[python]: how to get the string from the pointer by using ctypes?

Question

Here is the thing, I wrote a program using windows api EnumWindows which requires a callback func as the first arg, my poor code is as follows:

User32 = WinDLL('User32.dll')
LPARAM = wintypes.LPARAM

HWND = wintypes.HWND
BOOL = wintypes.BOOL

def Proc(hwnd, lparam):
    print("hwnd = {}, lparam = {}".format(hwnd, cast(lparam, c_char_p)))
    return True

WNDPROCFUNC = WINFUNCTYPE(BOOL, HWND, LPARAM)  #用winfunctype 比cfunctype 好
cb_proc = WNDPROCFUNC(Proc)

EnumWindows = User32.EnumWindows
EnumWindows.restype = BOOL

EnumWindows(cb_proc, 'abcd')

then I ran the program, but it just print

hwnd = 65820, lparam = c_char_p(b'a')
hwnd = 65666, lparam = c_char_p(b'a')
hwnd = 65588, lparam = c_char_p(b'a')
hwnd = 65592, lparam = c_char_p(b'a')
hwnd = 1311670, lparam = c_char_p(b'a')
hwnd = 591324, lparam = c_char_p(b'a')
hwnd = 66188, lparam = c_char_p(b'a')
hwnd = 393862, lparam = c_char_p(b'a')

why not b'abcd'?

David Heffernan · Accepted Answer · 2011-08-11 09:04:55Z

2

Because you are using Python 3 which is treating abcd as a Unicode string which ctypes encodes with UTF-16. But you then cast it assuming it is a single byte ANSI string.

You can make the program behave the way you want by one of the following methods:

Use Python 2.x
Call EnumWindows like so: EnumWindows(cb_proc, b'abcd')
Use c_wchar_p in the case: cast(lparam, c_wchar_p)

answered Aug 11, 2011 at 9:04

David Heffernan

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2 Comments

Alcott Over a year ago

thank you very much, it works. BTW, why ctypes encodes the string with utf-16 not utf-8? Wouldn't it be convenient to use utf-8?

David Heffernan Over a year ago

@Alcott Windows APIs use UTF16 rather than UTF8.

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