Both advisory and mandatory locking are supported on the following types of file systems:
ufs--the default disk-based file system
fifofs--a pseudo file system of named pipe files that give processes common access to data
namefs--a pseudo file system used mostly by STREAMS for dynamic mounts of file descriptors on top of file
specfs--a pseudo file system that provides access to special character and block devices
Only advisory file locking is supported on NFS.
File locking is not supported for the proc and fd file systems.