时间戳

首页 > 解决方案 > loader 实例 `cdll` 的属性是如何创建的？

python - loader 实例 `cdll` 的属性是如何创建的？

问题描述

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

共享库也可以通过使用预制对象之一来加载，这些对象是 LibraryLoader 类的实例，或者通过调用 LoadLibrary() 方法，或者通过检索库作为加载器实例的属性。

我找到了第一种方式的示例Free the open ctypes library in Python

我想知道如何使用第二种方式？特别是loader实例的属性是如何cdll创建的？我的问题来自为什么加载 libc 共享库有“'LibraryLoader' 对象不可调用”错误？

LibraryLoader 的全部意义在于它会在您访问它时为您创建库。并且cdll.LoadLibrary("foo")不创建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'>}

标签： pythonpython-3.xshared-librariesctypesdynamic-loading

解决方案

示例（发生了什么）：

>>> 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']

它是如何发生的——都在“${PYTHON_SRC_DIR}/Lib/ctypes/__init__.py”
（[GitHub]：python/cpython -（master）cpython/Lib/ctypes/__init__.py）中。我正在粘贴代码，cdll因为windll（我在上面的示例中使用）只是它的一个包装器（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（至少是Lnx ）上，事情并不是那么好：

[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（相对于[MSDN]: LoadLibrary function），[man7]: DLOPEN(3)不会将（默认）库扩展名（.so）附加到文件名（如果它不包含它）已经）。

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，无法在Ux上复制Win行为（不幸的是，（dot）不能成为属性名称的一部分，以克服这一点）。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.so和.dylib都可以）。.

推荐阅读