FLOCK(2)                  (2017-09-15)                   FLOCK(2)

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

          #include <sys/file.h>

          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

              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 descrip-
          tors 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 exclu-
          sive) on a file.  Subsequent flock() calls on an already
          locked file will convert an existing lock to the new lock

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     FLOCK(2)                  (2017-09-15)                   FLOCK(2)

          Locks created by flock() are preserved across an execve(2).

          A shared or exclusive lock can be placed on a file regard-
          less of the mode in which the file was opened.

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

               fd is not an open file descriptor.

               While waiting to acquire a lock, the call was inter-
               rupted by delivery of a signal caught by a handler; see

               operation is invalid.

               The kernel ran out of memory for allocating lock

               The file is locked and the LOCK_NB flag was selected.

          4.4BSD (the flock() call first appeared in 4.2BSD).  A ver-
          sion of flock(), possibly implemented in terms of fcntl(2),
          appears on most UNIX systems.

          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 permis-
          sions 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.

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          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 lock-
          ing, 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 inter-
          act with one another over NFS.  It also means that in order
          to place an exclusive lock, the file must be opened for

          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).

          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)

          This page is part of release 5.10 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/.

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