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28 .\" @(#)flock.2 8.2 (Berkeley) 12/11/93
36 .Nd "apply or remove an advisory lock on an open file"
41 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
42 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
43 .Fd "#define LOCK_NB 0x04 /* do not block when locking */"
44 .Fd "#define LOCK_UN 0x08 /* unlock file */"
46 .Fn flock "int fd" "int operation"
50 system call applies or removes an
52 lock on the file associated with the file descriptor
54 A lock is applied by specifying an
56 argument that is one of
60 with the optional addition of
68 Advisory locks allow cooperating processes to perform
69 consistent operations on files, but do not guarantee
70 consistency (i.e., processes may still access files
71 without using advisory locks possibly resulting in
74 The locking mechanism allows two types of locks:
79 At any time multiple shared locks may be applied to a file,
80 but at no time are multiple exclusive, or both shared and exclusive,
81 locks allowed simultaneously on a file.
85 to an exclusive lock, and vice versa, simply by specifying
86 the appropriate lock type; this results in the previous
87 lock being released and the new lock applied (possibly
88 after other processes have gained and released the lock).
90 Requesting a lock on an object that is already locked
91 normally causes the caller to be blocked until the lock may be
97 then this will not happen; instead the call will fail and
102 Locks are on files, not file descriptors.
103 That is, file descriptors
108 do not result in multiple instances of a lock, but rather multiple
109 references to a single lock.
110 If a process holding a lock on a file
111 forks and the child explicitly unlocks the file, the parent will
119 locks are compatible.
120 Processes using different locking interfaces can cooperate
121 over the same file safely.
122 However, only one of such interfaces should be used within
124 If a file is locked by a process through
126 any record within the file will be seen as locked
127 from the viewpoint of another process using
133 Processes blocked awaiting a lock may be awakened by signals.
139 system call fails if:
141 .It Bq Er EWOULDBLOCK
142 The file is locked and the
144 option was specified.
148 is an invalid descriptor.
152 refers to an object other than a file.
156 refers to an object that does not support file locking.
170 system call appeared in