The lckpwdf() and ulckpwdf() functions enable modification access to the password databases through the lock file. A process first uses lckpwdf()
to lock the lock file, thereby gaining exclusive rights to modify the /etc/passwd or /etc/shadow password database. See passwd(4) and shadow(4). Upon completing modifications, a process should release the lock on the lock file using ulckpwdf(). This mechanism prevents simultaneous modification
of the password databases. The lock file, /etc/.pwd.lock, is used to coordinate modification access to the password databases /etc/passwd and /etc/shadow.
If lckpwdf() is successful in locking the file within 15 seconds, it returns 0. If unsuccessful (for example, /etc/.pwd.lock is already
locked), it returns -1.
If ulckpwdf() is successful in unlocking the file /etc/.pwd.lock, it returns 0. If unsuccessful (for example, /etc/.pwd.lock is already unlocked), it returns -1.
