man2/flock.2

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
   2 .\" and Copyright 2002 Michael Kerrisk
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  25 .\"
  26 .\" Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esr@thyrsus.com>
  27 .\" Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamie@imbolc.ucc.ie>
  28 .\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Substantial rewrites and additions
  30 .\" 2005-05-10 mtk, noted that lock conversions are not atomic.
  31 .\"
  32 .\" FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE
  33 .\" which only have effect for SAMBA.
  34 .\"
  35 .TH FLOCK 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  36 .SH NAME
  37 flock \- apply or remove an advisory lock on an open file
  38 .SH SYNOPSIS
  39 .B #include <sys/file.h>
  40 .PP
  41 .BI "int flock(int " fd ", int " operation );
  42 .SH DESCRIPTION
  43 Apply or remove an advisory lock on the open file specified by
  44 .IR fd .
  45 The argument
  46 .I operation
  47 is one of the following:
  48 .RS 4
  49 .TP 9
  50 .B LOCK_SH
  51 Place a shared lock.
  52 More than one process may hold a shared lock for a given file
  53 at a given time.
  54 .TP
  55 .B LOCK_EX
  56 Place an exclusive lock.
  57 Only one process may hold an exclusive lock for a given
  58 file at a given time.
  59 .TP
  60 .B LOCK_UN
  61 Remove an existing lock held by this process.
  62 .RE
  63 .PP
  64 A call to
  65 .BR flock ()
  66 may block if an incompatible lock is held by another process.
  67 To make a nonblocking request, include
  68 .B LOCK_NB
  69 (by ORing)
  70 with any of the above operations.
  71 .PP
  72 A single file may not simultaneously have both shared and exclusive locks.
  73 .PP
  74 Locks created by
  75 .BR flock ()
  76 are associated with an open file description (see
  77 .BR open (2)).
  78 This means that duplicate file descriptors (created by, for example,
  79 .BR fork (2)
  80 or
  81 .BR dup (2))
  82 refer to the same lock, and this lock may be modified
  83 or released using any of these file descriptors.
  84 Furthermore, the lock is released either by an explicit
  85 .B LOCK_UN
  86 operation on any of these duplicate file descriptors, or when all
  87 such file descriptors have been closed.
  88 .PP
  89 If a process uses
  90 .BR open (2)
  91 (or similar) to obtain more than one file descriptor for the same file,
  92 these file descriptors are treated independently by
  93 .BR flock ().
  94 An attempt to lock the file using one of these file descriptors
  95 may be denied by a lock that the calling process has
  96 already placed via another file descriptor.
  97 .PP
  98 A process may hold only one type of lock (shared or exclusive)
  99 on a file.
 100 Subsequent
 101 .BR flock ()
 102 calls on an already locked file will convert an existing lock to the new
 103 lock mode.
 104 .PP
 105 Locks created by
 106 .BR flock ()
 107 are preserved across an
 108 .BR execve (2).
 109 .PP
 110 A shared or exclusive lock can be placed on a file regardless of the
 111 mode in which the file was opened.
 112 .SH RETURN VALUE
 113 On success, zero is returned.
 114 On error, \-1 is returned, and
 115 .I errno
 116 is set appropriately.
 117 .SH ERRORS
 118 .TP
 119 .B EBADF
 120 .I fd
 121 is not an open file descriptor.
 122 .TP
 123 .B EINTR
 124 While waiting to acquire a lock, the call was interrupted by
 125 delivery of a signal caught by a handler; see
 126 .BR signal (7).
 127 .TP
 128 .B EINVAL
 129 .I operation
 130 is invalid.
 131 .TP
 132 .B ENOLCK
 133 The kernel ran out of memory for allocating lock records.
 134 .TP
 135 .B EWOULDBLOCK
 136 The file is locked and the
 137 .B LOCK_NB
 138 flag was selected.
 139 .SH CONFORMING TO
 140 4.4BSD (the
 141 .BR flock ()
 142 call first appeared in 4.2BSD).
 143 A version of
 144 .BR flock (),
 145 possibly implemented in terms of
 146 .BR fcntl (2),
 147 appears on most UNIX systems.
 148 .SH NOTES
 149 Since kernel 2.0,
 150 .BR flock ()
 151 is implemented as a system call in its own right rather
 152 than being emulated in the GNU C library as a call to
 153 .BR fcntl (2).
 154 With this implementation,
 155 there is no interaction between the types of lock
 156 placed by
 157 .BR flock ()
 158 and
 159 .BR fcntl (2),
 160 and
 161 .BR flock ()
 162 does not detect deadlock.
 163 (Note, however, that on some systems, such as the modern BSDs,
 164 .\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3
 165 .BR flock ()
 166 and
 167 .BR fcntl (2)
 168 locks
 169 .I do
 170 interact with one another.)
 171 .PP
 172 .BR flock ()
 173 places advisory locks only; given suitable permissions on a file,
 174 a process is free to ignore the use of
 175 .BR flock ()
 176 and perform I/O on the file.
 177 .PP
 178 .BR flock ()
 179 and
 180 .BR fcntl (2)
 181 locks have different semantics with respect to forked processes and
 182 .BR dup (2).
 183 On systems that implement
 184 .BR flock ()
 185 using
 186 .BR fcntl (2),
 187 the semantics of
 188 .BR flock ()
 189 will be different from those described in this manual page.
 190 .PP
 191 Converting a lock
 192 (shared to exclusive, or vice versa) is not guaranteed to be atomic:
 193 the existing lock is first removed, and then a new lock is established.
 194 Between these two steps,
 195 a pending lock request by another process may be granted,
 196 with the result that the conversion either blocks, or fails if
 197 .B LOCK_NB
 198 was specified.
 199 (This is the original BSD behavior,
 200 and occurs on many other implementations.)
 201 .\" Kernel 2.5.21 changed things a little: during lock conversion
 202 .\" it is now the highest priority process that will get the lock -- mtk
 203 .SS NFS details
 204 In Linux kernels up to 2.6.11,
 205 .BR flock ()
 206 does not lock files over NFS
 207 (i.e., the scope of locks was limited to the local system).
 208 Instead, one could use
 209 .BR fcntl (2)
 210 byte-range locking, which does work over NFS,
 211 given a sufficiently recent version of
 212 Linux and a server which supports locking.
 213 .PP
 214 Since Linux 2.6.12, NFS clients support
 215 .BR flock ()
 216 locks by emulating them as
 217 .BR fcntl (2)
 218 byte-range locks on the entire file.
 219 This means that
 220 .BR fcntl (2)
 221 and
 222 .BR flock ()
 223 locks
 224 .I do
 225 interact with one another over NFS.
 226 It also means that in order to place an exclusive lock,
 227 the file must be opened for writing.
 228 .PP
 229 Since Linux 2.6.37,
 230 .\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2
 231 the kernel supports a compatibility mode that allows
 232 .BR flock ()
 233 locks (and also
 234 .BR fcntl (2)
 235 byte region locks) to be treated as local;
 236 see the discussion of the
 237 .I "local_lock"
 238 option in
 239 .BR nfs (5).
 240 .SH SEE ALSO
 241 .BR flock (1),
 242 .BR close (2),
 243 .BR dup (2),
 244 .BR execve (2),
 245 .BR fcntl (2),
 246 .BR fork (2),
 247 .BR open (2),
 248 .BR lockf (3),
 249 .BR lslocks (8)
 250 .PP
 251 .I Documentation/filesystems/locks.txt
 252 in the Linux kernel source tree
 253 .RI ( Documentation/locks.txt
 254 in older kernels)