How to make tcl background proc wait on a file?

Question

I'm trying to write a proc that waits in the background on a file and launches a command and keeps on getting stuck while waiting
My code:

proc stuck_on_kuku {} {  
      while {![file exists kuku]} {   
          after 3000    
      }  
   echo done sleeping ; # here my command should appear
}

Now, when I call this proc in background mode, it jams my shell until done. The shell is recording key strokes, but will only consider them after the kuku file is created
> after idle stuck_on_kuku
Also tried to create an uplevel version of the proc, but still gets the same behavior.
Isn't there a way to wait for a file creation without jamming the tcl shell?
My goal is to have 2-3 of these background runs in parallel, each waiting on something else and launching something else.

Oh, and - worth a mention - I have no access to the Threads package on this env, so must make do with vanilla tcl 8.6

Use the second argument to after to emulate threading. In other languages this is called green threads. In Javascript and Tcl it's called the event loop — slebetman

slebetman slebetman · Accepted Answer · 2020-12-01T22:37:10

The after command has a blocking and non-blocking mode. You're using it in a blocking way. Simply use it's non-blocking mode:

proc stuck_on_kuku {} {
    if {[file exists kuku]} {
        echo done sleeping ; # here your command should appear
    } else {
        after 3000 stuck_on_kuku; # simulated "threading" without threads
    }
}

Note that with this set-up you can check the condition much more often and not consume much CPU time. Try after 100 to check 10 times per second and use almost 0% CPU time.

How to make tcl background proc wait on a file?

1 Answers