flock(2)



FLOCK(2)                   Linux Programmer's Manual                  FLOCK(2)

NAME
       flock - apply or remove an advisory lock on an open file

SYNOPSIS
       #include <sys/file.h>

       int flock(int fd, int operation);

DESCRIPTION
       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 ex-
                    clusive 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.   Fur-
       thermore,  the lock is released either by an explicit LOCK_UN operation
       on any of these duplicate file descriptors, or when all such  file  de-
       scriptors have been closed.

       If a process uses open(2) (or similar) to obtain more than one file de-
       scriptor for the same file, these file descriptors are treated indepen-
       dently 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  al-
       ready 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 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.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

ERRORS
       EBADF  fd is not an open file descriptor.

       EINTR  While waiting to acquire a lock, the call was interrupted by de-
              livery 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.

       EWOULDBLOCK
              The file is locked and the LOCK_NB flag was selected.

CONFORMING TO
       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.

NOTES
       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 fc-
       ntl(2).  With this implementation, there is no interaction between  the
       types  of lock placed by flock() and fcntl(2), and flock() does not de-
       tect deadlock.  (Note, however, that on some systems, such as the  mod-
       ern 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 de-
       scribed in this manual page.

       Converting a lock (shared to exclusive, or vice versa) is  not  guaran-
       teed  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  origi-
       nal 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  sup-
       ports 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  fc-
       ntl(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 al-
       lows 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).

SEE ALSO
       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)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          FLOCK(2)

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