[[add pointer to this tcl 8.5 core man page and describe the purpose of it.]]

http://www.tcl.tk/man/tcl8.5/TclCmd/refchan.htm

[DKF]: '''Ref'''lected '''chan'''nels are channels that are implemented in Tcl code. They're also a very new feature.

[PT]: Here is an implementation of a [memchan] type memory channel. That is a channel interface to a variable holding some data that you write into it. We can use this in the vfs package to provide various implementations of [memchan] channels depending upon what packages are loaded into what version of Tcl. It illustrates the necessary construction of a simple reflected channel.

    proc vfs::memchan {} {
        # Create the channel and obtain a generated channel identifier
        set fd [chan create {read write} [namespace origin memchan_handler]]
        # Initialize the data and seek position
        set ::vfs::_memchan_buf($fd) ""
        set ::vfs::_memchan_pos($fd) 0
        return $fd
    }

    proc vfs::memchan_handler {cmd chan args} {
        upvar 1 ::vfs::_memchan_buf($chan) buf
        upvar 1 ::vfs::_memchan_pos($chan) pos
        switch -exact -- $cmd {
            initialize {
                foreach {mode} $args break
                return [list initialize finalize watch read write seek]
            }
            finalize {
                unset buf pos
            }
            seek {
                foreach {offset base} $args break
                switch -exact -- $base {
                    current { incr offset $pos }
                    end     { incr offset [string length $buf] }
                }
                return [set pos $offset]
            }
            read {
                foreach {count} $args break
                set r [string range $buf $pos [expr {$pos + $count - 1}]]
                incr pos [string length $r]
                return $r
            }
            write {
                foreach {data} $args break
                set count [string length $data]
                if { $pos >= [string length $buf] } {
                    append buf $data
                } else {
                    set last [expr { $pos + $count - 1 }]
                    set buf [string replace $buf $pos $last $data]
                }
                incr pos $count
                return $count
            }
            watch {
                # We are required to implement 'watch' but are doing nothing.
                foreach {eventspec} $args break
            }
        }
    }

Usage is something like:
 set chan [vfs::memchan]
 puts $chan "data from somewhere"
 seek $chan 0
 read $chan
 close $chan


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