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LOCKF(3P)		   POSIX Programmer's Manual		     LOCKF(3P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       lockf — record locking on files

SYNOPSIS
       #include <unistd.h>

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

DESCRIPTION
       The lockf() function shall lock sections of a file  with	 advisory-mode
       locks.  Calls  to lockf() from threads in other processes which attempt
       to lock the locked file section shall either return an error  value  or
       block  until  the section becomes unlocked. All the locks for a process
       are removed when the process terminates. Record	locking	 with  lockf()
       shall  be  supported  for  regular files and may be supported for other
       files.

       The fildes argument is an open file descriptor.	To  establish  a  lock
       with this function, the file descriptor shall be opened with write-only
       permission (O_WRONLY) or with read/write permission (O_RDWR).

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

	      ┌─────────┬──────────────────────────────────────────────┐
	      │Function │		  Description		       │
	      ├─────────┼──────────────────────────────────────────────┤
	      │F_ULOCK	│ Unlock locked sections.		       │
	      │F_LOCK	│ Lock a section for exclusive use.	       │
	      │F_TLOCK	│ Test and lock a section for exclusive use.   │
	      │F_TEST	│ Test a section for locks by other processes. │
	      └─────────┴──────────────────────────────────────────────┘
       F_TEST shall detect if a lock by another	 process  is  present  on  the
       specified section.

       F_LOCK  and  F_TLOCK shall both lock a section of a file if the section
       is available.

       F_ULOCK shall remove locks from a section of the file.

       The size argument is the number of contiguous bytes  to	be  locked  or
       unlocked.   The	section to be locked or unlocked starts at the current
       offset in the file and extends forward for a positive size or  backward
       for  a  negative	 size (the preceding bytes up to but not including the
       current offset). If size is 0, the  section  from  the  current	offset
       through the largest possible file offset shall be 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 to be locked because 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 locked sections would	occur,
       the  sections  shall  be	 combined into a single locked section. If the
       request would cause the number of  locks	 to  exceed  a	system-imposed
       limit, the request shall fail.

       F_LOCK and F_TLOCK requests differ only by the action taken if the sec‐
       tion is not available. F_LOCK shall block the calling thread until  the
       section	is  available. F_TLOCK shall cause the function to fail if the
       section is already locked by another process.

       File locks shall be released on first close by the locking  process  of
       any file descriptor for the file.

       F_ULOCK	requests  may  release	(wholly or in part) one or more locked
       sections controlled by the process. Locked sections shall  be  unlocked
       starting	 at  the current file offset through size bytes or to the end-
       of-file if size is (off_t)0. When  all  of  a  locked  section  is  not
       released (that is, when the beginning or end of the area to be unlocked
       falls within a locked section), the remaining portions of that  section
       shall  remain  locked by the process. Releasing the center portion of a
       locked section shall cause the remaining locked beginning and end  por‐
       tions  to  become  two  separate	 locked sections. If the request would
       cause the number of locks in the	 system	 to  exceed  a	system-imposed
       limit, the request shall fail.

       A potential for deadlock occurs if the threads of a process controlling
       a locked section are blocked by accessing a locked section  of  another
       process. If the system detects that deadlock would occur, lockf() shall
       fail with an [EDEADLK] error.

       The interaction between fcntl() and lockf() locks is unspecified.

       Blocking on a section shall be interrupted by any signal.

       An F_ULOCK request in which size is non-zero and the offset of the last
       byte  of	 the  requested	 section is the maximum value for an object of
       type off_t, when the process has an existing lock in which  size	 is  0
       and  which  includes  the  last byte of the requested section, shall be
       treated as a request to unlock from the start of the requested  section
       with  a size equal to 0. Otherwise, an F_ULOCK request shall attempt to
       unlock only the requested section.

       Attempting to lock a section of	a  file	 that  is  associated  with  a
       buffered stream produces unspecified results.

RETURN VALUE
       Upon successful completion, lockf() shall return 0. Otherwise, it shall
       return −1, set errno to indicate an error, and existing locks shall not
       be changed.

ERRORS
       The lockf() function shall fail if:

       EBADF  The  fildes  argument  is	 not  a valid open file descriptor; or
	      function is F_LOCK or F_TLOCK and fildes is  not	a  valid  file
	      descriptor open for writing.

       EACCES or EAGAIN
	      The  function  argument  is F_TLOCK or F_TEST and the section is
	      already locked by another process.

       EDEADLK
	      The function argument is F_LOCK and a deadlock is detected.

       EINTR  A signal was caught during execution of the function.

       EINVAL The function argument is not one of F_LOCK, F_TLOCK, F_TEST,  or
	      F_ULOCK; or size plus the current file offset is less than 0.

       EOVERFLOW
	      The offset of the first, or if size is not 0 then the last, byte
	      in the requested section cannot be represented correctly	in  an
	      object of type off_t.

       The lockf() function may fail if:

       EAGAIN The  function  argument  is  F_LOCK  or  F_TLOCK and the file is
	      mapped with mmap().

       EDEADLK or ENOLCK
	      The function argument is F_LOCK, F_TLOCK, or  F_ULOCK,  and  the
	      request  would  cause  the  number  of locks to exceed a system-
	      imposed limit.

       EOPNOTSUPP or EINVAL
	      The implementation does not support the locking of files of  the
	      type indicated by the fildes argument.

       The following sections are informative.

EXAMPLES
   Locking a Portion of a File
       In  the	following  example, a file named /home/cnd/mod1 is being modi‐
       fied. Other processes that use locking are prevented from  changing  it
       during  this  process.  Only  the first 10000 bytes are locked, and the
       lock call fails if another process has any part	of  this  area	locked
       already.

	   #include <fcntl.h>
	   #include <unistd.h>

	   int fildes;
	   int status;
	   ...
	   fildes = open("/home/cnd/mod1", O_RDWR);
	   status = lockf(fildes, F_TLOCK, (off_t)10000);

APPLICATION USAGE
       Record-locking  should  not  be	used  in combination with the fopen(),
       fread(), fwrite(), and other stdio functions. Instead, the more	primi‐
       tive,  non-buffered  functions  (such  as open()) should be used. Unex‐
       pected 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 stdio functions are the most common	source	of  unexpected
       buffering.

       The  alarm()  function  may  be	used  to provide a timeout facility in
       applications requiring it.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm(), chmod(), close(), creat(), fcntl(), fopen(),  mmap(),  open(),
       read(), write()

       The Base Definitions volume of POSIX.1‐2008, <unistd.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the	referee document. The original Standard can be obtained online
       at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors,	 see  https://www.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			     LOCKF(3P)
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