Programming Perl, 4th Edition

flock

flock FILEHANDLE, OPERATION

The flock function is Perl’s portable file-locking interface. It locks only entire files, not individual records. The function manages locks on the file associated with FILEHANDLE, returning true for success and false otherwise. To avoid the possibility of lost data, Perl flushes your FILEHANDLE before locking or unlocking it. Perl might implement its flock using flock(2), fcntl(2), lockf(3), or some other platform-specific lock mechanism; if none of these is available, calling flock raises an exception. See the section File Locking in Chapter 15.

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly or‘d with LOCK_NB. These constants are traditionally valued 1, 2, 8, and 4, but you can use the symbolic names if you import them from the Fcntl module, either individually or as a group using the :flock tag.

LOCK_SH requests a shared lock, so it’s typically used for reading. LOCK_EX requests an exclusive lock, so it’s typically used for writing. LOCK_UN releases a previously requested lock; closing the file also releases any locks. If the LOCK_NB bit is used with LOCK_SH or LOCK_EX, flock returns immediately rather than waiting for an unavailable lock. ...