Utility functions:
ctypes.addressof(obj)
Returns the address of the memory buffer as integer. obj must be an instance of a ctypes type.
ctypes.alignment(obj_or_type)
Returns the alignment requirements of a ctypes type. obj_or_type must be a ctypes type or instance.
ctypes.byref(obj[, offset])
Returns a light-weight pointer to obj, which must be an instance of a ctypes type. offset defaults to zero, and must be an integer that will be added to the internal pointer value.
byref(obj, offset) corresponds to this C code:
(((char *)&obj) + offset)
The returned object can only be used as a foreign function call parameter. It behaves similar to pointer(obj), but the construction is a lot faster.
New in version 2.6: The offset optional argument was added.
ctypes.cast(obj, type)
This function is similar to the cast operator in C. It returns a new instance of type which points to the same memory block as obj. type must be a pointer type, and obj must be an object that can be interpreted as a pointer.
1、addressof與byref關係
>>>str1=c_char_p("hello world")
>>>p1=addressof(str1)
>>>p2=byref(str1)
>>>print hex(p1)
0x107a26a30
>>>print p2
<cparam 'P' (0x107a26a30)>
#其內存地址爲0x107a26a302、c_char_p、c_int、pointer內存中形式
>>>str1=c_char_p("hello world")
>>>i1=c_int(3333）
>>>p1=pointer(i1)str1的內存爲一個指針，指向一個字符串；
i1的內存爲一個整數；
p1的內存爲一個數據結構;
在windbg查看以上對象的內存內容發現c_int的內存形式與c語言的整數形式一樣。c_char_p的內存內容爲一個指針，指針指向一個字符串，而c語言中一個字符串變量的內存內容直接是字符串。pointer對象有其自己結構，其第三個DWORD爲其contents指針，contents內容爲pointer指向對象的指針。假如指向對象爲c_int，則contents中內容爲c_int對象的地址。
>>>str1=c_char_("hello world")
>>>p1=cast(str1,POINTER(c_int))
>>>print p1.contents.value
0x6c6c6568 #爲hell的十六進制形式假如我們要移位輸出str1的話，那怎麼辦呢？比如獲得“world”這個字符串。
>>>i1=cast(byref(str1),POINTER(c_int))#str1內容是字符串的指針，我們要對此指針進行加操作，先得獲取此指針，就是將其轉換成整數，cast只接受指針，所以先獲取他們的指針，再轉。
>>>offset1=i1.contents.value+6 #加偏移6,便是字符串world的地址，接下來需要將此地址轉成c_char_p
>>>i2=c_int(offset1) #首先得得到它的對象
>>>str2=cast(byref(i2),POINTER(c_char_p)) #將i2轉成c_char_p，由於cast只接受指針，所以先取其指針，再轉。
>>>print str2.contents.value
world挺無聊，其實就是想通過這個例子弄清楚ctypes中常用類型的類型轉換。