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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 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

       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

       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.)

       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).


       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)


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