Sometimes called virtual channel or stacked channel, not to be confused with virtual file systems.
Tcl 8.5 provides a facility to implement channels (as usually returned by open and socket) in pure tcl (see reflectedchan).
This enables a library to implement transformations over channels (see stacked channel) or present a file-like interface (read/puts/chan event etc.) to any data structure.
Reflected channels are created with chan create and their implementation is documented there, and http://www.tcl.tk/man/tcl8.5/TclCmd/refchan.htm
And a reflected channel example.
Meanwhile, here is an implementation of a simple reflected channel which acts as a buffer.
package provide rchan 1.0 namespace eval rchan { variable chan ;# set of known channels array set chan {} proc initialize {chanid args} { variable chan set chan($chanid) "" puts [info level 0] set map [dict create] dict set map finalize [list ::rchan::finalize $chanid] dict set map watch [list ::rchan::watch $chanid] dict set map seek [list ::rchan::seek $chanid] dict set map write [list ::rchan::write $chanid] if { 1 } { dict set map read [list ::rchan::read $chanid] dict set map cget [list ::rchan::cget $chanid] dict set map cgetall [list ::rchan::cgetall $chanid] dict set map configure [list ::rchan::configure $chanid] dict set map blocking [list ::rchan::blocking $chanid] } namespace ensemble create -map $map -command ::$chanid return "initialize finalize watch read write configure cget cgetall blocking" } proc finalize {chanid} { variable chan unset chan($chanid) puts [info level 0] } variable watching array set watching {read 0 write 0} proc watch {chanid events} { variable watching puts [info level 0] if {$events eq {}} { foreach event {read write} { set watching($event) 0 } } else { foreach event $events { set watching($event) 1 } } } proc read {chanid count} { variable chan puts [info level 0] if {[string length $chan($chanid)] < $count} { set result $chan($chanid); set chan($chanid) "" } else { set result [string range $chan($chanid) 0 $count-1] set chan($chanid) [string range $chan($chanid) $count end] } # implement max buffering variable watching variable max if {$watching(write) && ([string length $chan($chanid)] < $max)} { chan postevent $chanid write } return $result } variable max 1048576 ;# maximum size of the reflected channel proc write {chanid data} { variable chan variable max variable watching puts [info level 0] set left [expr {$max - [string length $chan($chanid)]}] ;# bytes left in buffer set dsize [string length $data] if {$left >= $dsize} { append chan($chanid) $data if {$watching(write) && ([string length $chan($chanid)] < $max)} { # inform the app that it may still write chan postevent $chanid write } } else { set dsize $left append chan($chanid) [string range $data $left] } # inform the app that there's something to read if {$watching(read) && ($chan($chanid) ne "")} { puts "post event read" chan postevent $chanid read } return $dsize ;# number of bytes actually written } proc blocking { chanid args } { variable chan puts [info level 0] } proc cget { chanid args } { variable chan puts [info level 0] } proc cgetall { chanid args } { variable chan puts [info level 0] } proc configure { chanid args } { variable chan puts [info level 0] } namespace export -clear * namespace ensemble create -subcommands {} }
catch {console show} set fd [chan create [list read write] rchan] puts $fd "Hello World" $fd write "Hello World" puts [gets $fd] proc GetData { fd args } { puts [info level 0] puts [gets $fd] } fileevent $fd readable [list GetData $fd] puts $fd "Hello Moon!" $fd write "Hello Moon!" fconfigure $fd -buffering line
::rchan::initialize rc0 {read write} ::rchan::write rc0 {Hello World} ::rchan::read rc0 4096 ::rchan::read rc0 4085 Hello World ::rchan::watch rc0 read ::rchan::write rc0 {Hello Moon!} post event read GetData rc0 ::rchan::read rc0 4096 ::rchan::read rc0 4085 Hello Moon!
[Using reflected channels to do the equivalent of
open "|simple1 |& cat" r
using exec instead of open and the "cat" helper?]
Zarutian 27. january 2006: are reflected chans just other name over rechans in Rchan allows channels to be implemented in Tcl?
Lars H: Same idea, different implementations. From the TIP discussions, I recall it being mentioned that the handling of threads and errors were tricky points for a reflected channel API.
hae 2009-26-05 Strange. The code above works so far. But puts and fconfigure do not call rchan::write and rchan::configure. However with
fconfigure $fd -blocking 0
the rchan::blocking is called.
APN 2010-07-15 If you call puts, you need to do a flush or fconfigure the channel buffering to be line (it is full by default).
APN 2010-07-15 Fixed what I think was a bug in the watch procedure when events is empty.