Sending strings between c++ libraries and python - c++

Question

I am attempting to pass a string from python to a c++ library. However, I have been crashing with variations of segfaults std::bad_alloc, and invalid type messages while I have been attempting to do so. Here is the code that I am attempting to use in c++:
#define DLLEXPORT extern "C"
DLLEXPORT std::string returnAString()
{
std::string ret = "Returning string from lib";
return ret;
}
DLLEXPORT char* returnACharArray()
{
return "Returning char* from lib";
}
DLLEXPORT void passInAString(std::string incomingString)
{
printf("Recieved message in passInAString\n");
printf("Recieved incoming message: %s", incomingString);
}
DLLEXPORT void passInACharArray(char* incomingString)
{
printf("Recieved message in passInACharArray\n");
printf("Recieved incoming message: %s", incomingString);
}
Realistically, with what I am doing I can work with either the char* or the std::string once it gets in my c++ code, and I don't really have a preference either way. Here is what I am doing in python:
from ctypes import *
import os
libPath = os.path.join(os.getcwd(), "mylib.so")
lib = cdll.LoadLibrary(libPath)
string = "hello from python"
lib.passInAString(string)
#lib.passInACharArray(string)
#ret = lib.returnAString()
#print("recieved string: " + string)
#ret = lib.returnACharArray()
#print("recieved char*: " + string)
here, I will uncomment whichever line I am attempting to test. When passing in a string, I will get my first printf statement, I will get std::bad_alloc. When I pass in a char*, I get a segfault. When I attempt to receive a message, I get back a number (I am assuming that this is the pointer), but I am unable to decode this message into an actual string message.
I have attempted to use c_char_p to convert my python string into a char* to pass to my library, but when I do that I get "invalid type". Trying to convert the returned message by doing c_char_p(lib.returnACharArray) and then print(str(string.value)) then gives me the hex value of 4 bytes... which is not what I am returning.
What am I missing to make this functionality work?

Answer 1

Your DLL has the wrong argument types.
Since you're passing a string from Python the function should take a char*. You can convert this to a std::string within the function if you want to.
There's a table in the Python docs showing the corresponding types in both C and Python here: https://docs.python.org/3.6/library/ctypes.html#fundamental-data-types
Edit: Just realized the string types differ for Python 3 meaning the function needs to be declared differently.
e.g.
// Python2, or Python3 passing bytes
DLLEXPORT void passInAString(char* incomingString)
{
printf("Received message in passInAString\n");
printf("Received incoming message: %s", incomingString);
std::string myStr = incomingString;
// and do stuff with myStr if needed
printf("Incoming message as std::string is %s", myStr.c_str());
}
// Python3 passing strings
DLLEXPORT void passStringPy3(wchar_t* wideString)
{
printf("Message >>%ls<<\n", wideString);
}