LOCKF(3C)							     LOCKF(3C)

NAME    [Toc]    [Back]

     lockf - record locking on files

C SYNOPSIS    [Toc]    [Back]

     #include <unistd.h>

     int lockf (int fildes, int	function, off_t	size);

DESCRIPTION    [Toc]    [Back]

     The lockf command will allow sections of a	file to	be locked; advisory or
     mandatory write locks depending on	the mode bits of the file [see
     chmod(2)].	 Locking calls from other processes which attempt to lock the
     locked file section will either return an error value or be put to	sleep
     until the resource	becomes	unlocked.  All the locks for a process are
     removed when the process terminates.  [See	fcntl(2) for more information
     about record locking.]

     Fildes is an open file descriptor.	 The file descriptor must have
     O_WRONLY or O_RDWR	permission in order to establish lock with this
     function call.

     Function is a control value which specifies the action to be taken.  The
     permissible values	for function are defined in <unistd.h> as follows:

     #define   F_ULOCK	 0   /*	Unlock a previously locked section */
     #define   F_LOCK	 1   /*	Lock a section for exclusive use */
     #define   F_TLOCK	 2   /*	Test and lock a	section	for exclusive use */
     #define   F_TEST	 3   /*	Test section for other processes locks */


     All other values of function are reserved for future extensions and will
     result in an error	return if not implemented.

     F_TEST is used to detect if a lock	by another process is present on the
     specified section.	 F_LOCK	and F_TLOCK both lock a	section	of a file if
     the section is available.	F_ULOCK	removes	locks from a section of	the
     file.

     Size is the number	of contiguous bytes to be locked or unlocked.  The
     resource to be locked starts at the current offset	in the file and
     extends forward for a positive size and backward for a negative size (the
     preceding bytes up	to but not including the current offset).  If size is
     zero, the section from the	current	offset through the largest file	offset
     is	locked (i.e., from the current offset through the present or any
     future end-of-file).  An area need	not be allocated to the	file in	order
     to	be locked as such locks	may exist past the end-of-file.

     The sections locked with F_LOCK or	F_TLOCK	may, in	whole or in part,
     contain or	be contained by	a previously locked section for	the same
     process.  When this occurs, or if adjacent	sections occur,	the sections
     are combined into a single	section.  If the request requires that a new
     element be	added to the table of active locks and this table is already



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LOCKF(3C)							     LOCKF(3C)



     full, an error is returned, and the new section is	not locked.

     F_LOCK and	F_TLOCK	requests differ	only by	the action taken if the
     resource is not available.	 F_LOCK	will cause the calling process to
     sleep until the resource is available.  F_TLOCK will cause	the function
     to	return a -1 and	set errno to [EACCES] error if the section is already
     locked by another process.

     F_ULOCK requests may, in whole or in part,	release	one or more locked
     sections controlled by the	process.  When sections	are not	fully
     released, the remaining sections are still	locked by the process.
     Releasing the center section of a locked section requires an additional
     element in	the table of active locks.  If this table is full, an
     [EDEADLK] error is	returned and the requested section is not released.

     A potential for deadlock occurs if	a process controlling a	locked
     resource is put to	sleep by accessing another process's locked resource.
     Thus calls	to lockf or fcntl scan for a deadlock prior to sleeping	on a
     locked resource.  An error	return is made if sleeping on the locked
     resource would cause a deadlock.

     Sleeping on a resource is interrupted with	any signal.  The alarm(2)
     command may be used to provide a timeout facility in applications which
     require this facility.

     The lockf utility will fail if one	or more	of the following are true:


     [EBADF]
	  Fildes is not	a valid	open file.

     [EACCES]
	  Cmd is F_TLOCK or F_TEST and the section is already locked by
	  another process.

     [EDEADLK]
	  Cmd is F_LOCK	and a deadlock would occur.  Also the cmd is either
	  F_LOCK, F_TLOCK, or F_ULOCK and the number of	entries	in the lock
	  table	would exceed the number	allocated on the system.

NOTES    [Toc]    [Back]

     Locks are on files, not file descriptors.	That is, file descriptors
     duplicated	through	dup(3C)	do not result in multiple instances of locks,
     but rather	multiple references to the same	locks.	Thus if	any of the
     descriptors associated with the same file are closed, the locks
     associated	with the file are lost.

SEE ALSO    [Toc]    [Back]

     chmod(2), close(2), creat(2), fcntl(2), intro(2), open(2),	read(2),
     write(2).





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LOCKF(3C)							     LOCKF(3C)

DIAGNOSTICS    [Toc]    [Back]

     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of	-1 is returned and errno is set	to indicate the	error.

WARNINGS    [Toc]    [Back]

     Unexpected	results	may occur in processes that do buffering in the	user
     address space.  The process may later read/write data which is/was
     locked.  The standard I/O package is the most common source of unexpected
     buffering.

     Because in	the future the variable	errno will be set to EAGAIN rather
     than EACCES when a	section	of a file is already locked by another
     process, portable application programs should expect and test for either
     value.


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