1FLOCK(2)                   Linux Programmer's Manual                  FLOCK(2)
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NAME

6       flock - apply or remove an advisory lock on an open file
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SYNOPSIS

9       #include <sys/file.h>
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11       int flock(int fd, int operation);
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DESCRIPTION

14       Apply or remove an advisory lock on the open file specified by fd.  The
15       argument operation is one of the following:
16
17           LOCK_SH  Place a shared lock.  More than one  process  may  hold  a
18                    shared lock for a given file at a given time.
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20           LOCK_EX  Place an exclusive lock.  Only one process may hold an ex‐
21                    clusive lock for a given file at a given time.
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23           LOCK_UN  Remove an existing lock held by this process.
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25       A call to flock() may block if an incompatible lock is held by  another
26       process.   To  make  a  nonblocking request, include LOCK_NB (by ORing)
27       with any of the above operations.
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29       A single file may not simultaneously have  both  shared  and  exclusive
30       locks.
31
32       Locks  created  by flock() are associated with an open file description
33       (see open(2)).  This means that duplicate file descriptors (created by,
34       for  example,  fork(2) or dup(2)) refer to the same lock, and this lock
35       may be modified or released using any of these file descriptors.   Fur‐
36       thermore,  the lock is released either by an explicit LOCK_UN operation
37       on any of these duplicate file descriptors, or when all such  file  de‐
38       scriptors have been closed.
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40       If a process uses open(2) (or similar) to obtain more than one file de‐
41       scriptor for the same file, these file descriptors are treated indepen‐
42       dently by flock().  An attempt to lock the file using one of these file
43       descriptors may be denied by a lock that the calling  process  has  al‐
44       ready placed via another file descriptor.
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46       A  process  may  hold  only one type of lock (shared or exclusive) on a
47       file.  Subsequent flock() calls on an already locked file will  convert
48       an existing lock to the new lock mode.
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50       Locks created by flock() are preserved across an execve(2).
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52       A  shared  or  exclusive lock can be placed on a file regardless of the
53       mode in which the file was opened.
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RETURN VALUE

56       On success, zero is returned.  On error, -1 is returned, and  errno  is
57       set to indicate the error.
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ERRORS

60       EBADF  fd is not an open file descriptor.
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62       EINTR  While waiting to acquire a lock, the call was interrupted by de‐
63              livery of a signal caught by a handler; see signal(7).
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65       EINVAL operation is invalid.
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67       ENOLCK The kernel ran out of memory for allocating lock records.
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69       EWOULDBLOCK
70              The file is locked and the LOCK_NB flag was selected.
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CONFORMING TO

73       4.4BSD (the flock() call first  appeared  in  4.2BSD).   A  version  of
74       flock(),  possibly  implemented  in  terms of fcntl(2), appears on most
75       UNIX systems.
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NOTES

78       Since kernel 2.0, flock() is implemented as a system call  in  its  own
79       right  rather than being emulated in the GNU C library as a call to fc‐
80       ntl(2).  With this implementation, there is no interaction between  the
81       types  of lock placed by flock() and fcntl(2), and flock() does not de‐
82       tect deadlock.  (Note, however, that on some systems, such as the  mod‐
83       ern BSDs, flock() and fcntl(2) locks do interact with one another.)
84
85       flock()  places  advisory  locks  only; given suitable permissions on a
86       file, a process is free to ignore the use of flock() and perform I/O on
87       the file.
88
89       flock()  and  fcntl(2)  locks  have different semantics with respect to
90       forked processes and dup(2).  On systems that implement  flock()  using
91       fcntl(2),  the  semantics  of  flock() will be different from those de‐
92       scribed in this manual page.
93
94       Converting a lock (shared to exclusive, or vice versa) is  not  guaran‐
95       teed  to  be atomic: the existing lock is first removed, and then a new
96       lock is established.  Between these two steps, a pending  lock  request
97       by  another process may be granted, with the result that the conversion
98       either blocks, or fails if LOCK_NB was specified.  (This is the  origi‐
99       nal BSD behavior, and occurs on many other implementations.)
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101   NFS details
102       In  Linux  kernels  up  to 2.6.11, flock() does not lock files over NFS
103       (i.e., the scope of locks was limited to the local  system).   Instead,
104       one  could  use  fcntl(2) byte-range locking, which does work over NFS,
105       given a sufficiently recent version of Linux and a  server  which  sup‐
106       ports locking.
107
108       Since Linux 2.6.12, NFS clients support flock() locks by emulating them
109       as fcntl(2) byte-range locks on the entire file.  This means  that  fc‐
110       ntl(2)  and  flock()  locks  do interact with one another over NFS.  It
111       also means that in order to place an exclusive lock, the file  must  be
112       opened for writing.
113
114       Since  Linux  2.6.37, the kernel supports a compatibility mode that al‐
115       lows flock() locks (and also fcntl(2) byte region locks) to be  treated
116       as local; see the discussion of the local_lock option in nfs(5).
117
118   CIFS details
119       In Linux kernels up to 5.4, flock() is not propagated over SMB.  A file
120       with such locks will not appear locked for remote clients.
121
122       Since Linux 5.5, flock() locks are emulated with SMB  byte-range  locks
123       on  the  entire  file.   Similarly to NFS, this means that fcntl(2) and
124       flock() locks interact with one another.  Another important side-effect
125       is  that  the  locks  are not advisory anymore: any IO on a locked file
126       will always fail with EACCES when done from a separate file descriptor.
127       This  difference  originates from the design of locks in the SMB proto‐
128       col, which provides mandatory locking semantics.
129
130       Remote and mandatory locking semantics  may  vary  with  SMB  protocol,
131       mount options and server type.  See mount.cifs(8) for additional infor‐
132       mation.
133

SEE ALSO

135       flock(1), close(2),  dup(2),  execve(2),  fcntl(2),  fork(2),  open(2),
136       lockf(3), lslocks(8)
137
138       Documentation/filesystems/locks.txt  in  the  Linux  kernel source tree
139       (Documentation/locks.txt in older kernels)
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COLOPHON

142       This page is part of release 5.13 of the Linux  man-pages  project.   A
143       description  of  the project, information about reporting bugs, and the
144       latest    version    of    this    page,    can     be     found     at
145       https://www.kernel.org/doc/man-pages/.
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149Linux                             2021-03-22                          FLOCK(2)
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