TCL/TK - how does one capture stderr output from a package? - redirect

Question

I have an application making use of some packages. Looking at their source code I see they're doing a simple puts stderr ... to dump out debug information. The problem is if you wrap the program with something like FreeWrap or the TDK, you lose access to the console; so we'd like to forward that stderr output to a file instead so we can see what's being printed.
I saw somewhere on StackOverflow that you can simply close the stderr channel, open a new one, and it should automatically replace the most recently closed channel like so:
close stderr
set out [open "outfile.txt" w]
puts stderr "hello world" # should output to the file
Unfortunatly this doesn't work. When I try it I get the error message: can not find channel named "stderr"

Answer 1

You can override puts so that printing to stderr can be intercepted:
set error_file [open "outfile.txt" w]
rename puts __tcl__puts
proc puts {args} {
if {[llength $args] == 2 && [lindex $args 0] eq "stderr"} {
set args [list $::error_file [lindex $args end]]
}
__tcl__puts {*}$args
}

Answer 2

This has been addressed a number of times: Use a channel interceptor (covered before, for capturing Tcl test suite output):
A channel interceptor is implemented as a channel transform; and has been covered here before.
Step 1: Define a Channel Interceptor
oo::class create ChannelSink {
variable fileHandle
method initialize {handle mode} {
set fileHandle [open "outfile.txt" w]
fconfigure $fileHandle -translation binary
return {finalize initialize write}
}
method finalize {handle} {
catch {close $fileHandle}
}
method write {handle bytes} {
puts -nonewline $fileHandle $bytes
return
}
}
The above snippet was derived from Donal's.
Step 2: Register the interceptor with stderr around your printing code
set cs [ChannelSink new]
chan push stderr $cs
puts stderr "hello world"
chan pop stderr