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 string.
It is working fine, but, because i manually allocated memory for the string, i should free it after use.

I have something like this:

extern "C" char* DLL_EXPORT func(const char* str1, const char* str2)
{
    return getSomeString(str1, str2);
}

// Goal is to call this function correctly from Python.    
extern "C" void DLL_EXPORT freeMem(void *mem)
    {
        if(mem!=NULL)
            delete mem;
    }

But i don't have any idea, how can i pass received pointer back for deletion in Python?

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 POINTER(c_char) and use cast to retrieve the string value, which leaves the return value a LP_c_char object that can be freed.

Here's an example. Note that declaring the correct .restype is especially important for 64-bit Python, since the default return type is c_int (32-bit) and a 64-bit pointer could be truncated. This code is tested with both 32- and 64-bit builds.

test.c

#include <string.h>
#include <stdlib.h>

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

API char* func(const char* str1, const char* str2) {
    size_t len = strlen(str1) + strlen(str2) + 1;
    char* tmp = malloc(len);
    strcpy_s(tmp, len, str1);
    strcat_s(tmp, len, str2);
    return tmp;
}

API void freeMem(void *mem) {
    free(mem);
}

test.py

import ctypes as ct

dll = ct.CDLL('./test')
dll.func.argtypes = ct.c_char_p,ct.c_char_p
dll.func.restype = ct.POINTER(ct.c_char)
dll.freeMem.argtypes = ct.c_void_p,
dll.freeMem.restype = None

# Helper function to extract the return value as a Python object
# and always free the pointer.
def freeMem(a,b):
    p = dll.func(b'abcdef', b'ghijkl')
    print(p)
    s = ct.cast(p, ct.c_char_p).value
    dll.freeMem(p)
    return s

print(freeMem(b'abcdef', b'ghijkl'))

Output:

<ctypes.LP_c_char object at 0x00000279D7959DC0>
b'abcdefghijkl'