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FLOCK(2)                   Linux Programmer's Manual                  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 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 non-blocking request, include LOCK_NB (by ORing) with any of the above oper-

       A single file may not simultaneously have both shared and exclusive locks.

       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 oper-
       ation 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.   Subse-
       quent  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 set appropri-

       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 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).  This yields clas-
       sical BSD semantics: 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 modern BSDs, flock() and fcntl(2) locks do interact with one another.)

       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 sup-
       port flock() locks by emulating them as byte-range locks on the entire file.   This
       means that fcntl(2) and flock() locks do interact with one another over NFS.  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() 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 pro-
       cesses 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  speci-
       fied.   (This  is  the  original BSD behavior, and occurs on many other implementa-

       close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)

       See  also  Documentation/filesystem/locks.txt  in  the  kernel  source  (Documenta-
       tion/locks.txt in older kernels).

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2009-07-25                          FLOCK(2)

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