FLOCK(2) Linux Programmer's Manual FLOCK(2)
flock - apply or remove an advisory lock on an open file
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 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
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 inde-
pendently 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
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.
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
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
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 guaran-
teed 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 origi-
nal BSD behavior, and occurs on many other implementations.)
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 sup-
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
flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2),
Documentation/filesystems/locks.txt in the Linux kernel source tree
(Documentation/locks.txt in older kernels)
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latest version of this page, can be found at
Linux 2017-09-15 FLOCK(2)
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