flock(2) System Calls Manual flock(2)
NAME
flock - apply or remove an advisory lock on an open file
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/file.h>
int flock(int fd, int op);
DESCRIPTION
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 ex-
clusive 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. Fur-
thermore, the lock is released either by an explicit LOCK_UN operation
on any of these duplicate file descriptors, or when all such file de-
scriptors have been closed.
If a process uses open(2) (or similar) to obtain more than one file de-
scriptor for the same file, these file descriptors are treated indepen-
dently 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set to indicate the error.
ERRORS
EBADF fd is not an open file descriptor.
EINTR While waiting to acquire a lock, the call was interrupted by de-
livery 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.
VERSIONS
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 fc-
ntl(2). With this implementation, there is no interaction between the
types of lock placed by flock() and fcntl(2), and flock() does not de-
tect 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 dif-
ference 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
BSD.
HISTORY
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 suffi-
ciently 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 fc-
ntl(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 lo-
cal; see the discussion of the local_lock option in nfs(5).
NOTES
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 de-
scribed 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), lslocks(8)
Documentation/filesystems/locks.txt in the Linux kernel source tree
(Documentation/locks.txt in older kernels)
Linux man-pages 6.9.1 2024-05-02 flock(2)
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