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 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  exam-
       ple, 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 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.

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

       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 conver-
       sion either blocks, or fails if LOCK_NB was specified.  (This is the original BSD behavior, and occurs on many other implementations.)

SEE ALSO

       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).

COLOPHON

       This page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2009-07-25								  FLOCK(2)