flock (2)
Leading comments
Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and and Copyright 2002 Michael Kerrisk %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a p...
NAME
flock - apply or remove an advisory lock on an open fileSYNOPSIS
#include <sys/file.h>int flock(int fd, int operation);
DESCRIPTION
Apply or remove an advisory lock on the open file specified by fd. The argument operation is one of the following:-
- LOCK_SH
- Place a shared lock. More than one process may hold a shared lock for a given file at a given time.
- LOCK_EX
- Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
- LOCK_UN
- Remove an existing lock held by this process.
A call to
flock()
may block if an incompatible lock is held by another process.
To make a nonblocking request, include
LOCK_NB
(by ORing)
with any of the above operations.
A single file may not simultaneously have both shared and exclusive locks.
Locks created by
flock()
are associated with an open file description (see
open(2)).
This means that duplicate file descriptors (created by, for example,
fork(2)
or
dup(2))
refer to the same lock, and this lock may be modified
or released using any of these descriptors.
Furthermore, the lock is released either by an explicit
LOCK_UN
operation on any of these duplicate descriptors, or when all
such descriptors have been closed.
If a process uses
open(2)
(or similar) to obtain more than one descriptor for the same file,
these descriptors are treated independently by
flock().
An attempt to lock the file using one of these file descriptors
may be denied by a lock that the calling process has
already placed via another descriptor.
A process may hold only one type of lock (shared or exclusive)
on a file.
Subsequent
flock()
calls on an already locked file will convert an existing lock to the new
lock mode.
Locks created by
flock()
are preserved across an
execve(2).
A shared or exclusive lock can be placed on a file regardless of the
mode in which the file was opened.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.ERRORS
- EBADF
- fd is not an open file descriptor.
- EINTR
- While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).
- EINVAL
- operation is invalid.
- ENOLCK
- The kernel ran out of memory for allocating lock records.
- EWOULDBLOCK
- The file is locked and the LOCK_NB flag was selected.
CONFORMING TO
4.4BSD (the flock() call first appeared in 4.2BSD). A version of flock(), possibly implemented in terms of fcntl(2), appears on most UNIX systems.NOTES
Since kernel 2.0, flock() is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to fcntl(2). With this implementation, there is no interaction between the types of lock placed by flock() and fcntl(2), and flock() does not detect deadlock. (Note, however, that on some systems, such as the modern BSDs, flock() and fcntl(2) locks do interact with one another.)In Linux kernels up to 2.6.11, flock() does not lock files over NFS (i.e., the scope of locks was limited to the local system). Instead, one could use fcntl(2) byte-range locking, which does work over NFS, given a sufficiently recent version of Linux and a server which supports locking. Since Linux 2.6.12, NFS clients support flock() locks by emulating them as byte-range locks on the entire file. This means that fcntl(2) and flock() locks do interact with one another over NFS. Since Linux 2.6.37, the kernel supports a compatibility mode that allows flock() locks (and also fcntl(2) byte region locks) to be treated as local; see the discussion of the local_lock option in nfs(5).
flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.
flock() and fcntl(2) locks have different semantics with respect to forked processes and dup(2). On systems that implement flock() using fcntl(2), the semantics of flock() will be different from those described in this manual page.
Converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is first removed, and then a new lock is established. Between these two steps, a pending lock request by another process may be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified. (This is the original BSD behavior, and occurs on many other implementations.)
SEE ALSO
flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)Documentation/filesystems/locks.txt in the Linux kernel source tree (Documentation/locks.txt in older kernels)