more gc gotchas
- From: William Adams <william_a_adams@xxxxxxx>
- To: <luajit@xxxxxxxxxxxxx>
- Date: Mon, 28 May 2012 13:53:13 +0000
LuaJIT offers some patterns that you don't see in regular Lua. As discussed
here previously, you have to be very careful with ffi.cast. I had code that
looked similar to the following:
local alltext = file:read("*all")
local bytes = ffi.cast("const uint8_t *", alltext)
local size = string.len(alltext)
local rowsize = somenumber
callfunc(bytes, size, rowsize)
function callfunc(bytes, size, rowsize)
local bytearray = ffi.new("uint8_t["..size.."]");
if bytesalign(bytes, size, 4) then
ffi.copy(bytearray, bytes, size)
else
-- not aligned on 4 byte boundary, so
-- copy one row at a time
for row=1,height do
ffi.copy(dst, src, rowsize);
end
end
end
It's not a complete example, but it serves for this explanation. In the
callfunc(), when I was copying memory, one big chunk at a time, there was no
problem. I'm assuming because the GC does not kick in any time before that
operation occurs. But, when the slower operation of copying a "row" at a time
was utilized, at some point (mysteriously after 64 iterations through the
loop), the thing would just crash. Seeing no obvious problem, I started to
think about the GC and casts. In this case, the 'bytes = ffi.cast...' code was
the culprit. I changed the code such that the cast now happens withinn
callfunc() instead, and all is right with the world.
local alltext = file:read("*all")
local rowsize = somenumber
callfunc(alltext, rowsize)
function callfunc(str, rowsize)
local bytes = ffi.cast("const uint8_t *", str)
local size = string.len(str)
...
This way, the "alltext" will stick around at least as long as the callfunc()
function is being run. The cast within the function will also stick around
long enough, and the underlying buffer won't be cleaned up out from under it as
was happening before.
It's one of those subtle things that a slow brain like mine takes a while to
catch. So, I put it here as an example for those who may follow. Beware the
gc...
-- William
===============================
- Shaping clay is easier than digging it out of the ground.
http://williamaadams.wordpress.com
http://www.thingiverse.com/WilliamAAdams
https://github.com/Wiladams
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