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32 .\" @(#)flock.2 8.2 (Berkeley) 12/11/93
33 .\" $FreeBSD: src/lib/libc/sys/flock.2,v 1.8.2.8 2001/12/14 18:34:00 ru Exp $
40 .Nd "apply or remove an advisory lock on an open file"
45 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
46 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
47 .Fd "#define LOCK_NB 0x04 /* don't block when locking */"
48 .Fd "#define LOCK_UN 0x08 /* unlock file */"
50 .Fn flock "int fd" "int operation"
55 lock on the file associated with the file descriptor
57 A lock is applied by specifying an
59 parameter that is one of
63 with the optional addition of
71 Advisory locks allow cooperating processes to perform
72 consistent operations on files, but do not guarantee
73 consistency (i.e., processes may still access files
74 without using advisory locks possibly resulting in
77 The locking mechanism allows two types of locks:
82 At any time multiple shared locks may be applied to a file,
83 but at no time are multiple exclusive, or both shared and exclusive,
84 locks allowed simultaneously on a file.
88 to an exclusive lock, and vice versa, simply by specifying
89 the appropriate lock type; this results in the previous
90 lock being released and the new lock applied (possibly
91 after other processes have gained and released the lock).
93 Requesting a lock on an object that is already locked
94 normally causes the caller to be blocked until the lock may be
99 then this will not happen; instead the call will fail and
104 Locks are on files, not file descriptors. That is, file descriptors
109 do not result in multiple instances of a lock, but rather multiple
110 references to a single lock. If a process holding a lock on a file
111 forks and the child explicitly unlocks the file, the parent will
114 Processes blocked awaiting a lock may be awakened by signals.
122 .It Bq Er EWOULDBLOCK
123 The file is locked and the
125 option was specified.
129 is an invalid descriptor.
133 refers to an object other than a file.
137 refers to an object that does not support file locking.
148 function call appeared in