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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.

       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

       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 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 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), lockf(3)

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


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