1close(n) Tcl Built-In Commands close(n)
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8 close - Close an open channel
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11 close channelId ?r(ead)|w(rite)?
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15 Closes or half-closes the channel given by channelId.
16 ChannelId must be an identifier for an open channel such as a Tcl stan‐
18 dard channel (stdin, stdout, or stderr), the return value from an invo‐
19 cation of open or socket, or the result of a channel creation command
20 provided by a Tcl extension.
21 The single-argument form is a simple “full-close”: all buffered output
23 is flushed to the channel's output device, any buffered input is dis‐
24 carded, the underlying file or device is closed, and channelId becomes
25 unavailable for use.
26 If the channel is blocking, the command does not return until all out‐
28 put is flushed. If the channel is nonblocking and there is unflushed
29 output, the channel remains open and the command returns immediately;
30 output will be flushed in the background and the channel will be closed
31 when all the flushing is complete.
32 If channelId is a blocking channel for a command pipeline then close
34 waits for the child processes to complete.
35 If the channel is shared between interpreters, then close makes chan‐
37 nelId unavailable in the invoking interpreter but has no other effect
38 until all of the sharing interpreters have closed the channel. When
39 the last interpreter in which the channel is registered invokes close,
40 the cleanup actions described above occur. See the interp command for a
41 description of channel sharing.
42 Channels are automatically closed when an interpreter is destroyed and
44 when the process exits. From 8.6 on (TIP#398), nonblocking channels │
45 are no longer switched to blocking mode when exiting; this guarantees a │
46 timely exit even when the peer or a communication channel is stalled. │
47 To ensure proper flushing of stalled nonblocking channels on exit, one │
48 must now either (a) actively switch them back to blocking or (b) use │
49 the environment variable TCL_FLUSH_NONBLOCKING_ON_EXIT, which when set │
50 and not equal to "0" restores the previous behavior.
51 The command returns an empty string, and may generate an error if an
53 error occurs while flushing output. If a command in a command pipeline
54 created with open returns an error, close generates an error (similar
55 to the exec command.)
56 The two-argument form is a “half-close”: given a bidirectional channel │
58 like a socket or command pipeline and a (possibly abbreviated) direc‐ │
59 tion, it closes only the sub-stream going in that direction. This means │
60 a shutdown() on a socket, and a close() of one end of a pipe for a com‐ │
61 mand pipeline. Then, the Tcl-level channel data structure is either │
62 kept or freed depending on whether the other direction is still open. │
63 A single-argument close on an already half-closed bidirectional channel │
65 is defined to just “finish the job”. A half-close on an already closed │
66 half, or on a wrong-sided unidirectional channel, raises an error. │
67 In the case of a command pipeline, the child-reaping duty falls upon │
69 the shoulders of the last close or half-close, which is thus allowed to │
70 report an abnormal exit error. │
71 Currently only sockets and command pipelines support half-close. A fu‐ │
73 ture extension will allow reflected and stacked channels to do so.
74
76 This illustrates how you can use Tcl to ensure that files get closed
77 even when errors happen by combining catch, close and return:
78 proc withOpenFile {filename channelVar script} {
80 upvar 1 $channelVar chan
81 set chan [open $filename]
82 catch {
83 uplevel 1 $script
84 } result options
85 close $chan
86 return -options $options $result
87 }
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90 file(n), open(n), socket(n), eof(n), Tcl_StandardChannels(3)
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93 blocking, channel, close, nonblocking, half-close
94Tcl 7.5 close(n)