lockf man page on HP-UX

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lockf(2)							      lockf(2)

       lockf - provide semaphores and record locking on files

       The  function  allows regions of a file to be used as semaphores (advi‐
       sory locks) or restricts access to only the locking  process  (enforce‐
       ment-mode  record  locks).   Other processes that attempt to access the
       locked resource either return an error  or  sleep  until	 the  resource
       becomes	unlocked.  All locks for a process are released upon the first
       close of the file, even if the process still has the file  opened,  and
       all locks held by a process are released when the process terminates.

       fildes  is an open file descriptor.  The file descriptor must have been
       opened with write-only permission or read-write permission in order  to
       establish a lock with this function call (see open(2)).

       If  the	calling	 process is a member of a group that has the privilege
       (see getprivgrp(2)), it can also use to lock files  opened  with	 read-
       only permission

       function	 is  a	control	 value	that specifies the action to be taken.
       Permissible values for function are defined in as follows:

	      #define F_ULOCK	0      /* unlock a region */
	      #define F_LOCK	1      /* lock a region */
	      #define F_TLOCK	2      /* test and lock a region */
	      #define F_TEST	3      /* test region for lock */

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

       is  used	 to detect whether a lock by another process is present on the
       specified region.  returns zero if the region is accessible and	if  it
       is not; in which case is set to and both lock a region of a file if the
       region is available.  removes locks from a region 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 region from the current offset through the end of the
       largest	possible  file	is  locked  (that  is, 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, because such locks can
       exist past the end of the file.

       Regions locked with or can, in whole or in part,	 contain  or  be  con‐
       tained  by  a previously locked region for the same process.  When this
       occurs or if adjacent regions occur, the regions are  combined  into  a
       single  region.	If the request requires that a new element be added to
       the table of active locks but the table is already full,	 an  error  is
       returned, and the new region is not locked.

       and  requests  differ  only  by the action taken if the resource is not
       available: causes the calling process to sleep until  the  resource  is
       available,  whereas returns an error if the region is already locked by
       another process.

       requests can, in whole or part, release one or more locked regions con‐
       trolled	by  the	 process.   When  regions  are not fully released, the
       remaining regions are still locked by the process.  Releasing the  cen‐
       ter  section  of	 a locked region requires an additional element in the
       table of active locks.  If this table is full, an  error	 is  returned,
       and the requested region is not released.

       Regular	files  with  the file mode of not having the group execute bit
       set,  will  have	 an  enforcement  policy  enabled.   With  enforcement
       enabled, reads and writes that would access a locked region sleep until
       the entire region is available if is clear, but return −1 with  set  if
       is set.	File access by other system functions, such as are not subject
       to the enforcement policy.  Locks on directories,  pipes,  and  special
       files are advisory only; no enforcement policy is used.

       A  potential  for  deadlock  occurs  if	a process controlling a locked
       resource is put to sleep by accessing the locked	 resource  of  another
       process.	  Thus,	 calls	to  or	(see  fcntl(2), lockf(2), read(2), and
       write(2)) scan for a deadlock prior to sleeping on a  locked  resource.
       Deadlock	 is  not  checked  for	the  and system calls (see wait(2) and
       pause(2)), so potential for deadlock is not eliminated.	A call	or  an
       call  with the and flags set on a regular file returns error if another
       process has locked part of the  file  and  the  file  is	 currently  in
       enforcement mode.

       The  advisory record-locking capabilities of are implemented throughout
       the network by the ``network lock daemon''  (see	 lockd(1M)).   If  the
       file  server  crashes  and  is  rebooted,  the  lock daemon attempts to
       recover all locks associated with the crashed server.  If a lock cannot
       be reclaimed, the process that held the lock is issued a signal.

       Only advisory record locking is implemented for NFS files.

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

       fails if any of the following occur:

	      [EACCES]	     function is or and the region is  already	locked
			     by another process.

	      [EBADF]	     fildes is not a valid, open file descriptor.

	      [EDEADLK]	     A	deadlock  would occur or the number of entries
			     in the system lock table would exceed  a  system-
			     dependent	maximum.   HP-UX guarantees this value
			     to be at least 50.

	      [EINTR]	     A signal was caught during the system call.

	      [EINVAL]	     Either function is not one of the functions spec‐
			     ified above, or size plus current offset produces
			     a negative offset into the file.

	      [EINVAL]	     size plus current offset  cannot  be  represented
			     correctly by an object of size

	      [ENOLCK]	     Either function is or and the file is an NFS file
			     with access bits set for enforcement mode, or the
			     file  is  an NFS file and a system error occurred
			     on the remote node.

       Deadlock conditions may arise when either the or system calls are  used
       in  conjunction	with  enforced	locking	 (see wait(2) and pause(2) for

       When a file descriptor is closed, all locks on the file from the	 call‐
       ing  process  are deleted, even if other file descriptors for that file
       (obtained through or for example) still exist.

       Unexpected results may occur in processes that use buffers in the  user
       address	space.	 The  process may later read or write data which is or
       was locked.  The standard I/O package, stdio(3S), is  the  most	common
       source of unexpected buffering.

       In  a hostile environment, locking can be misused by holding key public
       resources locked.  This is particularly true  with  public  read	 files
       that have enforcement enabled.

       It  is  not  recommended that the capability be used because it is pro‐
       vided for backward compatibility only.  This feature may be modified or
       dropped from future HP-UX releases.

       Locks  default  to advisory mode unless the bit of the file permissions
       is set.

   Application Usage
       Because in the future the variable will be set to rather	 than  when  a
       section of a file is already locked by another process, portable appli‐
       cation programs should expect and test for either value.	 For example:

	      if (lockf(fd, F_TLOCK, siz) == -1)
		  if ((errno == EAGAIN) || (errno == EACCES))
		  * section locked by another process
		  * check for either EAGAIN or EACCES
		  * due to different implementations
		  else if ...
		  * check for other errors

       lockd(1M),   statd(1M),	 chmod(2),   close(2),	 creat(2),   fcntl(2),
       creat64(2),  open(2),  pause(2),	 read(2),  stat(2), wait(2), write(2),


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