python lockf: LOCK_EX works but LOCK_SH doesn't

Question

I'm puzzled by the behaviour of the lockf function from the python fcntl library: I can't get a shared lock, while the exclusive one works:

In [1]: import fcntl                                                            

In [2]: f = open('file', 'w')                                                   

In [3]: fcntl.lockf(f, fcntl.LOCK_SH | fcntl.LOCK_NB)                           
---------------------------------------------------------------------------
OSError                                   Traceback (most recent call last)
<ipython-input-3-5d23c6a5f968> in <module>
----> 1 fcntl.lockf(f, fcntl.LOCK_SH | fcntl.LOCK_NB)

OSError: [Errno 9] Bad file descriptor

In [4]: fcntl.lockf(f, fcntl.LOCK_EX | fcntl.LOCK_NB)                           

In [5]: 🤔

The error code corresponds to EBADF from the http://man7.org/linux/man-pages/man3/lockf.3.html , which does not make much sense, as f is a writable open file descriptor.

Any ideas?

(Python 3.6.9, Ubuntu 18.04.4 LTS)

Answer 1

fcntl.lockf sure looks like it should be a wrapper around POSIX lockf , but it's not. POSIX lockf doesn't even have shared locks.

fcntl.lockf is a wrapper around POSIX fcntl . LOCK_SH corresponds to F_RDLCK , which requires a file descriptor opened for reading .

While you're at it, you might want to read about the problems with file locking .