Lockdev is a setgid binary, which provides a reliable way to put an exclusive lock in /var/lock to devices (e.g. ttyS0) using both FSSTND and SVr4 methods, so regular users don't need write access there.
This task is achieved through calls to the liblockdev(3) API.
/usr/sbin/lockdev [<operation>] <device>
The lockdev command can perform one of the following operations (calls to liblockdev(3) API):
-l - lock device with dev_lock() function
-u - unlock device with dev_unlock() function
no operation - test lock with dev_testlock() function
Value dev_lock dev_unlock dev_testlock
0 OK OK not locked
1 locked other locked other locked
255 error error error