1FLOCK(2) Linux Programmer's Manual FLOCK(2)
2
6 flock - apply or remove an advisory lock on an open file
7
9 #include <sys/file.h>
10 int flock(int fd, int operation);
12
14 Apply or remove an advisory lock on the open file specified by fd. The
15 argument operation is one of the following:
16 LOCK_SH Place a shared lock. More than one process may hold a
18 shared lock for a given file at a given time.
19 LOCK_EX Place an exclusive lock. Only one process may hold an
21 exclusive lock for a given file at a given time.
22 LOCK_UN Remove an existing lock held by this process.
24 A call to flock() may block if an incompatible lock is held by another
26 process. To make a non-blocking request, include LOCK_NB (by ORing)
27 with any of the above operations.
28 A single file may not simultaneously have both shared and exclusive
30 locks.
31 Locks created by flock() are associated with an open file table entry.
33 This means that duplicate file descriptors (created by, for example,
34 fork(2) or dup(2)) refer to the same lock, and this lock may be modi‐
35 fied or released using any of these descriptors. Furthermore, the lock
36 is released either by an explicit LOCK_UN operation on any of these
37 duplicate descriptors, or when all such descriptors have been closed.
38 If a process uses open(2) (or similar) to obtain more than one descrip‐
40 tor for the same file, these descriptors are treated independently by
41 flock(). An attempt to lock the file using one of these file descrip‐
42 tors may be denied by a lock that the calling process has already
43 placed via another descriptor.
44 A process may only hold one type of lock (shared or exclusive) on a
46 file. Subsequent flock() calls on an already locked file will convert
47 an existing lock to the new lock mode.
48 Locks created by flock() are preserved across an execve(2).
50 A shared or exclusive lock can be placed on a file regardless of the
52 mode in which the file was opened.
53
55 On success, zero is returned. On error, -1 is returned, and errno is
56 set appropriately.
57
59 EBADF fd is not an open file descriptor.
60 EINTR While waiting to acquire a lock, the call was interrupted by
62 delivery of a signal caught by a handler; see signal(7).
63 EINVAL operation is invalid.
65 ENOLCK The kernel ran out of memory for allocating lock records.
67 EWOULDBLOCK
69 The file is locked and the LOCK_NB flag was selected.
70
72 4.4BSD (the flock() call first appeared in 4.2BSD). A version of
73 flock(), possibly implemented in terms of fcntl(2), appears on most
74 Unix systems.
75
77 flock() does not lock files over NFS. Use fcntl(2) instead: that does
78 work over NFS, given a sufficiently recent version of Linux and a
79 server which supports locking.
80 Since kernel 2.0, flock() is implemented as a system call in its own
82 right rather than being emulated in the GNU C library as a call to
83 fcntl(2). This yields true BSD semantics: there is no interaction
84 between the types of lock placed by flock() and fcntl(2), and flock()
85 does not detect deadlock.
86 flock() places advisory locks only; given suitable permissions on a
88 file, a process is free to ignore the use of flock() and perform I/O on
89 the file.
90 flock() and fcntl(2) locks have different semantics with respect to
92 forked processes and dup(2). On systems that implement flock() using
93 fcntl(2), the semantics of flock() will be different from those
94 described in this manual page.
95 Converting a lock (shared to exclusive, or vice versa) is not guaran‐
97 teed to be atomic: the existing lock is first removed, and then a new
98 lock is established. Between these two steps, a pending lock request
99 by another process may be granted, with the result that the conversion
100 either blocks, or fails if LOCK_NB was specified. (This is the origi‐
101 nal BSD behavior, and occurs on many other implementations.)
102
104 close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)
105 See also Documentation/filesystem/locks.txt in the kernel source (Docu‐
107 mentation/locks.txt in older kernels).
108
110 This page is part of release 3.22 of the Linux man-pages project. A
111 description of the project, and information about reporting bugs, can
112 be found at http://www.kernel.org/doc/man-pages/.
113Linux 2009-07-25 FLOCK(2)