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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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flock(2) System Calls Manual flock(2)
flock - apply or remove an advisory lock on an open file
Standard C library (libc, -lc)
#include <sys/file.h>
int flock(int fd, int op);
Apply or remove an advisory lock on the open file specified by fd.
The argument op 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 to indicate the error.
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 op 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.
Since Linux 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.)
CIFS details
Up to Linux 5.4, flock() is not propagated over SMB. A file with
such locks will not appear locked for remote clients.
Since Linux 5.5, flock() locks are emulated with SMB byte-range
locks on the entire file. Similarly to NFS, this means that
fcntl(2) and flock() locks interact with one another. Another
important side-effect is that the locks are not advisory anymore:
any IO on a locked file will always fail with EACCES when done
from a separate file descriptor. This difference originates from
the design of locks in the SMB protocol, which provides mandatory
locking semantics.
Remote and mandatory locking semantics may vary with SMB protocol,
mount options and server type. See mount.cifs(8) for additional
information.
BSD.
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.
NFS details
Up to Linux 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() 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.)
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 flock(2)
Pages that refer to this page: flock(1), chown(2), fcntl(2), fcntl_locking(2), fork(2), getrlimit(2), syscalls(2), dbopen(3), flockfile(3), lockf(3), nfs(5), proc_locks(5), tmpfiles.d(5), landlock(7), signal(7), cryptsetup(8), fsck(8), lslocks(8), systemd-pcrphase.service(8), systemd-tmpfiles(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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