CFFI: defining array within a struct from numpy array

Question

I'm trying to allocate memory to an array variable of a struct defined in C++ through CFFI.

In NVCC/C++, I have:

struct array_struct {
  int ndim;
  int len;
  int* shape;
  float* array;
};

extern "C" void allocate(
  struct array_struct* host_array,
  struct array_struct* device_array
) {

  device_array->ndim = host_array->ndim;
  device_array->len  = 1;
  device_array->shape = new int[host_array->ndim];
  printf("Printing device_array info:\n");
  printf("device_array->ndim = %d\n", device_array->ndim);
  printf("device_array->shape = (");
  for (int i = 0; i < host_array->ndim; ++i) { 
    device_array->shape[i] = host_array->shape[i];
    device_array->len     *= host_array->shape[i];
    printf("%d, ", device_array->shape[i]);
  }
  printf(")\n");
  printf("device_array->len = %d\n", device_array->len);
}

In Python, I define the ffi.cdefs and try to allocate memory to the shape and array fields of the array_struct, and copy the numpy array information across:

ffi.cdef(
"""
struct array_struct {
    int ndim;
    int len;
    int* shape;
    float* array;
};
""")

ffi.cdef(
"""
void allocate(
  struct array_struct *host_array,
  struct array_struct *device_array
);
""")

lib = ffi.dlopen("./cupid/src/libAlg.so")

numpy_array = np.zeros((5,6),dtype=float)

host_array_struct       = ffi.new("struct array_struct *")
host_array_struct.ndim  = numpy_array.ndim
host_array_struct.len   = numpy_array.size

host_array_struct.shape = ffi.new(f"int[{numpy_array.ndim}]")
contiguous_shape_array  = np.ascontiguousarray(numpy_array.shape)
ffi.memmove(host_array_struct.shape, ffi.cast("int*", ffi.from_buffer(contiguous_shape_array)), numpy_array.ndim * ffi.sizeof("int"))
    
host_array_struct.array = ffi.new(f"float[{host_array_struct.len}]")
contiguous_numpy_array  = np.ascontiguousarray(numpy_array)
ffi.memmove(host_array_struct.array, ffi.cast("float*", ffi.from_buffer(contiguous_numpy_array)), host_array_struct.len * ffi.sizeof("float"))

device_array_struct = ffi.new("struct array_struct *")
lib.allocate(host_array_struct, device_array_struct)

However, not only is it not giving the correct device_array->shape, it's also producing a memory corruption issue in the output. I wondered whether this was to do with the device_array not having memory allocated to it, but that hasn't helped either.

Printing device_array info:
device_array->ndim = 2
device_array->shape = (5, 0, )
device_array->len = 0
*** Error in `python': corrupted size vs. prev_size: 0x00005619fc092850 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x7f7c4)[0x2ad30b6bd7c4]
/lib64/libc.so.6(+0x818bb)[0x2ad30b6bf8bb]
python(PyObject_Free+0x199)[0x5619fa064969]
...

Thanks in advance.

Answer 1

I managed to fix this issue by specifying the data types more explicitly. I'd forgotten that C++ float != Python float, so needed to specify np.float32 when creating (or casting) the np.array:

host_array_struct = ffi.new("struct array_struct*")
host_array_struct.ndim = numpy_array.ndim
host_array_struct.length = numpy_array.size
host_array_struct.shape = ffi.cast("int *", np.ascontiguousarray(numpy_array.shape, np.int32).ctypes.data)
host_array_struct.data = ffi.cast("float *", np.ascontiguousarray(numpy_array, np.float32).ctypes.data)

device_array_struct = ffi.new("struct array_struct *")

lib.allocate(host_array_struct, device_array_struct)