One of the neatest ways of doing this is to stack on a channel transform that redirects stdout to where you want it to go. This works even if the write to stdout happens from C code or in a different thread as it plugs into the channel machinery. The code is a little bit long (and requires Tcl 8.6) but is reliable and actually mostly very simple.
package require Tcl 8.6; # *REQUIRED* for [chan push] and [chan pop]
proc RedirectorCallback {targetHandle op args} {
# The switch/lassign pattern is simplest way of doing this in one procedure
switch $op {
initialize {
lassign $args handle mode
# Sanity check
if {$mode ne "write"} {
close $targetHandle
error "this is just a write transform"
}
# List of supported subcommands
return {initialize finalize write}
}
finalize {
lassign $args handle
# All we need to do here is close the target file handle
close $targetHandle
}
write {
lassign $args handle buffer
# Write the data to *real* destination; this does the redirect
puts -nonewline $targetHandle $buffer
# Stop the data going to *true* stdout by returning empty string
return ""
# If we returned the data instead, this would do a 'tee'
}
default {
error "unsupported subcommand"
}
}
}
# Here's a wrapper to make the transform easy to use
proc redirectStdout {file script} {
# Stack the transform onto stdout with the file handle to write to
# (which is going to be $targetHandle in [redirector])
chan push stdout [list RedirectorCallback [open $file "wb"]]
# Run the script and *definitely* pop the transform after it finishes
try {
uplevel 1 $script
} finally {
chan pop stdout
}
}
How would we actually use this? It's really very easy in practice:
# Exactly the code you started with
proc pptable {l1 l2} {
foreach i1 $l1 i2 $l2 {
puts " [format %6.2f $i1]\t[format %6.2f $i2]"
}
}
# Demonstrate that stdout is working as normal
puts "before"
# Our wrapped call that we're capturing the output from; pick your own filename!
redirectStdout "foo.txt" {
pptable {1.2 1.3 1.4} {6.9 6.8 6.7}
}
# Demonstrate that stdout is working as normal again
puts "after"
When I run that code, I get this:
bash$ tclsh8.6 stdout-redirect-example.tcl
before
after
bash$ cat foo.txt
1.20 6.90
1.30 6.80
1.40 6.70
I believe that's precisely what you are looking for.
You can do this with less code if you use Tcllib and TclOO to help deal with the machinery:
package require Tcl 8.6
package require tcl::transform::core
oo::class create WriteRedirector {
superclass tcl::transform::core
variable targetHandle
constructor {targetFile} {
set targetHandle [open $targetFile "wb"]
}
destructor {
close $targetHandle
}
method write {handle buffer} {
puts -nonewline $targetHandle $buffer
return ""
}
# This is the wrapper, as a class method
self method redirectDuring {channel targetFile script} {
chan push $channel [my new $targetFile]
try {
uplevel 1 $script
} finally {
chan pop $channel
}
}
}
Usage example:
proc pptable {l1 l2} {
foreach i1 $l1 i2 $l2 {
puts " [format %6.2f $i1]\t[format %6.2f $i2]"
}
}
puts "before"
WriteRedirector redirectDuring stdout "foo.txt" {
pptable {1.2 1.3 1.4 1.5} {6.9 6.8 6.7 6.6}
}
puts "after"
