test chan+exec

#!/usr/bin/env tclsh

package require Tk

pack \
    [button .send -text Send -command {puts [lindex $pIn 1] $inData}] \
    [entry .b -textvariable inData] \
    [text .a]

set pIn [chan pipe]
set pOut [chan pipe]

set pr [exec cat <@ [lindex $pIn 0] >@ [lindex $pOut 1] &]

close [lindex $pIn 0]
close [lindex $pOut 1]

fconfigure [lindex $pIn 1] -buffering none
fconfigure [lindex $pOut 0] -blocking 0

fileevent [lindex $pOut 0] readable {
    .a insert end [read [lindex $pOut 0] 1000]
}