C function with pointer of points wrapping

Question

I have a method define in C/C++ DLL that takes 2 args

void SetLines(char** args,int argCount);

I need to call it from Python, what is the proper way to do so.

from ctypes import *
path="test.dll"
lib = cdll.LoadLibrary(path)
Lines=["line 2","line 2"]
lib.SetLines(Lines,len(lines))
print(code)

Excuting the Python code gives the following error:

Traceback (most recent call last):
  File "<test.py>", line 6, in <module>
ctypes.ArgumentError: argument 1: <class 'TypeError'>: Don't know how to convert parameter 1

if figure it out.. using ctypes.Array(c_char_p,len)#len integer value, charpp =ctypes.Array(c_char_p,len)--> charpp[i]=c_char_p(Lines[i].encode("utf-8")) — Aladdin
– Aladdin, Commented May 21, 2015 at 6:22
indeed you have to use ctypes conversion for the list of strings. If you find the Answer, you could write it so new comers can find it — Assem
– Assem, Commented May 21, 2015 at 7:07

Aladdin · Accepted Answer · 2015-05-21 07:20:50Z

1

After some code digging I figure it out:

any C/C++ parameter that accepts a pointer to a list of values should be wrapped in python with

MyType=ctypes.ARRAY(/*any ctype*/,len)
MyList=MyType()

and filled with

MyList[index]=/*that ctype*/

in mycase the solution was:

from ctypes import *
path="test.dll"
lib = cdll.LoadLibrary(path)

Lines=["line 1","line 2"]
string_pointer= ARRAY(c_char_p,len(Lines)) 
c_Lines=string_pointer()
for i in range(len(Lines)):
    c_Lines[i]=c_char_p(Lines[i].encode("utf-8"))

lib.SetLines(c_Lines,len(lines))

answered May 21, 2015 at 7:20

Aladdin

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4 Comments

Aladdin Over a year ago

Lines[i].encode("<any encoding name>") is neccary because c_char_p accepts bytes not str (as I tested under Python 3.4.3)

Eryk Sun Over a year ago

ARRAY is deprecated. Create array types using the * operator. For example: c_lines = (c_char_p * len(Lines))(*(s.encode("utf-8") for s in Lines)).

Aladdin Over a year ago

both ways leads to the same result printing c_lines in both ways gives <code.c_char_p_Array_2 object at 0x0267F670> <code.c_char_p_Array_2 object at 0x0267F620>

Eryk Sun Over a year ago

ARRAY is kept around for backward compatibility with very old code. It's currently defined as just def ARRAY(typ, len): return typ * len. It's not documented and not even mentioned in the docs.

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