tcl/TK question

[Brian St. Pierre]

On Thu, Jun 16, 2011 at 12:41 PM, Jerry Feldman <gaf at gapps.blu.org> wrote:
> One possible solution that seems to work. In my wrapper script:
> <command> --arg1 --arg2.... 2>&1
>
> And in the tcl script:
>
> if [catch {open "|$command"} input] {
>
> I still get my zombie on the start command, but I query the open file list in tcl (file channels) and close any open files other than stdxxx.
>
> It's fun trying to write production code when you don't know the language :-)

The section in the man page for open called COMMAND PIPELINES doesn't
really spell it out. The `open` command returns a file descriptor. You
need to close that file descriptor to wait for the process. Here's a
quick demo:

#!/usr/bin/tclsh

# This produces a zombie, the file descriptor is "lost".
if [catch {open "|echo foo"} input] {
   puts "something went wrong: $input"
}

puts "Press ctrl-z now and do a ps to check for zombies"
after 3000

# This does not:
if [catch {set f [open "|echo bar"]} input] {
   puts "something else went wrong: $input"
}

# (The zombie is here.)
puts "Press ctrl-z now and do a ps to check for zombies"
after 3000

close $f

# (And now it's not.)
puts "Press ctrl-z now and do a ps to check for zombies"
after 3000

# Closing the file descriptor from this `open` will cause an error.
# (See the man page.)
if [catch {set f [open "|/bin/false"]} input] {
   puts "something different went wrong: $input"
}

close $f

HTH.

--
Brian St. Pierre