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       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  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   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 closed.

       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  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.


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


       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).  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.)

       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 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

       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.)


       flock(1),  close(2),  dup(2),  execve(2),  fcntl(2),  fork(2), open(2),

       Documentation/filesystems/locks.txt in the  Linux  kernel  source  tree
       (Documentation/locks.txt in older kernels)


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