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28 .\" @(#)flock.2 8.2 (Berkeley) 12/11/93
35 .Nd "apply or remove an advisory lock on an open file"
40 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
41 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
42 .Fd "#define LOCK_NB 0x04 /* do not block when locking */"
43 .Fd "#define LOCK_UN 0x08 /* unlock file */"
45 .Fn flock "int fd" "int operation"
49 system call applies or removes an
51 lock on the file associated with the file descriptor
53 A lock is applied by specifying an
55 argument that is one of
59 with the optional addition of
67 Advisory locks allow cooperating processes to perform
68 consistent operations on files, but do not guarantee
69 consistency (i.e., processes may still access files
70 without using advisory locks possibly resulting in
73 The locking mechanism allows two types of locks:
78 At any time multiple shared locks may be applied to a file,
79 but at no time are multiple exclusive, or both shared and exclusive,
80 locks allowed simultaneously on a file.
84 to an exclusive lock, and vice versa, simply by specifying
85 the appropriate lock type; this results in the previous
86 lock being released and the new lock applied (possibly
87 after other processes have gained and released the lock).
89 Requesting a lock on an object that is already locked
90 normally causes the caller to be blocked until the lock may be
96 then this will not happen; instead the call will fail and
101 Locks are on files, not file descriptors.
102 That is, file descriptors
107 do not result in multiple instances of a lock, but rather multiple
108 references to a single lock.
109 If a process holding a lock on a file
110 forks and the child explicitly unlocks the file, the parent will
118 locks are compatible.
119 Processes using different locking interfaces can cooperate
120 over the same file safely.
121 However, only one of such interfaces should be used within
123 If a file is locked by a process through
125 any record within the file will be seen as locked
126 from the viewpoint of another process using
132 Processes blocked awaiting a lock may be awakened by signals.
138 system call fails if:
140 .It Bq Er EWOULDBLOCK
141 The file is locked and the
143 option was specified.
147 is an invalid descriptor.
151 refers to an object other than a file.
155 refers to an object that does not support file locking.
157 A lock was requested, but no locks are available.
171 system call appeared in