Python ctypes:如何释放内存？获得无效指针错误

Question

我想从带有ctypes的C/C++库中获取一些字符串到python中.我的代码看起来像这样:

lib中的代码:

const char* get(struct something *x) 
{
    [...]
    // buf is a stringstream
   return strdup(buf.str().c_str());
}

void freeme(char *ptr)
{
    free(ptr);
}

Python代码:

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) 

如果我打印()c_str,一切都在那里.问题是(或似乎是)在最后一个Python行中.我无法释放内存 - 库得到错误的指针.我在这做错了什么？(请不要建议使用boost :: python等).

*** glibc detected *** python2: munmap_chunk(): invalid pointer: 0x00000000026443fc ***

Answer 1

正如David Schwartz指出的那样,如果将restype设置为c_char_p,则ctypes会返回一个常规的Python字符串对象.解决这个问题的一个简单方法是使用a void *并转换结果:

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用法:

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

您还可以使用的子类c_char_p.事实证明,ctypes不会调用getfuncfor类的简单类型.

class c_char_p_sub(c_char_p):
    pass

lib.get.restype = c_char_p_sub

该value属性返回字符串.您可以将参数保留freeme为更通用的参数c_void_p.它接受任何指针类型或整数地址.