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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 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 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.


       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.)


       flock(1), close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), lockf(3)

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


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