flock

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



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

SYNOPSIS
       #include <sys/file.h>

       int flock(int fd, int operation);

DESCRIPTION
       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.

       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
       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
              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.

       EWOULDBLOCK
              The file is locked and the LOCK_NB flag was selected.

CONFORMING TO
       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
       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 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 implementations.)

   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 nfs(5).

   CIFS details
       In Linux kernels up to 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
       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.

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)

COLOPHON
       This page is part of release 5.13 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                           FLOCK(2)