Python ctypes: how to free memory? Getting invalid pointer error

c, c++, ctypes, python

Solution

As David Schwartz pointed out, if you set restype to `c_char_p`, ctypes returns a regular Python string object. A simple way to get around this is to use a `void *` and cast the result:

string.c:

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

char *get(void)
{
    char *buf = "Hello World";
    char *new_buf = strdup(buf);
    printf("allocated address: %p\n", new_buf);
    return new_buf;
}

void freeme(char *ptr)
{
    printf("freeing address: %p\n", ptr);
    free(ptr);
}

Python usage:

from ctypes import *

lib = cdll.LoadLibrary('./string.so')
lib.freeme.argtypes = c_void_p,
lib.freeme.restype = None
lib.get.argtypes = []
lib.get.restype = c_void_p

>>> ptr = lib.get()
allocated address: 0x9facad8
>>> hex(ptr)
'0x9facad8'
>>> cast(ptr, c_char_p).value
'Hello World'
>>> lib.freeme(ptr)
freeing address: 0x9facad8

You can also use a subclass of `c_char_p`. It turns out that ctypes doesn't call the `getfunc` for a subclass of a simple type.

class c_char_p_sub(c_char_p):
    pass

lib.get.restype = c_char_p_sub

The `value` attribute returns the string. You can leave the parameter for `freeme` as the more generic `c_void_p`. That accepts any pointer type or integer address.

Problem

I want to get some string from a C/C++ library with ctypes into python. My code looks like this: Code in lib: ``` const char* get(struct something *x) { [...] // buf is a stringstream return strdup(buf.str().c_str()); } void freeme(char *ptr) { free(ptr); } ``` Python code: ``` fillprototype(lib.get, c_char_p, POINTER(some_model)]) fillprototype(lib.freeme, None, [c_char_p]) // what i want to do here: get a string into python so that i can work // with it and release the memory in the lib. c_str = lib.get(some_model) y = ''.join(c_str) lib.freeme(c_str) ``` If i print() c_str, everything is there. Problem is (or seems to be) in the last Python line. I cannot free the memory - the library is getting a wrong pointer. What I am doing wrong here? (And please don't suggest boost::python or so). ``` *** glibc detected *** python2: munmap_chunk(): invalid pointer: 0x00000000026443fc *** ```

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