Python Language Common pitfalls


Example

Failing to load a file

The first possible error is failing to load the library. In that case an OSError is usually raised.

This is either because the file doesn't exists (or can't be found by the OS):

>>> cdll.LoadLibrary("foobar.so")
Traceback (most recent call last):
File "<stdin>", 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: foobar.so: cannot open shared object file: No such file or directory

As you can see, the error is clear and pretty indicative.

The second reason is that the file is found, but is not of the correct format.

>>> cdll.LoadLibrary("libc.so")
Traceback (most recent call last):
File "<stdin>", 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: /usr/lib/i386-linux-gnu/libc.so: invalid ELF header

In this case, the file is a script file and not a .so file. This might also happen when trying to open a .dll file on a Linux machine or a 64bit file on a 32bit python interpreter. As you can see, in this case the error is a bit more vague, and requires some digging around.

Failing to access a function

Assuming we successfully loaded the .so file, we then need to access our function like we've done on the first example.

When a non-existing function is used, an AttributeError is raised:

>>> libc.foo
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.5/ctypes/__init__.py", line 360, in __getattr__
    func = self.__getitem__(name)
File "/usr/lib/python3.5/ctypes/__init__.py", line 365, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: /lib/i386-linux-gnu/libc.so.6: undefined symbol: foo