Re: Table crash on callback

[Sam Roberts <sroberts@...>]

On Thu, Mar 22, 2007 at 11:58:52AM -0700, Wesley Smith wrote:
> Hi,
> I'm trying to track down a nasty crashing problem with Lua tables.  In
> the LuaODE module, there is a callback for collision detection.  I
> pass in to C a reference to a Lua function and if a collision is
> detected, I call that function.  As an example:
> 
> function collisionDetection(data, o1, o2)
>      print(data)
>      print(tostring(o1) .. " " .. tostring(o2))
> end
> 
> I'm having problems when I try to create a new field in a table within
> the callback like this:
> 
> strings = {}
> function collisionDetection(data, o1, o2)
>      strings[ tostring(o1) ]
> end

Where is there a new field being created in that code?

It is invalid syntax:

  Lua 5.1.1  Copyright (C) 1994-2006 Lua.org, PUC-Rio
  > strings={}
  > function collisionDetection(data, o1, o2)
  >> strings[ tostring(o1) ]
  >> end
  stdin:3: '=' expected near 'end'

Do you mean:

  function collisionDetection(data, o1, o2)
    return strings[tostring(o1)]
  end

or

  function collisionDetection(data, o1, o2)
    strings[tostring(o1)] = o2
  end

?

> Here's the crash stack trace.
> Exception:  EXC_BAD_ACCESS (0x0001)
> Codes:      KERN_PROTECTION_FAILURE (0x0002) at 0x06b3afd8
> 0   com.yourcompany.Lua            	0x050a1f38 newkey + 208
> 1   com.yourcompany.Lua            	0x050a3ab8 luaV_settable + 80
> 2   com.yourcompany.Lua            	0x050a48b0 luaV_execute + 1012
> 3   com.yourcompany.Lua            	0x05093c38 luaD_call + 136
> 4   com.yourcompany.Lua            	0x05088304 lua_call + 44

This is a strange backtrace, what tool is it from? Aren't you on OS X?
Can you provide a backtrace from gdb with a debug build that has line
numbers?

As a guess, you look like you are doing a lua_call(), I suspect that lua
is raising an error on the invalid syntax, and since you aren't using
lua_pcall(), the exception is not being caught, and your app is dying.

Cheers,
Sam