flock

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 pre-
       cisely, 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.   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/Documen-
       tation.



Linux				  2002-04-24			     FLOCK(2)