Ctypes: transferring string from C(++) to Python

Question

I have been struggling with this for almost a week, maybe someone can help me here? To be short, I wrote a program in C++ and want to link it with Python (to build a web application with Flask or Django).

My C functions which would return an int, char or void work fine, the problem is when they should return a string. I more or less understand what is the cause of the problem (my C function returns a pointer, which my Python function can't handle as a useful data).

So here is an example code that I wrote. In my main.cpp file I have the following the functions:

extern "C" {
  int addint(int a) {
    return a + 1;
  }

  const char* hello() {
    return "hello";
  }
}

and in my pywrapper.py file I have the following:

def addint(number):
    result = lib.addint(c_int(number))
    return result

def saystr():
    result = lib.hello()
    return result

if __name__ == "__main__":
    print('addint: ', addint(5))
    print('saystr: ', saystr())

My output is:

addint:  6
saystr:  -354261249

So the first function works fine. But what can I do for the second function to return 'hello'??

Useless · Accepted Answer · 2017-01-20 11:06:37Z

1

As the ctypes documentation shows, you need to explicitly tell it how to handle the return type:

def saystr():
    lib.hello.restype = c_char_p # pointer to a string
    result = lib.hello()
    return result

otherwise the default return type is int (and the returned pointer is converted to an integer address).

answered Jan 20, 2017 at 11:06

Useless

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