NAME

       close - Close an open channel

SYNOPSIS

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

       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 2
       on (TIP#398), nonblocking channels are no longer switched to blocking mode when exiting; this  guarantees 2
       a  timely  exit  even  when  the peer or a communication channel is stalled. To ensure proper flushing of 2
       stalled nonblocking channels on exit, one must now either (a) actively switch them back  to  blocking  or 2
       (b)  use  the  environment  variable  TCL_FLUSH_NONBLOCKING_ON_EXIT,  which when set and not equal to "0" 2
       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 2
       and a (possibly abbreviated) direction, it closes only the sub-stream going in that direction. This means 2
       a  shutdown() on a socket, and a close() of one end of a pipe for a command pipeline. Then, the Tcl-level 2
       channel data structure is either kept or freed depending on whether the other direction is still open.    2

       A single-argument close on an already half-closed bidirectional channel is defined to  just  “finish  the 2
       job”.   A  half-close  on  an  already closed half, or on a wrong-sided unidirectional channel, raises an 2
       error.                                                                                                    2

       In the case of a command pipeline, the child-reaping duty falls upon the shoulders of the last  close  or 2
       half-close, which is thus allowed to report an abnormal exit error.                                       2

       Currently  only sockets and command pipelines support half-close. A future extension will allow reflected 2
       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(3tcl), open(3tcl), socket(3tcl), eof(3tcl), Tcl_StandardChannels(3tcl)

KEYWORDS

       blocking, channel, close, nonblocking, half-close

Tcl                                                    7.5                                           close(3tcl)