man2/flock.2

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
   2 .\" and Copyright 2002 Michael Kerrisk
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .\" Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esr@thyrsus.com>
   7 .\" Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamie@imbolc.ucc.ie>
   8 .\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
   9 .\"     Substantial rewrites and additions
  10 .\" 2005-05-10 mtk, noted that lock conversions are not atomic.
  11 .\"
  12 .\" FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE
  13 .\" which only have effect for SAMBA.
  14 .\"
  15 .TH FLOCK 2 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
  16 .SH NAME
  17 flock \- apply or remove an advisory lock on an open file
  18 .SH LIBRARY
  19 Standard C library
  20 .RI ( libc ", " \-lc )
  21 .SH SYNOPSIS
  22 .nf
  23 .B #include <sys/file.h>
  24 .PP
  25 .BI "int flock(int " fd ", int " operation );
  26 .fi
  27 .SH DESCRIPTION
  28 Apply or remove an advisory lock on the open file specified by
  29 .IR fd .
  30 The argument
  31 .I operation
  32 is one of the following:
  33 .RS 4
  34 .TP 9
  35 .B LOCK_SH
  36 Place a shared lock.
  37 More than one process may hold a shared lock for a given file
  38 at a given time.
  39 .TP
  40 .B LOCK_EX
  41 Place an exclusive lock.
  42 Only one process may hold an exclusive lock for a given
  43 file at a given time.
  44 .TP
  45 .B LOCK_UN
  46 Remove an existing lock held by this process.
  47 .RE
  48 .PP
  49 A call to
  50 .BR flock ()
  51 may block if an incompatible lock is held by another process.
  52 To make a nonblocking request, include
  53 .B LOCK_NB
  54 (by ORing)
  55 with any of the above operations.
  56 .PP
  57 A single file may not simultaneously have both shared and exclusive locks.
  58 .PP
  59 Locks created by
  60 .BR flock ()
  61 are associated with an open file description (see
  62 .BR open (2)).
  63 This means that duplicate file descriptors (created by, for example,
  64 .BR fork (2)
  65 or
  66 .BR dup (2))
  67 refer to the same lock, and this lock may be modified
  68 or released using any of these file descriptors.
  69 Furthermore, the lock is released either by an explicit
  70 .B LOCK_UN
  71 operation on any of these duplicate file descriptors, or when all
  72 such file descriptors have been closed.
  73 .PP
  74 If a process uses
  75 .BR open (2)
  76 (or similar) to obtain more than one file descriptor for the same file,
  77 these file descriptors are treated independently by
  78 .BR flock ().
  79 An attempt to lock the file using one of these file descriptors
  80 may be denied by a lock that the calling process has
  81 already placed via another file descriptor.
  82 .PP
  83 A process may hold only one type of lock (shared or exclusive)
  84 on a file.
  85 Subsequent
  86 .BR flock ()
  87 calls on an already locked file will convert an existing lock to the new
  88 lock mode.
  89 .PP
  90 Locks created by
  91 .BR flock ()
  92 are preserved across an
  93 .BR execve (2).
  94 .PP
  95 A shared or exclusive lock can be placed on a file regardless of the
  96 mode in which the file was opened.
  97 .SH RETURN VALUE
  98 On success, zero is returned.
  99 On error, \-1 is returned, and
 100 .I errno
 101 is set to indicate the error.
 102 .SH ERRORS
 103 .TP
 104 .B EBADF
 105 .I fd
 106 is not an open file descriptor.
 107 .TP
 108 .B EINTR
 109 While waiting to acquire a lock, the call was interrupted by
 110 delivery of a signal caught by a handler; see
 111 .BR signal (7).
 112 .TP
 113 .B EINVAL
 114 .I operation
 115 is invalid.
 116 .TP
 117 .B ENOLCK
 118 The kernel ran out of memory for allocating lock records.
 119 .TP
 120 .B EWOULDBLOCK
 121 The file is locked and the
 122 .B LOCK_NB
 123 flag was selected.
 124 .SH STANDARDS
 125 4.4BSD (the
 126 .BR flock ()
 127 call first appeared in 4.2BSD).
 128 A version of
 129 .BR flock (),
 130 possibly implemented in terms of
 131 .BR fcntl (2),
 132 appears on most UNIX systems.
 133 .SH NOTES
 134 Since kernel 2.0,
 135 .BR flock ()
 136 is implemented as a system call in its own right rather
 137 than being emulated in the GNU C library as a call to
 138 .BR fcntl (2).
 139 With this implementation,
 140 there is no interaction between the types of lock
 141 placed by
 142 .BR flock ()
 143 and
 144 .BR fcntl (2),
 145 and
 146 .BR flock ()
 147 does not detect deadlock.
 148 (Note, however, that on some systems, such as the modern BSDs,
 149 .\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3
 150 .BR flock ()
 151 and
 152 .BR fcntl (2)
 153 locks
 154 .I do
 155 interact with one another.)
 156 .PP
 157 .BR flock ()
 158 places advisory locks only; given suitable permissions on a file,
 159 a process is free to ignore the use of
 160 .BR flock ()
 161 and perform I/O on the file.
 162 .PP
 163 .BR flock ()
 164 and
 165 .BR fcntl (2)
 166 locks have different semantics with respect to forked processes and
 167 .BR dup (2).
 168 On systems that implement
 169 .BR flock ()
 170 using
 171 .BR fcntl (2),
 172 the semantics of
 173 .BR flock ()
 174 will be different from those described in this manual page.
 175 .PP
 176 Converting a lock
 177 (shared to exclusive, or vice versa) is not guaranteed to be atomic:
 178 the existing lock is first removed, and then a new lock is established.
 179 Between these two steps,
 180 a pending lock request by another process may be granted,
 181 with the result that the conversion either blocks, or fails if
 182 .B LOCK_NB
 183 was specified.
 184 (This is the original BSD behavior,
 185 and occurs on many other implementations.)
 186 .\" Kernel 2.5.21 changed things a little: during lock conversion
 187 .\" it is now the highest priority process that will get the lock -- mtk
 188 .SS NFS details
 189 In Linux kernels up to 2.6.11,
 190 .BR flock ()
 191 does not lock files over NFS
 192 (i.e., the scope of locks was limited to the local system).
 193 Instead, one could use
 194 .BR fcntl (2)
 195 byte-range locking, which does work over NFS,
 196 given a sufficiently recent version of
 197 Linux and a server which supports locking.
 198 .PP
 199 Since Linux 2.6.12, NFS clients support
 200 .BR flock ()
 201 locks by emulating them as
 202 .BR fcntl (2)
 203 byte-range locks on the entire file.
 204 This means that
 205 .BR fcntl (2)
 206 and
 207 .BR flock ()
 208 locks
 209 .I do
 210 interact with one another over NFS.
 211 It also means that in order to place an exclusive lock,
 212 the file must be opened for writing.
 213 .PP
 214 Since Linux 2.6.37,
 215 .\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2
 216 the kernel supports a compatibility mode that allows
 217 .BR flock ()
 218 locks (and also
 219 .BR fcntl (2)
 220 byte region locks) to be treated as local;
 221 see the discussion of the
 222 .I "local_lock"
 223 option in
 224 .BR nfs (5).
 225 .SS CIFS details
 226 In Linux kernels up to 5.4,
 227 .BR flock ()
 228 is not propagated over SMB.
 229 A file with such locks will not appear locked for remote clients.
 230 .PP
 231 Since Linux 5.5,
 232 .BR flock ()
 233 locks are emulated with SMB byte-range locks on the entire file.
 234 Similarly to NFS, this means that
 235 .BR fcntl (2)
 236 and
 237 .BR flock ()
 238 locks interact with one another.
 239 Another important side-effect is that the locks are not advisory anymore:
 240 any IO on a locked file will always fail with
 241 .B EACCES
 242 when done from a separate file descriptor.
 243 This difference originates from the design of locks in the SMB protocol,
 244 which provides mandatory locking semantics.
 245 .PP
 246 Remote and mandatory locking semantics may vary with
 247 SMB protocol, mount options and server type.
 248 See
 249 .BR mount.cifs (8)
 250 for additional information.
 251 .SH SEE ALSO
 252 .BR flock (1),
 253 .BR close (2),
 254 .BR dup (2),
 255 .BR execve (2),
 256 .BR fcntl (2),
 257 .BR fork (2),
 258 .BR open (2),
 259 .BR lockf (3),
 260 .BR lslocks (8)
 261 .PP
 262 .I Documentation/filesystems/locks.txt
 263 in the Linux kernel source tree
 264 .RI ( Documentation/locks.txt
 265 in older kernels)