Re: luajit - ffi c call with multiple return values - some thoughts
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- Subject: Re: luajit - ffi c call with multiple return values - some thoughts
- From: "Robert G. Jakabosky" <bobby@...>
- Date: 2011年2月22日 23:51:10 -0800
On Tuesday 22, David Manura wrote:
>
> ffi.cdef [[ void swap(double, double, double*, double*); ]]
> function swap(x,y)
> local t = ffi.new'double[2]' -- warning: memory alloc not hoisted
> ffi.C.swap(x,y, t,t+1)
> return t[0], t[1]
> end
You could try to use a temp. value allocated outside the function:
ffi.cdef [[ void swap(double, double, double*, double*); ]]
local tmp = ffi.new'double[2]' -- allocate one instance and re-use
function swap(x,y)
ffi.C.swap(x,y, tmp, tmp+1)
return tmp[0], tmp[1]
end
As long as the C function doesn't store a reference to those pointers for
later use, it should be safe (If you are careful).
In the ZeroMQ FFI-based bindings I had a similar problem with a temp. message
structure needing an allocate (in the C bindings the structure is stack
allocated not heap allocated).
It would be nice if FFI provided a way to 'stack' allocate a cdata (i.e. mark
it as only valid for the current scope). I would think this should make it
easier for LuaJIT to hoist the alloc.
ffi.cdef [[ void swap(double, double, double*, double*); ]]
function swap(x,y)
local tmp = ffi.stack'double[2]' -- stack allocate temp. variable.
ffi.C.swap(x,y, tmp, tmp+1)
return tmp[0], tmp[1]
end
--
Robert G. Jakabosky