Ctypes read data from a double pointer

Question

I am working on a C++ Dll with C wrapper and I am creating a Python wrapper for future user (I discover ctypes since monday). One of the method of my Dll (because it is a class) return an unsigned short **, call data, which corresponds to an image. On C++, I get the value of a pixel using data[row][column].

I create in Python a function on the following model :

mydll.cMyFunction.argtypes = [c_void_p]
mydll.cMyFunction.restype = POINTER(POINTER(c_ushort))

When I call this function, I have result = <__main__.LP_LP_c_ushort at 0x577fac8> and when I try to see the data at this address (using result.contents.contents) I get the correct value of the first pixel. But I don't know how to access values of the rest of my image. Is there a easy way to do something like C++ (data[i][j]) ?

Mark Tolonen · Accepted Answer · 2018-02-15 20:11:20Z

2

Yes, just use result[i][j]. Here's a contrived example:

>>> from ctypes import *
>>> ppus = POINTER(POINTER(c_ushort))
>>> ppus
<class '__main__.LP_LP_c_ushort'>
>>> # This creates an array of pointers to ushort[5] arrays
>>> x=(POINTER(c_ushort)*5)(*[cast((c_ushort*5)(n,n+1,n+2,n+3,n+4),POINTER(c_ushort)) for n in range(0,25,5)])
>>> a = cast(x,ppus) # gets a ushort**
>>> a
<__main__.LP_LP_c_ushort object at 0x00000000026F39C8>
>>> a[0] # deref to get the first ushort[5] array
<__main__.LP_c_ushort object at 0x00000000026F33C8>
>>> a[0][0] # get an item from a row
0
>>> a[0][1]
1
>>>
>>> a[1][0]
5

So if you are returning the ushort** correctly from C, it should "just work".

answered Feb 15, 2018 at 20:11

Mark Tolonen

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1 Comment

Mathieu Gauquelin Over a year ago

I don't know why I don't try this solution... Maybe the LP_LP_c_ushort confused me. Thank you.

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