如何在Python中操作C函数返回的C类型指针？

Question

这是在"Testlib.c"里的C代码。我只是从C函数返回一个指向结构体的指针。所有的C代码会被编译成一个库，以便被Python调用。

#include <stdio.h>
#include <stdlib.h>
typedef struct{
    int num;
    char c;
}aStruct;
aStruct* newStructPointer()
{
    aStruct* s = (aStruct*)malloc(sizeof(aStruct)) ;
    s->num = 3;
    s->c = 'f';
    return s;
}
int getNumField(aStruct* a)
{
    return a->num;
}
char getCharField(aStruct* a)
{
    return a->c;
}

这是Python代码。它非常简单，调用newStructPointer来获取一个新的结构体，然后获取它字段中的数字值。

#!/usr/bin/env python
# -*- coding: utf-8 -*- 
from ctypes import *
class testlib(object):
    def __init__(self):
        lib=cdll.LoadLibrary("libtestdylib.dylib")
        s=lib.newStructPointer()
        num=lib.getNumField(cast(s, c_void_p))

t=testlib()

然后，它崩溃了，因为指针已经无效了，这里是崩溃报告：

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libtestdylib.dylib              0x000000010638ef5c getNumField + 12 (interface.c:26)
1   libffi.dylib                    0x00007fff9354ef9c ffi_call_unix64 + 76
2   libffi.dylib                    0x00007fff9354f78e ffi_call + 790
3   _ctypes.so                      0x000000010637390e _ctypes_callproc + 794
4   _ctypes.so                      0x000000010636df80 0x10636b000 + 12160
5   org.python.python               0x0000000106085f72 PyObject_Call + 101
6   org.python.python               0x00000001060ffdf5 PyEval_EvalFrameEx + 15416
7   org.python.python               0x00000001060fc093 PyEval_EvalCodeEx + 1641
8   org.python.python               0x00000001060a3796 0x10607c000 + 161686
9   org.python.python               0x0000000106085f72 PyObject_Call + 101
10  org.python.python               0x00000001060909a7 0x10607c000 + 84391
11  org.python.python               0x0000000106085f72 PyObject_Call + 101
12  org.python.python               0x00000001060cb6ce 0x10607c000 + 325326
13  org.python.python               0x00000001060c7184 0x10607c000 + 307588
14  org.python.python               0x0000000106085f72 PyObject_Call + 101
15  org.python.python               0x00000001060ffdf5 PyEval_EvalFrameEx + 15416
16  org.python.python               0x00000001060fc093 PyEval_EvalCodeEx + 1641
17  org.python.python               0x00000001060fba24 PyEval_EvalCode + 54
18  org.python.python               0x000000010611ac2c 0x10607c000 + 650284
19  org.python.python               0x000000010611acd3 PyRun_FileExFlags + 137
20  org.python.python               0x000000010611a821 PyRun_SimpleFileExFlags + 718
21  org.python.python               0x000000010612b363 Py_Main + 2995
22  libdyld.dylib                   0x00007fff8e4215fd start + 1

我已经阅读了Python的文档，地址是 https://docs.python.org/2/library/ctypes.html ，但没有找到任何有用的信息。我该如何解决这个问题呢？

Answer 1

你需要正确地定义函数的原型，这样才能安全地调用这些函数：

# define your structure
class aStruct(ctypes.Structure):
    _fields_ = [('num', ctypes.c_int),
                ('c', ctypes.c_char]

# this defines aStructPtr to be a pointer
# to an aStruct
aStructPtr = ctypes.POINTER(aStruct)

# next, define the interface to your functions:
lib.newStructPointer.restype = aStructPtr
lib.newStructPointer.argtypes = [] # void, no arguments

# your getter functions both take the structure pointer as inputs
# and return an integer or char
lib.getNumField.restype = ctypes.c_int
lib.getNumField.argtypes = [aStructPtr]

lib.getCharField.restype = ctypes.c_char
lib.getCharField.argtypes = [aStructPtr]

注意，如果你在ctypes中正确声明了结构体，就不需要使用C语言的获取函数了。下面的代码也应该可以正常工作：

s = lib.newStructPointer()
num_value = s.contents.num
char_value = s.contents.c