The contents of a GtkTextBuffer can be saved as serialized binary file. Whilst converting such a file to ascii may be problematic if the buffer contains binary image data, saving text can be achieved quite simply as follows:
$txt save myfile.ser
set file [open "myfile.ser" rb]
set DATA [read $file]
close $file
set fp [open myfile.txt w]
puts $header
set i 0
foreach line [split $DATA \n] {
if {$i == 0} {
puts $fp ":GTKTEXTBUFFERCONTENTS-0001 <text_view_markup>"
set i 1
continue}
puts $fp $line
}
close $fp
