callback for a file event is executed in the context of $widget
70 with which fileevent was invoked. If an error occurs while executing
71 the callback then the Tk::Error mechanism is used to report the error.
72 In addition, the file event handler is deleted if it ever returns an
73 error; this is done in order to prevent infinite loops due to buggy
74 handlers.
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77 On windows platforms fileevent is limited in the types of filehandles
78 that behave correctly. Making filehandles non-blocking is only
79 implemented on a subset of UNIX platforms (see Tk::IO).
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82 fileevent is based on the addinput command created by Mark Diekhans.
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85 Tk::IO Tk::callbacks
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88 asynchronous I/O, blocking, filehandle, event handler, nonblocking,
89 readable, callback, writable.
90perl v5.34.0 2021-07-23 fileevent(3)