close − Close an open channel

close channelId ?r(ead)|w(rite)?

     Closes or half‐closes the channel given by channelId.

     ChannelId must be an identifier for an open channel
such as a Tcl standard channel (stdin, stdout, or stderr),
the return value from an invocation of open or socket, or
the result of a channel creation command provided by a Tcl

     The single‐argument form is a simple full‐close: 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 output 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

     If the channel is shared between interpreters, then
close makes channelId 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

     The two‐argument form is a half‐close: 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 finish the job.  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.

     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

file(n), open(n), socket(n), eof(n), Tcl_StandardChannels(3)

blocking, channel, close, nonblocking, half‐close