flock(2) — Linux manual page


FLOCK(2)                  Linux Programmer's Manual                 FLOCK(2)

NAME         top

       flock - apply or remove an advisory lock on an open file

SYNOPSIS         top

       #include <sys/file.h>

       int flock(int fd, int operation);

DESCRIPTION         top

       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.
       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.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

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).

       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.

CONFORMING TO         top

       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.

NOTES         top

       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

       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

   NFS details
       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
       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

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2017-09-15                         FLOCK(2)

Pages that refer to this page: flock(1)chown(2)chown32(2)fchown(2)fchown32(2)fchownat(2)fcntl(2)fcntl64(2)fork(2)getrlimit(2)lchown(2)lchown32(2)prlimit(2)prlimit64(2)setrlimit(2)syscalls(2)ugetrlimit(2)db(3)dbopen(3)flockfile(3)ftrylockfile(3)funlockfile(3)lockf(3)vlimit(3)nfs(5)proc(5)procfs(5)tmpfiles.d(5)signal(7)cryptsetup(8)fsck(8)lslocks(8)vipw(8@@util-linux)