Lock files should be stored within the /var/lock directory structure.

Lock files for devices and other resources shared by multiple applications, such as the serial device lock files that were originally found in either /usr/spool/locks or /usr/spool/uucp, must now be stored in /var/lock. The naming convention which must be used is "LCK.." followed by the base name of the device. For example, to lock /dev/ttyS0 the file "LCK..ttyS0" would be created. ^[43]

The format used for the contents of such lock files must be the HDB UUCP lock file format. The HDB format is to store the process identifier (PID) as a ten byte ASCII decimal number, with a trailing newline. For example, if process 1230 holds a lock file, it would contain the eleven characters: space, space, space, space, space, space, one, two, three, zero, and newline.