How to correctly initialize ctypes char***?

Question

I'm using ctype to call C code from Python.

The C function I need to call takes a char***, and so it's bind as using a ctypes.POINTER(ctypes.POINTER(ctypes.c_char)).

I don't understand how I should safely create and initialize such objects, because if I create one and immediatly try to iterate it, the Python program crashs:

import ctypes
names = ctypes.POINTER(ctypes.POINTER(ctypes.c_char))()
if names != None:
    for name in names:
        pass

Error is:

Traceback (most recent call last):
  File "example_sdetests_lib_bind_python_string_array.py", line 6, in <module>
    for name in names:
ValueError: NULL pointer access

How can I safely check a ctypes.POINTER(ctypes.POINTER(ctypes.c_char)) is NULL or not?

Answer 1

FYI: char*** does need three ctypes.POINTER.

To test for null, test like any other "falsey" object:

import ctypes as ct

# define char*** type
PPPCHAR = ct.POINTER(ct.POINTER(ct.POINTER(ct.c_char)))

# instantiate (default null)
names = PPPCHAR()
if not names:
    print('NULL')
    
# Simple initialization
c = ct.c_char(b'a')
# pointer(obj) creates a ctypes pointer instance to a ctypes object.
pc = ct.pointer(c)
ppc = ct.pointer(pc)
pppc = ct.pointer(ppc)

if pppc:
    print('non-NULL')

print(pppc.contents.contents.contents)

Output:

NULL
non-NULL
c_char(b'a')