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4 Mandatory File Locking For The Linux Operating System
22 	  A process may then see file data change even while a mandatory
24 	- Similarly, an exclusive lock may be granted on a file after
27 	  the file data in a state which should not have been visible
35 Mandatory locking is kernel enforced file locking, as opposed to the more usual
36 cooperative file locking used to guarantee sequential access to files among
37 processes. File locks are applied using the flock() and fcntl() system calls
39 normally a process' responsibility to check for locks on a file it wishes to
40 update, before applying its own lock, updating the file and unlocking it again.
52 block attempts by a process to write to a file that another process holds a
54 file that a process holds a "write " -or- "exclusive" lock on.
63    1. In saying "file" in the paragraphs above I am actually not telling
73 2. Marking a file for mandatory locking
76 A file is marked as a candidate for mandatory locking by setting the group-id
77 bit in its file mode but removing the group-execute bit. This is an otherwise
82 a setgid file is written to. This is a security measure. The kernel has been
96 All the reference systems reject all calls to open() for a file on which
101 modify the contents of the file.
103 HP-UX even disallows open() with O_TRUNC for a file with advisory locks, not
107 prevent mandatory locks from being applied to an mmap()'ed file, but  HP-UX
108 also disallows advisory locks for such a file. SVID actually specifies the
129 2. If a process has locked a region of a file with a mandatory read lock, then
132    released, unless the process has opened the file with the O_NONBLOCK
136 3. If a process has locked a region of a file with a mandatory write lock, all
138    unless a process has opened the file with the O_NONBLOCK flag in which case
141 4. Calls to open() with O_TRUNC, or to creat(), on a existing file that has
145 5. Attempts to apply a mandatory lock to a file that is memory mapped and
149 6. Attempts to create a shared memory map of a file (via mmap() with MAP_SHARED)
156 Those which modify a file's contents, not just the inode. That gives read(),
163 defined as the bytes of a file removed or added (we must also consider bytes
164 added, as a lock can specify just "the whole file", rather than a specific
175 havoc if they lock crucial files. The way around it is to change the file