结构

Question

我有以下包含可变C风格字符串的C结构。

typedef struct XMLCFG_PARAMS {
    unsigned long       ulMedia;                    // Save/load to file or buffer
    unsigned long       ulCfgFlags;                 // Flags say what type of info to save/load
    char*               pszBufOrFilename;           // The buffer or the filename 
    unsigned long       ulBufLen;                   // How long is the buffer (in bytes)
} XMLCFG_PARAMS;

我已经在Python中定义了这样的结构：

class XMLCFG_PARAMS(ctypes.Structure):
    _fields_ = [
        ("ulMedia", ctypes.c_ulong),
        ("ulCfgFlags", ctypes.c_ulong),
        ("pszBufOrFilename", ctypes.c_char_p),
        ("ulBufLen", ctypes.c_ulong),
    ]

然后实例化此结构：

xml_cfg = XMLCFG_PARAMS()
xml_cfg.ulMedia = XMLCFG_BUFFER
xml_cfg.ulCfgFlags = config_flags
xml_buffer = ctypes.create_string_buffer(xml_text, size=1024 * 1024)
xml_cfg.pszBufOrFilename = ctypes.c_char_p(ctypes.addressof(xml_buffer))
xml_cfg.ulBufLen = len(xml_buffer)

C调用返回错误WindowsError: exception: access violation reading 0x6E6F697B。

我怀疑我在实例化XMLCFG_PARAMS结构，特别是pszBufOrFilename字段的方式上做错了，因为这是我第一次遇到可变的C风格结构中的字符串。

C功能签名

BOOL Control(unsigned long   ulEngId,
             unsigned long   cmd_id,
             unsigned long   *ulParam0 = 0,
             unsigned long   *ulParam1 = 0,
             unsigned long   *ulParam2 = 0,
             unsigned long   *ulParam3 = 0);

Python ctypes调用

dll_handle.Control(engine_id, cmd_id, ctypes.pointer(xml_cfg), None, None, None)

hex(ctypes.addressof(xml_buffer))的输出为0x3940020，而异常持续引用0x6E6F697B。这是在Win XP上，所以我猜这个内存地址不会改变，因为操作系统缺少ASLR。