close(n) Tcl Built-In Commands close(n)
NAME
close - Close an open channel
SYNOPSIS
close channelId ?r(ead)|w(rite)?
DESCRIPTION
Closes or half-closes the channel given by channelId.
ChannelId must be an identifier for an open channel such as a Tcl stan-
dard channel (stdin, stdout, or stderr), the return value from an invo-
cation of open or socket, or the result of a channel creation command
provided by a Tcl extension.
The single-argument form is a simple all buffered output is flushed to
the channel's output device, any buffered input is discarded, the
underlying file or device is closed, and channelId becomes unavailable
for use.
If the channel is blocking, the command does not return until all out-
put is flushed. If the channel is nonblocking and there is unflushed
output, the channel remains open and the command returns immediately;
output will be flushed in the background and the channel will be closed
when all the flushing is complete.
If channelId is a blocking channel for a command pipeline then close
waits for the child processes to complete.
If the channel is shared between interpreters, then close makes chan-
nelId unavailable in the invoking interpreter but has no other effect
until all of the sharing interpreters have closed the channel. When
the last interpreter in which the channel is registered invokes close,
the cleanup actions described above occur. See the interp command for a
description of channel sharing.
Channels are automatically closed when an interpreter is destroyed and
when the process exits. From 8.6 on (TIP#398), nonblocking channels
are no longer switched to blocking mode when exiting; this guarantees a
timely exit even when the peer or a communication channel is stalled.
To ensure proper flushing of stalled nonblocking channels on exit, one
must now either (a) actively switch them back to blocking or (b) use
the environment variable TCL_FLUSH_NONBLOCKING_ON_EXIT, which when set
and not equal to "0" restores the previous behavior.
The command returns an empty string, and may generate an error if an
error occurs while flushing output. If a command in a command pipeline
created with open returns an error, close generates an error (similar
to the exec command.)
The two-argument form is a given a bidirectional channel like a socket
or command pipeline and a (possibly abbreviated) direction, it closes
only the sub-stream going in that direction. This means a shutdown() on
a socket, and a close() of one end of a pipe for a command pipeline.
Then, the Tcl-level channel data structure is either kept or freed
depending on whether the other direction is still open.
A single-argument close on an already half-closed bidirectional channel
is defined to just A half-close on an already closed half, or on a
wrong-sided unidirectional channel, raises an error.
In the case of a command pipeline, the child-reaping duty falls upon
the shoulders of the last close or half-close, which is thus allowed to
report an abnormal exit error.
Currently only sockets and command pipelines support half-close. A
future extension will allow reflected and stacked channels to do so.
EXAMPLE
This illustrates how you can use Tcl to ensure that files get closed
even when errors happen by combining catch, close and return:
proc withOpenFile {filename channelVar script} {
upvar 1 $channelVar chan
set chan [open $filename]
catch {
uplevel 1 $script
} result options
close $chan
return -options $options $result }
SEE ALSO
file(n), open(n), socket(n), eof(n), Tcl_StandardChannels(3)
KEYWORDS
blocking, channel, close, nonblocking, half-close
Tcl 7.5 close(n)
tcl 8.6.0 - Generated Tue Jan 8 06:31:08 CST 2013