Converting python string object to c char* using ctypes

Question

I am trying to send 2 strings from Python (3.2) to C using ctypes. This is a small part of my project on my Raspberry Pi. To test if the C function received the strings corr

Answer 1

Have you considered using SWIG? I haven't tried it myself but here's what it would look like, without changing your C source:

/*mymodule.i*/

%module mymodule
extern void my_c_function(const char* str1, const char* str2);

This would make your Python source as simple as (skipping compilation):

import mymodule

string1 = "my string 1"
string2 = "my string 2"
my_c_function(string1, string2)

Note I'm not certain .encode('utf-8') is necessary if your source file is already UTF-8.

Answer 2

Thanks to Eryksun the solution:

Python code

string1 = "my string 1"
string2 = "my string 2"

# create byte objects from the strings
b_string1 = string1.encode('utf-8')
b_string2 = string2.encode('utf-8')

# send strings to c function
my_c_function.argtypes = [ctypes.c_char_p, ctypes.char_p]
my_c_function(b_string1, b_string2)

Answer 3

I think you just need to use c_char_p() instead of create_string_buffer().

string1 = "my string 1"
string2 = "my string 2"

# create byte objects from the strings
b_string1 = string1.encode('utf-8')
b_string2 = string2.encode('utf-8')

# send strings to c function
my_c_function(ctypes.c_char_p(b_string1),
              ctypes.c_char_p(b_string2))

If you need mutable strings then use create_string_buffer() and cast those to c_char_p using ctypes.cast().