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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.)

       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), lslocks(8)

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


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