flock(2) - Linux manual page (original) (raw)
flock(2) System Calls Manual flock(2)
NAME top
flock - apply or remove an advisory lock on an open fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/file.h>**
**int flock(int** _fd_**, int** _op_**);**DESCRIPTION top
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)](../man2/open.2.html)). This means that duplicate file
descriptors (created by, for example, [fork(2)](../man2/fork.2.html) or [dup(2)](../man2/dup.2.html)) 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)](../man2/open.2.html) (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)](../man2/execve.2.html).
A shared or exclusive lock can be placed on a file regardless of
the mode in which the file was opened.RETURN VALUE top
On success, zero is returned. On error, -1 is returned, and _[errno](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
**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)](../man7/signal.7.html).
**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.VERSIONS top
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)](../man2/fcntl.2.html). With this implementation, there is no
interaction between the types of lock placed by **flock**() and
[fcntl(2)](../man2/fcntl.2.html), and **flock**() does not detect deadlock. (Note, however,
that on some systems, such as the modern BSDs, **flock**() and
[fcntl(2)](../man2/fcntl.2.html) 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)](../man2/fcntl.2.html) 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.STANDARDS top
BSD.HISTORY top
4.4BSD (the **flock**() call first appeared in 4.2BSD). A version of
**flock**(), possibly implemented in terms of [fcntl(2)](../man2/fcntl.2.html), 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)](../man2/fcntl.2.html) byte-range locks on the entire file. This means
that [fcntl(2)](../man2/fcntl.2.html) 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)](../man2/fcntl.2.html) byte region locks) to be
treated as local; see the discussion of the _locallock_ option in
[nfs(5)](../man5/nfs.5.html).NOTES top
**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)](../man2/fcntl.2.html) locks have different semantics with respect
to forked processes and [dup(2)](../man2/dup.2.html). On systems that implement **flock**()
using [fcntl(2)](../man2/fcntl.2.html), 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 top
[flock(1)](../man1/flock.1.html), [close(2)](../man2/close.2.html), [dup(2)](../man2/dup.2.html), [execve(2)](../man2/execve.2.html), [fcntl(2)](../man2/fcntl.2.html), [fork(2)](../man2/fork.2.html), [open(2)](../man2/open.2.html),
[lockf(3)](../man3/lockf.3.html), [lslocks(8)](../man8/lslocks.8.html)
_Documentation/filesystems/locks.txt_ in the Linux kernel source
tree (_Documentation/locks.txt_ in older kernels)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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_locks(5), tmpfiles.d(5), landlock(7), signal(7), cryptsetup(8), fsck(8), lslocks(8), systemd-pcrphase.service(8), systemd-tmpfiles(8)