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 parameter 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 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 table entry.  This means that duplicate file descrip-
       tors  (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.

       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

       EWOULDBLOCK
	      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.

CONFORMING TO

       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.

NOTES

       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 semantics: 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 dup(2).

SEE ALSO

       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)