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

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
}