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 table entry.
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 descriptors. Furthermore, the
lock is released either by an explicit LOCK_UN operation on any of
these duplicate descriptors, or when all such descriptors have been
If a process uses open(2) (or similar) to obtain more than one
descriptor for the same file, these 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 descriptor.
A process may only hold 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
flock() does not lock files over NFS. Use fcntl(2) instead: that does
work over NFS, given a sufficiently recent version of Linux and a
server which supports locking.
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). This yields true BSD semantics: there is no interaction
between the types of lock placed by flock() and fcntl(2), and flock()
does not detect deadlock.
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
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.)
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)
See also Documentation/filesystem/locks.txt in the kernel source
(Documentation/locks.txt in older kernels).
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.