Re: Simple Lua for scripts - Sumary

[Rici Lake <lua@...>]

On 25-Aug-05, at 6:12 PM, David Given wrote:

> On Thursday 25 August 2005 23:04, Ben Sunshine-Hill wrote:
> [...]
> > Limiting execution time: This is an OS-specific thing. The effect of
> > it, though, should be to set a line hook in the VM when you decide you
> > want a script to die suddenly. Then the script will break on the next
> > line.
>
> Not meaning to contradict you, but:
>
>   while true do end
>
> No line hooks!

He might have meant a counthook. You set the counthook to one and it 
will trigger on the next VM instruction. Setting the counthook to one 
inside of an alarm handler is a standard way of breaking Lua execution 
after a certain amount of time.

In any event, in Lua 5.0.2 at least, the above statement does trigger 
line hooks. The line hook is triggered by any backwards branch, 
regardless of line number:

function hook(what, where)
  print ("Hooked!", what, where)
  if what == "count" then debug.sethook(hook, "l")
    elseif count < 10 then count = count + 1; debug.sethook(hook, "c", 
100)
    else error("Done!")
  end
end

> count = 0
> debug.sethook(hook, "c", 100); while true do end
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
Hooked! count   nil
Hooked! line    1
stdin:5: Done!
stack traceback:
        [C]: in function `error'
        stdin:5: in function `hook'
        stdin:1: in main chunk
        [C]: ?