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 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 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 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 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.

       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 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.)

SEE ALSO

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

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

COLOPHON

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