Bi-directional pipe

a.tcl:
#!/usr/bin/tclsh             

set f [open "|./b.tcl" r+]
fconfigure $f -buffering line -blocking 0
fileevent $f readable [list data $f]     

proc data {fd} {
    if {[eof $fd]} {
        puts EOF    
        close $fd   
        exit        
    } elseif {[gets $fd line] != -1} {
        puts $line                    
    }                                 
}                                     

after 12000 puts $f reset
after 32000 puts $f quit 

vwait forever

###############################################################

b.tcl:
#!/usr/bin/tclsh             

fileevent stdin readable [list data stdin]
fconfigure stdin -blocking 0
fconfigure stdout -buffering line

proc data {fd} {
    if {[eof $fd]} {
        exit
    } elseif {[gets $fd line]} {
        if {$line eq "quit"} {
            exit
        } elseif {$line eq "reset"} {
            set ::counter 0
        }
    }
}

proc output {} {
    puts [incr ::counter]
    after 5000 output
}

output
vwait forever