How to pass pointer back in ctypes?

Question

Don\'t know much about ctypes, just recently began working with it.

I have a simple function in C-like dll which returns a pointer to the dynamically generated strin

Answer 1

Normally each function you use in ctypes should have its arguments and return type declared so Python can check for the correct number and type of arguments and convert Python object arguments to the correct C data objects. Unfortunately in this case, the normal return value for func would be c_char_p, but ctypes tries to be helpful and convert a c_char_p return value to a Python string, losing access to the raw C pointer value. Instead, you can declare the return type as c_void_p and use cast to retrieve the string value, which leaves the return value a c_char_p object.

Here's an example (Python 3):

import ctypes

func = ctypes.cdll.TestDLL.func
func.argtypes = [ctypes.c_char_p,ctypes.c_char_p]
func.restype = ctypes.c_void_p

freeMem = ctypes.cdll.TestDLL.freeMem
freeMem.argtypes = [ctypes.c_void_p]
freeMem.restype = None

s = func(b'abcdef',b'ghijkl')
s = ctypes.cast(s,ctypes.c_char_p)
print(s.value)
freeMem(s)

Answer 2

You're on the right track.

// TestDLL.cpp
#include <string.h> // strcpy

extern "C" __declspec(dllexport) char* stringdup(const char* str) {
    char* p = new char[strlen(str)+1];
    strcpy(p,str);
    return p;
}

// if you have no good reason to use void*, use the type
// you've allocated. while it usually works for built-in
// types, it wouldn't work for classes (it wouldn't call
// the destructor)
extern "C" __declspec(dllexport) void stringfree(char* ptr) {
    // you don't need to check for 0 before you delete it,
    // but if you allocate with new[], free with delete[] !
    delete [] ptr; 
}

And in python:

# Test.py
import ctypes

lib = ctypes.cdll.TestDLL

# this creates a c-style char pointer, initialized with a string whose
# memory is managed by PYTHON! do not attempt to free it through the DLL!
cstr = ctypes.c_char_p("hello ctypes")

# call the dll function that returns a char pointer 
# whose memory is managed by the DLL.
p = lib.stringdup(cstr)

# p is just an integer containing the memory address of the 
# char array. therefore, this just prints the address:
print p

# this prints the actual string
print ctypes.c_char_p(p).value

# free the memory through the DLL
lib.stringfree(p)