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4 Mandatory File Locking For The Linux Operating System
13 0. Why you should avoid mandatory locking
19 	- The write system call checks for a mandatory lock only once
22 	  A process may then see file data change even while a mandatory
32 1. What is  mandatory locking?
35 Mandatory locking is kernel enforced file locking, as opposed to the more usual
51 with a "mandatory" locking scheme, whereby the operating system kernel would
56 The System V mandatory locking scheme was intended to have as little impact as
58 as candidates for mandatory locking, and using the existing fcntl()/lockf()
66       addition to entire files, so the mandatory locking rules also have byte
69    2. POSIX.1 does not specify any scheme for mandatory locking, despite
70       borrowing the fcntl() locking scheme from System V. The mandatory locking
73 2. Marking a file for mandatory locking
76 A file is marked as a candidate for mandatory locking by setting the group-id
83 modified to recognize the special case of a mandatory lock candidate and to
85 to run mandatory lock candidates with setgid privileges.
90 I have considered the implementations of mandatory locking available with
97 another process has outstanding mandatory locks. This is in direct
104 just mandatory locks. That would appear to contravene POSIX.1.
107 prevent mandatory locks from being applied to an mmap()'ed file, but  HP-UX
112 only from mandatory locks - that is what is currently implemented.
115 mandatory locks, so reads and writes to locked files always block when they
125 1. Mandatory locks can only be applied via the fcntl()/lockf() locking
127    locks using flock() never result in a mandatory lock.
129 2. If a process has locked a region of a file with a mandatory read lock, then
136 3. If a process has locked a region of a file with a mandatory write lock, all
142    any mandatory locks owned by other processes will be rejected with the
145 5. Attempts to apply a mandatory lock to a file that is memory mapped and
150    that has any mandatory locks in effect will be rejected with the error status
159 for the purposes of mandatory locking.
167 Note 3: I may have overlooked some system calls that need mandatory lock
174 Not even root can override a mandatory lock, so runaway processes can wreak
181 Mandatory locking is disabled on all filesystems by default, and must be
185 Since kernel v4.5, it is possible to disable mandatory locking