flush - Flush buffered output for a channel 
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http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/flush.htm

    :   '''flush''' ''channelId''

Flushes any output that has been buffered for ''channelId''. ''ChannelId'' must be a [channel] identifier such as returned by a previous [open] or [socket] command, and it must have been opened for writing. If the channel is in blocking mode, the command does not return until all the buffered output has been flushed to the channel. If the channel is in nonblocking mode, the command may return before all buffered output has been flushed; the remainder will be flushed in the background as fast as the underlying file or device is able to absorb it.  (From: [TclHelp])
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Cases in which flushing is a good idea:
   * [puts] -nonewline'ing a string, e.g.,
======
        puts -nonewline "Enter value: "
        flush stdout
        set result [gets stdin]
======
   * when using unnamed pipe IPC 

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