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28 .\" @(#)flock.2 8.2 (Berkeley) 12/11/93
29 .\" $FreeBSD: src/lib/libc/sys/flock.2,v 1.8.2.8 2001/12/14 18:34:00 ru Exp $
36 .Nd "apply or remove an advisory lock on an open file"
42 .Fn flock "int fd" "int operation"
47 lock on the file associated with the file descriptor
49 A lock is applied by specifying an
51 parameter that is one of
55 with the optional addition of
63 Advisory locks allow cooperating processes to perform
64 consistent operations on files, but do not guarantee
65 consistency (i.e., processes may still access files
66 without using advisory locks possibly resulting in
69 The locking mechanism allows two types of locks:
74 At any time multiple shared locks may be applied to a file,
75 but at no time are multiple exclusive, or both shared and exclusive,
76 locks allowed simultaneously on a file.
80 to an exclusive lock, and vice versa, simply by specifying
81 the appropriate lock type; this results in the previous
82 lock being released and the new lock applied (possibly
83 after other processes have gained and released the lock).
85 Requesting a lock on an object that is already locked
86 normally causes the caller to be blocked until the lock may be
91 then this will not happen; instead the call will fail and
96 Locks are on files, not file descriptors. That is, file descriptors
101 do not result in multiple instances of a lock, but rather multiple
102 references to a single lock. If a process holding a lock on a file
103 forks and the child explicitly unlocks the file, the parent will
106 Processes blocked awaiting a lock may be awakened by signals.
114 .It Bq Er EWOULDBLOCK
115 The file is locked and the
117 option was specified.
121 is an invalid descriptor.
125 refers to an object other than a file.
129 refers to an object that does not support file locking.
140 function call appeared in