close(n)                      Tcl Built-In Commands                     close(n)


       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 extension.

       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 complete.

       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 command.)

       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

Tcl                                    7.5                              close(n)