Python | accessing dll using ctypes

Question

I'm trying to access some functions in a dll (nss3.dll) that ships with Firefox web browser. To handle this task I have used ctypes in Python. The problem is that it fails at the initial point which is when loading the dll in to the memory.

This is the code snippet that I have to do so.

>>> from ctypes import *
>>> windll.LoadLibrary("E:\\nss3.dll")

The exception I'm getting is

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    windll.LoadLibrary("E:\\nss3.dll")
  File "C:\Python26\lib\ctypes\__init__.py", line 431, in LoadLibrary
    return self._dlltype(name)
  File "C:\Python26\lib\ctypes\__init__.py", line 353, in __init__
    self._handle = _dlopen(self._name, mode)
WindowsError: [Error 126] The specified module could not be found

I also tried loading it from the Firefox installation path assuming that there maybe dependencies.

>>> windll.LoadLibrary("F:\\Softwares\\Mozilla Firefox\\nss3.dll")

But I'm getting the same exception as mentioned above.

Thanks.

Answer 1

nss3.dll is linked to the following DLLs, which are all located in the Firefox directory: nssutil3.dll, plc4.dll, plds4.dll, nspr4.dll, and mozcrt19.dll. The system library loader looks for these files in the DLL search path of the process, which includes the application directory, system directories, the current directory, and each of the directories listed in the PATH environment variable.

The simplest solution is to change the current directory to the DLL Firefox directory. However, that's not thread safe, so I wouldn't rely on it in general. Another option is to append the Firefox directory to the PATH environment variable, which is what I suggested in my original version of this answer. However, that's not much better than modifying the current directory.

Newer versions of Windows (NT 6.0+ with update KB2533623) allow the DLL search path to be updated in a thread-safe manner via SetDefaultDllDirectories, AddDllDirectory, and RemoveDllDirectory. But that approach would be over the top here.

In this case, for the sake of both simplicity and compatibility with older versions of Windows, it suffices to call LoadLibraryEx with the flag LOAD_WITH_ALTERED_SEARCH_PATH. You need to load the DLL using an absolute path, else the behavior is undefined. For convenience we can subclass ctypes.CDLL and ctypes.WinDLL to call LoadLibraryEx instead of LoadLibrary.

import os
import ctypes

if os.name == 'nt':
    from ctypes import wintypes

    kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)

    def check_bool(result, func, args):
        if not result:
            raise ctypes.WinError(ctypes.get_last_error())
        return args

    kernel32.LoadLibraryExW.errcheck = check_bool
    kernel32.LoadLibraryExW.restype = wintypes.HMODULE
    kernel32.LoadLibraryExW.argtypes = (wintypes.LPCWSTR,
                                        wintypes.HANDLE,
                                        wintypes.DWORD)

class CDLLEx(ctypes.CDLL):
    def __init__(self, name, mode=0, handle=None, 
                 use_errno=True, use_last_error=False):
        if os.name == 'nt' and handle is None:
            handle = kernel32.LoadLibraryExW(name, None, mode)
        super(CDLLEx, self).__init__(name, mode, handle,
                                     use_errno, use_last_error)

class WinDLLEx(ctypes.WinDLL):
    def __init__(self, name, mode=0, handle=None, 
                 use_errno=False, use_last_error=True):
        if os.name == 'nt' and handle is None:
            handle = kernel32.LoadLibraryExW(name, None, mode)
        super(WinDLLEx, self).__init__(name, mode, handle,
                                       use_errno, use_last_error)

Here are all of the available LoadLibraryEx flags:

DONT_RESOLVE_DLL_REFERENCES         = 0x00000001
LOAD_LIBRARY_AS_DATAFILE            = 0x00000002
LOAD_WITH_ALTERED_SEARCH_PATH       = 0x00000008
LOAD_IGNORE_CODE_AUTHZ_LEVEL        = 0x00000010  # NT 6.1
LOAD_LIBRARY_AS_IMAGE_RESOURCE      = 0x00000020  # NT 6.0
LOAD_LIBRARY_AS_DATAFILE_EXCLUSIVE  = 0x00000040  # NT 6.0

# These cannot be combined with LOAD_WITH_ALTERED_SEARCH_PATH.
# Install update KB2533623 for NT 6.0 & 6.1.
LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR    = 0x00000100
LOAD_LIBRARY_SEARCH_APPLICATION_DIR = 0x00000200
LOAD_LIBRARY_SEARCH_USER_DIRS       = 0x00000400
LOAD_LIBRARY_SEARCH_SYSTEM32        = 0x00000800
LOAD_LIBRARY_SEARCH_DEFAULT_DIRS    = 0x00001000

For example:

firefox_path = r'F:\Softwares\Mozilla Firefox'
nss3 = CDLLEx(os.path.join(firefox_path, 'nss3.dll'), 
              LOAD_WITH_ALTERED_SEARCH_PATH)

nss3.NSS_GetVersion.restype = c_char_p

>>> nss3.NSS_GetVersion()                 
'3.13.5.0 Basic ECC'