Everything you never wanted to know about file locking (2010)

4 months ago

File locking in Unix is complex and varies between systems, with three main APIs: flock(), fcntl(), and lockf().
flock() is simple but not POSIX-standardized, doesn't work over NFS, and has race conditions when upgrading locks.
fcntl() is POSIX-standardized, supports byte-range locks, but has insane behaviors like losing locks when any file descriptor to the inode is closed.
lockf() is also POSIX-standardized but may not be supported on all BSD systems and lacks features like querying lock ownership.
Mixing different lock types (flock(), fcntl(), lockf()) on the same file is undefined and non-portable.
Mandatory locks are unreliable and should be avoided; advisory locks are the only sensible choice.
Python's fcntl module provides access to all three lock types but has its own quirks and inconsistencies.
A bug in MacOS X 10.6's fcntl(F_SETLK) could cause silent file corruption, affecting programs like sqlite.
Windows 10 WSL 'implements' fcntl() locks by always returning success, leading to file corruption.
Linux introduced F_OFD_SETLK in 2015, a saner alternative to traditional fcntl() locks, but it's not yet widely portable.

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