Os File File Lock
DodaTech
1 min read
In this tutorial, you'll learn about How to Fix File Locking Errors. We cover key concepts, practical examples, and best practices.
Fix file locking errors when flock vs fcntl lock confusion leading to incorrect locking behavior.
Quick Fix
Wrong
import fcntl, os
f=open('data.txt','w')
fcntl.flock(f, fcntl.LOCK_EX) # advisory lock
# But fcntl.flock and fcntl.fcntl(F_SETLK) are different!
# flock: per-fd, not inherited by fork, released on ANY fd close
# fcntl: per-inode, inherited by fork, associated with process
flock releases lock when ANY fd pointing to same file is closed. Not fork-safe.
Right
import fcntl, os
f=open('data.txt','w')
# Use fcntl (POSIX) lock instead of flock:
lockdata=fcntl.fcntl(f, fcntl.F_GETLK, fcntl.flock(0,0,0,0,0))
if lockdata.l_type==fcntl.F_UNLCK:
# Acquire write lock:
lock=fcntl.flock(fcntl.F_WRLCK, fcntl.SEEK_SET, 0, 0, 0)
fcntl.fcntl(f, fcntl.F_SETLK, lock)
print('Lock acquired')
# flock vs fcntl summary:
# - flock: simple, per-fd, released on any close. Good for single-process.
# - fcntl: POSIX, per-process, works with NFS. Use for multi-process.
fcntl lock acquired. Process-specific. Survives fd close until process releases or dies.
Prevention
Use fcntl (POSIX) locks for cross-process synchronization. flock for per-process safety.
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