1fileevent(n) Tcl Built-In Commands fileevent(n)
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8 fileevent - Execute a script when a channel becomes readable or
9 writable
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12 fileevent channelId readable ?script?
13 fileevent channelId writable ?script?
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19 This command is used to create file event handlers. A file event han‐
20 dler is a binding between a channel and a script, such that the script
21 is evaluated whenever the channel becomes readable or writable. File
22 event handlers are most commonly used to allow data to be received from
23 another process on an event-driven basis, so that the receiver can con‐
24 tinue to interact with the user while waiting for the data to arrive.
25 If an application invokes gets or read on a blocking channel when there
26 is no input data available, the process will block; until the input
27 data arrives, it will not be able to service other events, so it will
28 appear to the user to “freeze up”. With fileevent, the process can
29 tell when data is present and only invoke gets or read when they will
30 not block.
31 The channelId argument to fileevent refers to an open channel such as a
33 Tcl standard channel (stdin, stdout, or stderr), the return value from
34 an invocation of open or socket, or the result of a channel creation
35 command provided by a Tcl extension.
36 If the script argument is specified, then fileevent creates a new event
38 handler: script will be evaluated whenever the channel becomes read‐
39 able or writable (depending on the second argument to fileevent). In
40 this case fileevent returns an empty string. The readable and writable
41 event handlers for a file are independent, and may be created and
42 deleted separately. However, there may be at most one readable and one
43 writable handler for a file at a given time in a given interpreter. If
44 fileevent is called when the specified handler already exists in the
45 invoking interpreter, the new script replaces the old one.
46 If the script argument is not specified, fileevent returns the current
48 script for channelId, or an empty string if there is none. If the
49 script argument is specified as an empty string then the event handler
50 is deleted, so that no script will be invoked. A file event handler is
51 also deleted automatically whenever its channel is closed or its inter‐
52 preter is deleted.
53 A channel is considered to be readable if there is unread data avail‐
55 able on the underlying device. A channel is also considered to be
56 readable if there is unread data in an input buffer, except in the spe‐
57 cial case where the most recent attempt to read from the channel was a
58 gets call that could not find a complete line in the input buffer.
59 This feature allows a file to be read a line at a time in nonblocking
60 mode using events. A channel is also considered to be readable if an
61 end of file or error condition is present on the underlying file or
62 device. It is important for script to check for these conditions and
63 handle them appropriately; for example, if there is no special check
64 for end of file, an infinite loop may occur where script reads no data,
65 returns, and is immediately invoked again.
66 A channel is considered to be writable if at least one byte of data can
68 be written to the underlying file or device without blocking, or if an
69 error condition is present on the underlying file or device.
70 Event-driven I/O works best for channels that have been placed into
72 nonblocking mode with the fconfigure command. In blocking mode, a puts
73 command may block if you give it more data than the underlying file or
74 device can accept, and a gets or read command will block if you attempt
75 to read more data than is ready; a readable underlying file or device
76 may not even guarantee that a blocking [read 1] will succeed (counter-
77 examples being multi-byte encodings, compression or encryption trans‐
78 forms ). In all such cases, no events will be processed while the com‐
79 mands block.
80 In nonblocking mode puts, read, and gets never block. See the documen‐
82 tation for the individual commands for information on how they handle
83 blocking and nonblocking channels.
84 Testing for the end of file condition should be done after any attempts
86 read the channel data. The eof flag is set once an attempt to read the
87 end of data has occurred and testing before this read will require an
88 additional event to be fired.
89 The script for a file event is executed at global level (outside the
91 context of any Tcl procedure) in the interpreter in which the fileevent
92 command was invoked. If an error occurs while executing the script
93 then the command registered with interp bgerror is used to report the
94 error. In addition, the file event handler is deleted if it ever
95 returns an error; this is done in order to prevent infinite loops due
96 to buggy handlers.
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99 In this setup GetData will be called with the channel as an argument
100 whenever $chan becomes readable. The read call will read whatever
101 binary data is currently available without blocking. Here the channel
102 has the fileevent removed when an end of file occurs to avoid being
103 continually called (see above). Alternatively the channel may be closed
104 on this condition.
105 proc GetData {chan} {
107 set data [read $chan]
108 puts "[string length $data] $data"
109 if {[eof $chan]} {
110 fileevent $chan readable {}
111 }
112 }
113 fconfigure $chan -blocking 0 -encoding binary
115 fileevent $chan readable [list GetData $chan]
116 The next example demonstrates use of gets to read line-oriented data.
118 proc GetData {chan} {
120 if {[gets $chan line] >= 0} {
121 puts $line
122 }
123 if {[eof $chan]} {
124 close $chan
125 }
126 }
127 fconfigure $chan -blocking 0 -buffering line -translation crlf
129 fileevent $chan readable [list GetData $chan]
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132 fileevent is based on the addinput command created by Mark Diekhans.
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135 fconfigure(n), gets(n), interp(n), puts(n), read(n), Tcl_StandardChan‐
136 nels(3)
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139 asynchronous I/O, blocking, channel, event handler, nonblocking, read‐
140 able, script, writable.
141Tcl 7.5 fileevent(n)