ctypes.c_void_p 指针在我的应用程序中没有引用正确的内存位置

Question

我的 Python 脚本是：

def xmlEncodeSpecialChars(doc: xmlDoc, input: str) -> str:
    '''
    Do a global encoding of a string, replacing the predefined entities this routine is reentrant, and result must be deallocated.

        doc:        the document containing the string
        input:      A string to convert to XML.
        Returns:    A newly allocated string with the substitution done.'''
        
    libXML = LibXML2() # DSO/DLL 

    libXML.xmlEncodeSpecialChars.restype = ctypes.c_char_p
    libXML.xmlEncodeSpecialChars.argtypes = ctypes.c_void_p, ctypes.c_char_p, 

    output: ctypes.c_char_p = libXML.xmlEncodeSpecialChars(doc, input.encode()); 
    Err = xmlGetLastError()
    if Err: raise LibErr(Err)
    ans = output.decode()
# free memory
    libXML.xmlMemFree.argtypes = ctypes.c_char_p, ;
    libXML.xmlMemFree(output)
    
    return ans

它正确地返回一个转义字符串，但是 xmlMemFree() 在 Python 中生成一个

Memory tag error

，尽管对同一个 DLL 的等效 C 调用工作得很好。

如何设置

ctypes

无内存操作？

Answer 1

在

ctypes

中，

c_char_p

转换一个空终止的C

char*

并返回一个Python

bytes

对象。指针丢失了。

使用

c_void_p

或

POINTER(c_char)

保持指针。

string_at

可用于读取返回地址处的空终止字符串。

未测试：

def xmlEncodeSpecialChars(doc: xmlDoc, input: str) -> str:
    '''
    Do a global encoding of a string, replacing the predefined entities this routine is reentrant, and result must be deallocated.

        doc:        the document containing the string
        input:      A string to convert to XML.
        Returns:    A newly allocated string with the substitution done.'''
    libXML = LibXML2() # DSO/DLL 

    libXML.xmlEncodeSpecialChars.restype = ctypes.c_void_p
    libXML.xmlEncodeSpecialChars.argtypes = ctypes.c_void_p, ctypes.c_char_p

    output: ctypes.c_void_p = libXML.xmlEncodeSpecialChars(doc, input.encode())
    Err = xmlGetLastError()
    if Err: raise LibErr(Err)
    ans = ctypes.string_at(output).decode()
# free memory
    libXML.xmlMemFree.argtypes = ctypes.c_void_p,
    libXML.xmlMemFree.restype = None
    libXML.xmlMemFree(output)
    return ans