I'm trying to build a python interface around some existing C code with CFFI. As usual with C code trimmed for performance, it is fraught with extensive macros and typedefs.
ATM I am working on replicating following struct
#define DIM 3
typedef double REAL;
struct Object_structure {
int numpoints;
REAL (* vertices)[DIM];
int * rings;
};
typedef struct Object_structure * Object;
The function I'm trying to call expects an argument of type Object.
REAL gjk_distance(
Object obj1, REAL (* tr1)[DIM+1],
Object obj2, REAL (* tr2)[DIM+1],
REAL wpt1[DIM], REAL wpt2[DIM],
struct simplex_point * simplex, int use_seed
);
I have written the following python class for representing such an object/struct, but I'm having trouble converting it into the expected cdata object. (For now I just considering a UnitCube, but ultimately I'll want to generalize that.)
class Box:
def __init__(self, pos):
self._weakkeydict = weakref.WeakKeyDictionary()
self.numpoints = 8
self.rings = [
8, 12, 16, 20, 24, 28, 32, 36,
3, 1, 4, -1,
0, 2, 5, -1,
1, 3, 6, -1,
2, 0, 7, -1,
7, 5, 0, -1,
4, 6, 1, -1,
5, 7, 2, -1,
6, 4, 3, -1]
x, y, z = pos
self.vertices = [
[x+0, y+0, z+0],
[x+1, y+0, z+0],
[x+1, y+1, z+0],
[x+0, y+1, z+0],
[x+0, y+0, z+1],
[x+1, y+0, z+1],
[x+1, y+1, z+1],
[x+0, y+1, z+1],
]
@property
def cdata(self):
self._weakkeydict.clear()
#ptr_numpoints = ffi.new("int", self.numpoints)
ptr_rings = ffi.new("int[]", self.rings)
vertices = [ffi.new("REAL[3]", v) for v in self.vertices]
ptr_vertices = ffi.new("REAL *[3]", vertices )
ptr_obj = ffi.new("Object", {
'numpoints': self.numpoints,
'rings': ptr_rings,
'vertices': ptr_vertices})
self._weakkeydict[ptr_obj] = (ptr_rings, ptr_vertices, vertices)
return ptr_obj
With the above I'm getting IndexError: too many initializers for 'double *[3]' (got 8) in line ptr_vertices = ffi.new("REAL *[3]", vertices ) when calling:
box1 = Box((0,0,0))
box2 = Box((10,0,0))
d = lib.gjk_distance(
[box1.cdata], ffi.NULL,
[box2.cdata], ffi.NULL,
ffi.NULL, ffi.NULL,
ffi.NULL, 0 )
To me it seems as if the dimensions got switched somehow. As it should be an 8 element array with 3 element items. I'm hoping someone can point me in the right direction here.
If you are creating a single item, use ffi.new('REAL(*)[3]', [1, 2, 3]). The parenthesis around the * is important.
In C, the type REAL(*)[3] means a pointer to array (size=3) of REAL, while REAL*[3] means an array (size=3) of pointer to real. See C pointer to array/array of pointers disambiguation for detail.
Now, you are creating an array of items, CFFI expects an array type instead, as you have already discovered. This can be compared as:
ffi.new('int*', 1) # ok
ffi.new('int[]', 1) # wrong
ffi.new('int*', [1, 2, 3]) # wrong
ffi.new('int[]', [1, 2, 3]) # ok
ffi.new('REAL(*)[3]', [0.1, 0.2, 0.3]) # ok
ffi.new('REAL[][3]', [0.1, 0.2, 0.3]) # wrong
ffi.new('REAL(*)[3]', [[0.1, 0.2, 0.3], [0.4, 0.5, 0.6]]) # wrong
ffi.new('REAL[][3]', [[0.1, 0.2, 0.3], [0.4, 0.5, 0.6]]) # ok