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Minix/1.7.5/MANUALS/CAT3/LOCKF.3

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+
+NAME
+	  lockf	- record locking on files
+SYNTAX
+	  # include <unistd.h>
+	  lockf	(fildes, function, size) long size; int	fildes,
+	  function;
+DESCRIPTION
+	  The lockf command will allow sections	of a file to be	locked
+	  (advisory write locks).  (Mandatory or enforcement mode
+	  record locks are not currently available.)  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_UNLOCK 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 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 [EACCESS] 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 lock 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 descriptor.
+	  [EACCESS]
+	       Cmd is F_TLOCK or F_TEST	and the	section	is already
+	       locked by another process.
+	  [EDEADLK]
+	       Cmd is F_LOCK or	F_TLOCK	and a deadlock would occur.
+	       Also the	cmd is either of the above or F_ULOCK and the
+	       number of entries in the	lock table would exceed	the
+	       number allocated	on the system.
+SEE ALSO
+	  alarm(2), close(2), creat(2),	fcntl(2), intro(2), open(2),
+	  read(2), write(2).
+     RETURN VALUE
+	  Upon successful completion, a	value of 0 is returned.
+	  Otherwise, a value of	-1 is returned and errno is set	to
+	  indicate the error.
+     CAVEATS
+	  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.
+