Sending strings between c++ libraries and python

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?

Hitobat · Accepted Answer · 2018-05-24 21:12:25Z

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);
}

edited May 24, 2018 at 21:12

answered May 24, 2018 at 20:38

Hitobat

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3 Comments

user3830784 Over a year ago

As you can see from the functions that I have available, I have the function passInACharArray(char* incomingString), which I have tested with. When I use that function though, the code prints out "Recieved message in passInACharArray", followed immediately after by a segfault. Since you say this is the proper method of doing this, how would you write the python that calls this function?

Hitobat Over a year ago

If you're using Python3 I think you need to convert the string to a bytestring. e.g. lib.passInACharArray(thestring.encode('utf-8'))

Hitobat Over a year ago

I've edited the code for proper strings passed from Py3.

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