close(3tcl)                  Tcl Built-In Commands                 close(3tcl)


       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(3tcl), open(3tcl), socket(3tcl), eof(3tcl),

       blocking, channel, close, nonblocking, half-close

Tcl                                   7.5                          close(3tcl)