Well, presumably somewhere you're calling "select_all". Try calling select_all($canvas), where $canvas is whatever variable is used to store/create your canvas.

One way to find out what is being passed is to add some debugging - try printing out the value of $c at the top of the "select_all" routine, running the program and using your menu command to 'Select All'. Hopefully you should be able to see what's being passed in, and emulate it in your own "select_all" call.

Hope this helps,
Ben.