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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 $
34 .\" $DragonFly: src/lib/libc/sys/flock.2,v 1.3 2007/12/03 20:13:10 swildner Exp $
41 .Nd "apply or remove an advisory lock on an open file"
46 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
47 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
48 .Fd "#define LOCK_NB 0x04 /* don't block when locking */"
49 .Fd "#define LOCK_UN 0x08 /* unlock file */"
51 .Fn flock "int fd" "int operation"
56 lock on the file associated with the file descriptor
58 A lock is applied by specifying an
60 parameter that is one of
64 with the optional addition of
72 Advisory locks allow cooperating processes to perform
73 consistent operations on files, but do not guarantee
74 consistency (i.e., processes may still access files
75 without using advisory locks possibly resulting in
78 The locking mechanism allows two types of locks:
83 At any time multiple shared locks may be applied to a file,
84 but at no time are multiple exclusive, or both shared and exclusive,
85 locks allowed simultaneously on a file.
89 to an exclusive lock, and vice versa, simply by specifying
90 the appropriate lock type; this results in the previous
91 lock being released and the new lock applied (possibly
92 after other processes have gained and released the lock).
94 Requesting a lock on an object that is already locked
95 normally causes the caller to be blocked until the lock may be
100 then this will not happen; instead the call will fail and
105 Locks are on files, not file descriptors. That is, file descriptors
110 do not result in multiple instances of a lock, but rather multiple
111 references to a single lock. If a process holding a lock on a file
112 forks and the child explicitly unlocks the file, the parent will
115 Processes blocked awaiting a lock may be awakened by signals.
123 .It Bq Er EWOULDBLOCK
124 The file is locked and the
126 option was specified.
130 is an invalid descriptor.
134 refers to an object other than a file.
138 refers to an object that does not support file locking.
149 function call appeared in