python - loader实例 `cdll`的属性是如何创建的？

Question

来自https://docs.python.org/3/library/ctypes.html#loading-shared-libraries

Shared libraries can also be loaded by using one of the prefabricated objects, which are instances of the LibraryLoader class, either by calling the LoadLibrary() method, or by retrieving the library as attribute of the loader instance.

我找到了第一种方式的例子Free the opened ctypes library in Python

我想知道如何使用第二种方式？特别是，加载器实例cdll的属性是如何创建的？我的问题来自Why does loading the libc shared library have "'LibraryLoader' object is not callable" error?

The whole point of the LibraryLoader is that it creates the library for you when you access it. And cdll.LoadLibrary("foo") doesn't create cdll.foo.

我的实验有问题吗？为什么cdll.libc永远不存在？

>>> from ctypes import *
>>> cdll.libc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/ctypes/__init__.py", line 418, in __getattr__
    dll = self._dlltype(name)
  File "/usr/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libc: cannot open shared object file: No such file or directory
>>> cdll.LoadLibrary("libc.so.6")
<CDLL 'libc.so.6', handle 7f6afe03a000 at 0x7f6afc1afac8>
>>> cdll.libc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/ctypes/__init__.py", line 418, in __getattr__
    dll = self._dlltype(name)
  File "/usr/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libc: cannot open shared object file: No such file or directory
>>> libc=cdll.LoadLibrary("libc.so.6")
>>> cdll.libc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/ctypes/__init__.py", line 418, in __getattr__
    dll = self._dlltype(name)
  File "/usr/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libc: cannot open shared object file: No such file or directory
>>> CDLL("libc.so.6")
<CDLL 'libc.so.6', handle 7f6afe03a000 at 0x7f6afc1af978>
>>> cdll.libc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/ctypes/__init__.py", line 418, in __getattr__
    dll = self._dlltype(name)
  File "/usr/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libc: cannot open shared object file: No such file or directory

>>> libc=CDLL("libc.so.6")
>>> cdll.libc
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/ctypes/__init__.py", line 418, in __getattr__
    dll = self._dlltype(name)
  File "/usr/lib/python3.6/ctypes/__init__.py", line 348, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libc: cannot open shared object file: No such file or directory
>>> cdll.__dict__
{'_dlltype': <class 'ctypes.CDLL'>}

Answer 1

示例(发生什么):

>>> import sys
>>> import ctypes
>>> print("Python {:s} on {:s}".format(sys.version, sys.platform))
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32
>>>
>>> [item for item in dir(ctypes.windll) if "__" not in item]
['LoadLibrary', '_dlltype', 'kernel32']
>>>
>>> user32_dll = ctypes.windll.LoadLibrary("user32")
>>> user32_dll
<WinDLL 'user32', handle 7ff882810000 at 0x2434399b4e0>
>>> [item for item in dir(ctypes.windll) if "__" not in item]
['LoadLibrary', '_dlltype', 'kernel32']
>>>
>>> user32_dll = ctypes.WinDLL("user32")
>>> user32_dll
<WinDLL 'user32', handle 7ff882810000 at 0x2434399b4a8>
>>> [item for item in dir(ctypes.windll) if "__" not in item]
['LoadLibrary', '_dlltype', 'kernel32']
>>>
>>> user32_dll = ctypes.windll.user32
>>> user32_dll
<WinDLL 'user32', handle 7ff882810000 at 0x24343984d68>
>>> [item for item in dir(ctypes.windll) if "__" not in item]
['LoadLibrary', '_dlltype', 'kernel32', 'user32']
>>>
>>> ctypes.windll.user32
<WinDLL 'user32', handle 7ff882810000 at 0x24343984d68>
>>>
>>> ctypes.windll.user321
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\install\x64\python\python\3.5\Lib\ctypes\__init__.py", line 421, in __getattr__
    dll = self._dlltype(name)
  File "c:\install\x64\python\python\3.5\Lib\ctypes\__init__.py", line 351, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: [WinError 126] The specified module could not be found
>>>
>>> dir(ctypes.windll)
['LoadLibrary', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattr__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', '_dlltype', 'kernel32', 'user32']

如何发生 - 一切都在“${PYTHON_SRC_DIR}/Lib/ctypes/__init__.py”
([GitHub]: python/cpython - (master) cpython/Lib/ctypes/__init__.py)中。我正在粘贴 cdll 的代码，因为 windll (我在上面的示例中使用的)只是它的包装器(并且需要更多代码):

# ...

class CDLL(object):

# ...

class LibraryLoader(object):
    def __init__(self, dlltype):
        self._dlltype = dlltype

    def __getattr__(self, name):
        if name[0] == '_':
            raise AttributeError(name)
        dll = self._dlltype(name)
        setattr(self, name, dll)  # @TODO - cfati: This is the key for always returning the same instance.
        return dll

# ...

cdll = LibraryLoader(CDLL)

# ...

@EDIT0:

嗯，在Ux(至少Lnx)方面，情况不太好:

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> ls
libcapi.so
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> LD_LIBRARY_PATH=$(pwd) python3 -c "import ctypes; ctypes.cdll.LoadLibrary('libcapi.so')"
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> LD_LIBRARY_PATH=$(pwd) python3 -c "import ctypes; ctypes.cdll.LoadLibrary('libcapi')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.5/ctypes/__init__.py", line 425, in LoadLibrary
    return self._dlltype(name)
  File "/usr/lib/python3.5/ctypes/__init__.py", line 347, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libcapi: cannot open shared object file: No such file or directory

那是因为显然(与 [MSDN]: LoadLibrary function 相对)， [man7]: DLOPEN(3)不会将(默认)库扩展名 (.so) 附加到文件名中(如果它尚未包含)。

代码.c:

#include <stdio.h>
#include <dlfcn.h>


int main(int argc, char *argv[]) {
    if (argc == 1) {
        printf("Dll name required\n");
        return 1;
    }
    void *handle = dlopen(argv[1], RTLD_NOW);
    if (handle == NULL) {
        printf("Could not load [%s]\n", argv[1]);
        return 2;
    } else {
        printf("Successfully loaded [%s]\n", argv[1]);
        dlclose(handle);
        return 0;
    }
}

输出:

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> gcc code.c -Wl,-ldl
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> ./a.out "libcapi.so"
Could not load [libcapi.so]
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> LD_LIBRARY_PATH=$(pwd) ./a.out "libcapi.so"
Successfully loaded [libcapi.so]
[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q050838633]> LD_LIBRARY_PATH=$(pwd) ./a.out "libcapi"
Could not load [libcapi]

因此，Win 行为无法在 Ux 上复制(不幸的是，.(dot)不能成为属性名称的一部分，以克服这个问题)。或者，也许链接器可以配置为隐式搜索 .so 文件？但这只能部分解决问题，因为许多库看起来像 libc.so.6 (或 AFAIK，在 OSX 上都是 .所以和.dylib是OK)。