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.
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. Channels are switched to blocking mode, to ensure that all output is correctly flushed before the process exits.
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.)
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
}