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Linux 2.6 - man page for flock (linux section 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 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.  Further-
       more, the lock is released either by an explicit LOCK_UN operation on any of these  dupli-
       cate 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 imple-
       mented 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 seman-
       tics: 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.)

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   (Documenta-
       tion/locks.txt in older kernels)

COLOPHON
       This  page  is  part of release 3.55 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					    2013-02-11					 FLOCK(2)


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