A '''channel transform''' is a mapping applied at the byte-level to a [channel]. Note that it does not operate on strings; Tcl converts to/from the channel's [encoding] at a level that is closer to the script level than transforms operate at.

The [TLS] extension uses transforms internally.

Transforms may be implemented in Tcl (from 8.6 onwards) with the help of [chan push] and [chan pop].

[PT]: Here is an example of a base64 channel transform using Tcl 8.6 features. You push this transform onto any other sort of channel and it will base64 encode the data as it passes through.

 # A base64 channel transform.
 #
 # usage:  chan push $channel ::tcl::binary::transform_base64
 #
 package require Tcl 8.6
 
 proc ::tcl::binary::K {a b} {set a}
 
 proc ::tcl::binary::transform_base64 {cmd handle args} {
    upvar #0 [namespace current]::_transform_r_$handle rstate
    upvar #0 [namespace current]::_transform_w_$handle wstate
    switch -exact -- $cmd {
        initialize {
            set rstate ""; set wstate ""
            return [list initialize finalize read write flush drain]
        }
        finalize {
            unset rstate wstate
            return
        }
        write {
            foreach {buffer} $args break
            append wstate $buffer
            set len [string length $wstate]
            set end [expr {$len - ($len % 3)}]
            set edge [expr {$end - 1}]
            set res [binary encode base64 [string range $wstate 0 $edge]]
            set wstate [string range $wstate $end end]
            return $res
        }
        flush {
            set buffer [K $wstate [set wstate ""]]
            set res [binary encode base64 $buffer]
            return $res
        }
        read {
            foreach {buffer} $args break
            append rstate $buffer
            set len [string length $rstate]
            set end [expr {$len - ($len % 4)}]
            set edge [expr {$end - 1}]
            set res [string range $rstate 0 $edge]
            set rstate [string range $rstate $end end]
            set resx [binary decode base64 $res]
            if {[string length $resx]%3 != 0} { puts stderr "alert: $res" }
            return $resx
        }
        drain {
            set res [K [binary decode base64 $rstate] [set rstate ""]]
            return $res
        }
    }
 }

----
See also [stacked channel].

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%| [Category Channel] | [Category Concept] |%
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