Reference counting issue
Posted: Mon Sep 19, 2011 8:57 pm
A recent bug in DSB was due to the fact that while Lua table references are reference counted, any pointers to userdata that they may contain are not. Bitmaps and other such things are implemented as Lua tables in DSB, so what this means is that if you are going to use a duplicate of a bitmap, you must be sure that it is regarded as the same table by Lua.
This code is fine, because bmp2 is simply a reference to the same DSB bitmap (i.e., a Lua table) as bmp. This use case also covers parameter passing and such, and pretty much most other uses of bitmaps. In other words, if you don't know if your code is fine or not, it probably is.
The problem lies here:
Making a copy of the table (as opposed to simply another reference to the same table) will cause the underlying bitmap to be destroyed as soon as any variable that contains it goes out of scope. This means that after bmp is set to nil, the garbage collector will destroy the underlying bitmap, and using bmp2 will cause DSB to crash.
This problem is esoteric enough I have no plans to fix it at the moment, but I thought at least the problem should be documented.
Code: Select all
bmp = dsb_get_bitmap("TEST")
bmp2 = bmp
bmp = nil
The problem lies here:
Code: Select all
bmp = dsb_get_bitmap("TEST")
bmp2 = { }
for i in pairs(bmp) do
bmp2[i] = bmp[i]
end
bmp = nilThis problem is esoteric enough I have no plans to fix it at the moment, but I thought at least the problem should be documented.