Cython Memoryviews -- From Array of Structs?

Question

I'd like to quickly fill with as few copies as possible a long array of structs that I'm receiving incrementally from C.

If my struct is only primary data types, like the following:

cdef packed struct oh_hi:
    int lucky
    char unlucky

Then the following works fine:

  DEF MAXPOWER = 1000000
  cdef oh_hi * hi2u = <oh_hi *>malloc(sizeof(oh_hi)*MAXPOWER)
  cdef oh_hi [:] hi2me = <oh_hi[:MAXPOWER]> hi2u

But once I change my struct to hold a character array:

cdef packed struct oh_hi:
    int lucky
    char unlucky[10]

The previous memoryview casting compiles but when run gives a:

  ValueError: Expected 1 dimension(s), got 1

Is there an easy way to do this in Cython? I'm aware that I could create a structured array, but afaik, that wouldn't let me assign the C structs straight into it.

This seems to be very similar to this issue with numpy structured arrays. The problem is that cython does not seem to like mixing arrays with scalar values if the first item in a struct is a scalar. The workaround is just to rearrange the items in the struct such that there is always an array member/item first. — CodeSurgeon
– CodeSurgeon, Commented Jan 28, 2018 at 23:35

radikalus · Accepted Answer · 2013-06-21 16:12:25Z

6

Actually, just building a structured numpy array and then a memoryview works just fine.

cdef np.ndarray hi2u = np.ndarray((MAXPOWER,),dtype=[('lucky','i4'),('unlucky','a10')])
cdef oh_hi [:] hi2me = hi2u

The performance of this seems quite good and this saves a later copy if you need the data back in python. As per usual, the numpy version is pretty good. =p

answered Jun 21, 2013 at 16:12

radikalus

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3 Comments

dashesy Over a year ago

This did not work for me. Maybe because I used fixed array of int32 and not string 'a10'. I ended up flattening the structures.

radikalus Over a year ago

I'll look into this, but how were you hoping these would be stored in the numpy structure?

dashesy Over a year ago

for 10 integers, something line ('lucky',(np.int32,10))

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