在 Python 中如何在转换为 ctypes c_wchar_p 时引用 function 字符串参数的值

Question

Context

I have used Python very occasionally in the past and am not familiar with its type system. This is the first time I am consuming the win32 API with Python ctypes.

Issue

With this code authentication succeeds and the process is started

def run_as_user(command):
  lpUsername            = ctypes.c_wchar_p("hardcoded username")
  lpDomain              = ctypes.c_wchar_p("hardcoded domain")
  lpPassword            = ctypes.c_wchar_p("hardcoded password")

  ... 

  ctypes.windll.advapi32.CreateProcessWithLogonW(
    lpUsername,
    lpDomain, 
    lpPassword, 
    ...
  )

However with this code authentication fails.

def run_as_user(username, domain, password, command):
  lpUsername            = ctypes.c_wchar_p(username)
  lpDomain              = ctypes.c_wchar_p(domain)
  lpPassword            = ctypes.c_wchar_p(password)

  ... 

  ctypes.windll.advapi32.CreateProcessWithLogonW(
    lpUsername,
    lpDomain, 
    lpPassword, 
    ...
  )

Questions

How can I fix the second code so that the conversion to ctypes c_wchar_p is performed on the values of the strings passed as function arguments instead of on their reference?

Where can I find resources about how strings are passed in Python function calls and ctypes casts?

Answer 1

A simple method, you only need to call the .value attribute of ctypes.c_wchar_p , you can get a Python native object, so as to call the function correctly.

def run_as_user(username, domain, password, command):
  lpUsername            = ctypes.c_wchar_p(username)
  lpDomain              = ctypes.c_wchar_p(domain)
  lpPassword            = ctypes.c_wchar_p(password)

  ... 

  ctypes.windll.advapi32.CreateProcessWithLogonW(
    lpUsername.value,
    lpDomain.value, 
    lpPassword.value, 
    ...
  )

More reference: casting into a Python string from a char[] returned by a DLL

Answer 2

Dude, seriously, your two examples as shown are doing the same thing. I don't know why your authentication fails (the function actually takes 11 parameters) and I'm not debugging it without the asked for reproducible example, but your handling of the parameters is identical. See this answer for a correct ctypes declaration.

Here's an actual minimal reproducible example showing handling of a wchar_t* parameter in various ways:

test.c (A function taking a wchar_t*, printing and returning it):

#include <stdio.h>

#define API __declspec(dllexport)

API wchar_t* demo(wchar_t* input) {
    printf("In demo: %S\n", input);
    return input;
}

test.py

from ctypes import *

dll = CDLL('./test')

def demo1():
    pdata = c_wchar_p("hard coded")   # Your "hard coded" example
    dll.demo(pdata)

def demo2(data):
        pdata = c_wchar_p(data)       # Your "not hard coded" example
        dll.demo(pdata)
        
demo1()                  # works
demo2("soft coded")      # also works

data = "example"

 # Other "accepted" answer is just extracting the original value from the wrapper
print(c_wchar_p(data).value == data)  # returns True

dll.demo('also no wrapper')  # no need for a wrapper at all actually
                             # ctypes "knows" to marshal `str` as `wchar_t*`
try:
    dll.demo(123)            # but no error checking without setting .argtypes
except Exception as e:       # as recommended crashes the C code
    print(e)

dll.demo.argtypes = c_wchar_p,   # but setting .argtypes as recommended
try:                             # gives a better error message
    dll.demo(123)
except Exception as e:
    print(e)

result = dll.demo('restype test')  # not setting .restype as recommended
print('no restype =',result)       # returns garbage

dll.demo.restype = c_wchar_p       # setting the restype as recommended
result = dll.demo('restype test')  # returns the right value
print('with restype =',result)

Output:

In demo: hard coded
In demo: soft coded
True
In demo: no wrapper
In demo: also no wrapper
exception: access violation reading 0x000000000000007B    # .argtypes not set
argument 1: <class 'TypeError'>: wrong type
no restype = 1373086064                                   # .restype not set
with restype = restype test
In demo: In demo: restype test
In demo: restype test