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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 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 operation 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.  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.


       flock() does not lock files over NFS.  Use fcntl(2) instead: that  does
       work  over  NFS,  given  a  sufficiently  recent version of Linux and a
       server which supports locking.

       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 true BSD  semantics:  there  is  no  interaction
       between  the  types of lock placed by flock() and fcntl(2), and flock()
       does not detect deadlock.

       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.)


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

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


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