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RedHat 9 (Linux i386) - man page for flock (redhat section 2)

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

       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 parameter opera-
       tion 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
       non-blocking 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 a file, or, more precisely, an open file ta-
       ble 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 opera-
       tion on any of these duplicate descriptors, or when all such descriptors have been closed.

       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

       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.

	      The file is locked and the LOCK_NB flag was selected.

       EBADF  fd is not a 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.

       EINVAL operation is invalid.

       ENOLCK The kernel ran out of memory for allocating lock records.

       4.4BSD  (the  flock(2)  call  first  appeared in 4.2BSD).  A version of flock(2), possibly
       implemented in terms of fcntl(2), appears on most Unices.

       flock(2) 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(2) 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(2) and fcntl(2),
       and flock(2) does not detect deadlock.

       flock(2) places advisory locks only; given suitable permissions on a file,  a  process  is
       free to ignore the use of flock(2) and perform I/O on the file.

       flock(2)  and fcntl(2) locks have different semantics with respect to forked processes and

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

       There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.

Linux					    2002-04-24					 FLOCK(2)

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