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Linux 2.6 - man page for lockf (linux section 3posix)

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

       lockf - record locking on files

       #include <unistd.h>

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

       The  lockf()  function  shall  lock  sections of a file with advisory-mode locks. Calls to
       lockf() from other threads 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 sup-
       ported 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 per-
       missible 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 includ-
       ing  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  con-
       tained  by a previously locked section for the same process. When this occurs, or if adja-
       cent locked sections would occur, the sections shall be combined into a single locked sec-
       tion. 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 section is  not  avail-
       able.  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  portions 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 another process' locked section. If the system detects that dead-
       lock 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 sec-
       tion, 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.

       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.

       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.


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

	      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.

	      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().


	      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.


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

       The following sections are informative.

   Locking a Portion of a File
       In the following example, a file named /home/cnd/mod1 is being modified.  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);

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

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



       alarm() , chmod() , close() , creat() , fcntl() , fopen() , mmap() ,  open()  ,	read()	,
       write() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 LOCKF(P)

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