luajit - ffi c call with multiple return values - some thoughts
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- Subject: luajit - ffi c call with multiple return values - some thoughts
- From: David Manura <dm.lua@...>
- Date: 2011年2月23日 02:10:03 -0500
Recording of LuaJIT C FFI calls is looking good:
double f(double, double);
but how can we make a C FFI call return multiple values?
ffi.cdef [[ double, double swap(double, double); ]] -- wishful thinking
x2,y2 = ffi.C.swap(x,y)
The traditional hack of passing pointers to the C function lacks the
typical Lua interface, and wrapping it doesn't provide the efficiency
either:
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
The struct return technique is closer:
ffi.cdef [[ typedef struct { double v[2] } tuple2_t; ]]
ffi.cdef [[ tuple2_t swap(double, double); ]]
function swap(x,y)
local t = ffi.C.swap(x,y) -- NYI: unsupported C function type
return t.v[0], t.v[1] --a.k.a. unpack(ffi.C.swap(x,y)) ?
end
except that we'd prefer LuaJIT to automatically do that unpacking.
Special attributes might force that behavior:
ffi.cdef [[ void swap(double, double, __luajit_return double *,
__luajit_return double *); ]] -- blah
ffi.cdef [[ __luajit_unpack tuple2_t swap(double, double); ]] -- hmm
ffi.cdef [[ boost::tuple<double, double> swap(double, double); ]] -- yuck
ffi.cdef [[ double, double swap(double, double); ]] -- why not?
There is also the question of handling a variable number of return
values, which the fixed-sized struct return technique doesn't simply
handle.