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

Question

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) 
{         


        

Answer 1

a cheap hack around freeing the memory is to allocate it statically in C and never free it. only works if you know the maximum amount you need, of course, and if you know that you're done with before a second call to your routine.

static char _externalStringBuffer[MAX_BUFFER_SIZE];
char *get() {
    // put characters in _externalStringBuffer
    return _externalStringBuffer;
}

Answer 2

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.