Fix ctypes usage with Python 3.9

[vincentbernat]

There is a regression in Python 3.9 with the find_library()
function:

>>> import ctypes.util
>>> ctypes.util.find_library("libc")
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "/usr/lib/python3.9/ctypes/util.py", line 341, in find_library
     _get_soname(_findLib_gcc(name)) or _get_soname(_findLib_ld(name))
   File "/usr/lib/python3.9/ctypes/util.py", line 147, in _findLib_gcc
     if not _is_elf(file):
   File "/usr/lib/python3.9/ctypes/util.py", line 99, in _is_elf
     with open(filename, 'br') as thefile:
 FileNotFoundError: [Errno 2] No such file or directory: b'liblibc.a'

A workaround is to use find_library("c") instead. It also works in
older versions of Python and that's already what's used in
contrib/isotp.py.

Python issue reported here:
https://bugs.python.org/issue42580

Signed-off-by: Vincent Bernat [email protected]

[guedou]

Thanks for the PR!

[guedou]

On which operating system did you get the issue? It works fine on macOS with Python3.9.

[vincentbernat]

On Debian unstable. If it helps, note find_library() does not expect to get a name prefixed with lib:

https://docs.python.org/3/library/ctypes.html

Try to find a library and return a pathname. name is the library name without any prefix like lib, suffix like .so, .dylib or version number (this is the form used for the posix linker option -l). If no library can be found, returns None.

[p-l-]

Python 3.9.0 (under ArchLinux) gets no error:

>>> import ctypes.util
>>> ctypes.util.find_library("libc")

But:

>>> ctypes.util.find_library("c")
'libc.so.6'

Which is probably what one expects.