from cffi import FFI
import numpy as np

s = '''
    typedef struct
    {
        int a;
        int b;
        float c;
        double d;
    } mystruct;
    '''
ffi = FFI()
ffi.cdef(s)

m_arr = ffi.new("mystruct[2]")

#create array and fill with dummy data
for k in range(2):

    m = m_arr[k]

    m.a = k
    m.b = k + 1
    m.c = k + 2.0
    m.d = k + 3.0

print(m_arr)

# dtype for structured array in Numpy
dt = np.dtype({'names': ('a', 'b', 'c', 'd'),
               'formats': ('i4', 'i4', 'f4', 'f8'),
               'offset': (ffi.offsetof('mystruct', 'a'),
                          ffi.offsetof('mystruct', 'b'),
                          ffi.offsetof('mystruct', 'c'),
                          ffi.offsetof('mystruct', 'd'),
                      )
              })
padding = ffi.sizeof('mystruct') - dt.itemsize
if padding:
    # this padding takes into account aligning arrays of structs
    dt = np.dtype({'names': ('a', 'b', 'c', 'd','padding'),
                   'formats': ('i4', 'i4', 'f4', 'f8', 'S%d' % padding),
                   'offset': (ffi.offsetof('mystruct', 'a'),
                              ffi.offsetof('mystruct', 'b'),
                              ffi.offsetof('mystruct', 'c'),
                              ffi.offsetof('mystruct', 'd'),
                          )
                   })


# member size, 20 bytes
print('size, manually', 4 + 4 + 4 + 8)

# total size of struct, 24 bytes
print('sizeof', ffi.sizeof(m_arr[0]))


#reason is member padding in structs

buf = ffi.buffer(m_arr)
print(buf)

x = np.frombuffer(buf, dtype=dt)
print(x)