Detect end of TCL background process in a TCL script

Question

I\'m working on a program the uses an EXEC command to run a make file. This can take a long time so I want to put it in the background so the GUI doesn\'t lock up. However I

Answer 1

You want to run the make process in a pipeline and use the event loop and fileevent to monitor its progress (see http://wiki.tcl.tk/880)

proc handle_make_output {chan} {
    # The channel is readable; try to read it.
    set status [catch { gets $chan line } result]
    if { $status != 0 } {
        # Error on the channel
        puts "error reading $chan: $result"
        set ::DONE 2
    } elseif { $result >= 0 } {
        # Successfully read the channel
        puts "got: $line"
    } elseif { [chan eof $chan] } {
        # End of file on the channel
        puts "end of file"
        set ::DONE 1
    } elseif { [chan blocked $chan] } {
        # Read blocked.  Just return
    } else {
        # Something else
        puts "can't happen"
        set ::DONE 3
    }
}

set chan [open "|make" r]
chan configure $chan -blocking false
chan event $chan readable [list handle_make_output $chan]
vwait ::DONE
close $chan

I'm not certain about the use of vwait within Tk's event loop. Perhaps an expert will help me out here.

Answer 2

As @glenn-jackman pointed out, the use of fileevent is preferred (because it should work everywhere).

proc handle_bgexec {callback chan} {
    append ::bgexec_data($chan) [read $chan]
    if {[eof $chan]} {
        # end of file, call the callback
        {*}$callback $::bgexec_data($chan)
        unset ::bgexec_data($chan)
    }
}

proc bgexec {callback args} {
    set chan [open "| $args" r]
    fconfigure $chan -blocking false
    fileevent $chan readable [list handle_bgexec $callback $chan]
    return
}

Invoke this as bgexec job_done cmd /c start /wait cmd /c make all-all. job_done gets called with the output of the command after it finishes.

It is also possible to use threads for this things, but this requires a threaded tcl build (which is now default for all platforms AFAIK, but older versions of Tcl esp. under unix don't build a threaded Tcl by default.) and the Thread package (which is included by default). An approach to use it with Threads would be:

thread::create "[list exec cmd /c start /wait cmd /c make all-all];[list thread::send [thread::id] {callback code}];thread::exit"

If you need to call this on a regular basis it might be worth to use only one worker thread instead of creating a new one for each job.

Edit: Add /wait as parameter for start the keep the first cmd running.

cmd /c start /wait cmd /c make all-all