Find_library () in ctypes

Question

I am trying to use the find_library () command from ctypes, but I am getting an error that I do not understand its cause. I am working on windows

This is the code:

import ctypes
from ctypes.util import find_library
import numpy
from string import atoi
from time import sleep

# Class constants
#nidaq = ctypes.windll.nicaiu
nidaq  = ctypes.cdll.LoadLibrary(find_library('NIDAQmx'))

And this is the error I get:

Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    nidaq  = ctypes.cdll.LoadLibrary(find_library('NIDAQmx'))
  File "C:\Python27\lib\ctypes\__init__.py", line 443, in LoadLibrary
    return self._dlltype(name)
  File "C:\Python27\lib\ctypes\__init__.py", line 365, in __init__
    self._handle = _dlopen(self._name, mode)
TypeError: expected string or Unicode object, NoneType found

Should I place NIDAQmx in a specific place, for example, so that it can be found? Or is it not connected?

Thank!

Answer 1

On Windows, it find_librarysearches for directories in an environment variable PATH, which is not the real search order for desktop applications used by the Windows boot loader. In particular, it find_librarydoes not include the application directory and the current directory.

Windows SearchPath , , DLL API, SetDllDirectory API SetDefaultDllDirectories AddDllDirectory.

, Windows, DLL CDLL (cdecl) WinDLL (stdcall):

nidaq_cdecl   = ctypes.CDLL('NIDAQmx')
nidaq_stdcall = ctypes.WinDLL('NIDAQmx')

DLL PATH ( Linux LD_LIBRARY_PATH ). , , DLL- "dlls" . :

import os

basepath = os.path.dirname(os.path.abspath(__file__))
dllspath = os.path.join(basepath, 'dlls')
os.environ['PATH'] = dllspath + os.pathsep + os.environ['PATH']

---

, GetDllDirectory SetDllDirectory, , . , , PATH, , . , , CreateProcess .

import os
import ctypes
from ctypes import wintypes
from contextlib import contextmanager

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

def check_dword(result, func, args):
    if result == 0:
        last_error = ctypes.get_last_error()
        if last_error != 0:
            raise ctypes.WinError(last_error)
    return args

def check_bool(result, func, args):
    if not result:
        last_error = ctypes.get_last_error()
        if last_error != 0:
            raise ctypes.WinError(last_error)
        else:
            raise OSError
    return args

kernel32.GetDllDirectoryW.errcheck = check_dword
kernel32.GetDllDirectoryW.argtypes = (wintypes.DWORD,  # _In_  nBufferLength
                                      wintypes.LPWSTR) # _Out_ lpBuffer

kernel32.SetDllDirectoryW.errcheck = check_bool
kernel32.SetDllDirectoryW.argtypes = (wintypes.LPCWSTR,) # _In_opt_ lpPathName

@contextmanager
def use_dll_dir(dll_dir):
    size = newsize = 0
    while newsize >= size:
        size = newsize
        prev = (ctypes.c_wchar * size)()
        newsize = kernel32.GetDllDirectoryW(size, prev)
    kernel32.SetDllDirectoryW(os.path.abspath(dll_dir))
    try:
        yield
    finally:
        kernel32.SetDllDirectoryW(prev)

:

if __name__ == '__main__':
    basepath = os.path.dirname(os.path.abspath(__file__))
    dllspath = os.path.join(basepath, 'dlls')
    with use_dll_dir(dllspath):
        nidaq = ctypes.CDLL('NIDAQmx')

, DLL , . SetDllDirectoryW .

---

LoadLibraryEx LOAD_WITH_ALTERED_SEARCH_PATH, DLL . DLL, , undefined. ctypes.CDLL ctypes.WinDLL LoadLibraryEx LoadLibrary.

import ctypes
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 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 handle is None:
            handle = kernel32.LoadLibraryExW(name, None, mode)
        super(WinDLLEx, self).__init__(name, mode, handle,
                                       use_errno, use_last_error)

LoadLibraryEx:

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

:

if __name__ == '__main__':
    basepath = os.path.dirname(os.path.abspath(__file__))
    dllpath = os.path.join(basepath, 'dlls', 'NIDAQmx.dll')
    nidaq = CDLLEx(dllpath, LOAD_WITH_ALTERED_SEARCH_PATH)

Answer 2

, , . find_library , , ctypes/macholib/dyld.py (. dyld_find).

, , /usr/lib, , , , DYLD_LIBRARY_PATH.