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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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FLOCK(2) Linux Programmer's Manual FLOCK(2)
flock - apply or remove an advisory lock on an open file
#include <sys/file.h>
int flock(int fd, int operation);
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 file descriptors.
Furthermore, the lock is released either by an explicit LOCK_UN
operation on any of these duplicate file descriptors, or when all
such file descriptors have been closed.
If a process uses open(2) (or similar) to obtain more than one file
descriptor for the same file, these file 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 file 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.
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
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.
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.
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.)
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.)
NFS details
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 fcntl(2) byte-range locks on the entire file. This means
that fcntl(2) and flock() locks do interact with one another over
NFS. It also means that in order to place an exclusive lock, the
file must be opened for writing.
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(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2),
lockf(3), lslocks(8)
Documentation/filesystems/locks.txt in the Linux kernel source tree
(Documentation/locks.txt in older kernels)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 FLOCK(2)
Pages that refer to this page: flock(1), chown(2), fcntl(2), fork(2), getrlimit(2), syscalls(2), dbopen(3), flockfile(3), lockf(3), nfs(5), proc(5), signal(7), cryptsetup(8), fsck(8), lslocks(8), vipw(8@@util-linux)
Copyright and license for this manual page
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