Ctypes pointer on element in ctypes array

Question

Is there a way to get a pointer to an element in the middle of an ctypes array? Example:

lib = ctypes.cdll.LoadLibrary('./lib.so')
arr = (ctypes.c_int32 * 100)()
lib.foo(arr)

Now I don't want to call foo with a pointer to the first element of arr, but on the 10th. That would be equivalent to C notation &arr[9]:

lib.foo(&arr[9])

Is there a smart way to do this?

Ok, and inside the func, I imagine there's some processing taking place. That processing involves accessing the elements of the array (passed as a pointer). ?How does the function when to stop? meaning that If i pass a 100 values array how does it know not to access 101th element? — CristiFati
– CristiFati, Commented Jul 9, 2018 at 14:54

Mark Tolonen · Accepted Answer · 2018-07-10 04:35:48Z

6

byref has an optional parameter to add a byte offset to the address.

test.dll (Windows)

__declspec(dllexport) int foo(int* arr)
{
    return *arr;
}

Example:

>>> from ctypes import *
>>> lib = CDLL('test')
>>> arr = (c_int * 100)(*range(100))
>>> arr[9]
9
>>> lib.foo(arr)
0
>>> lib.foo(byref(arr,sizeof(c_int) * 9))
9

answered Jul 10, 2018 at 4:35

Mark Tolonen

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