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LOCKF(3C)							     LOCKF(3C)

       lockf - record locking on files

       #include <unistd.h>

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

       The  lockf()  function allows sections of a file to be locked; advisory
       or mandatory write locks depending  on the mode bits of the  file  (see
       chmod(2)). Calls to lockf() from other threads that 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 informa‐
       tion about record locking.

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

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

	 #define   F_ULOCK   0	 /* unlock previously locked section */
	 #define   F_LOCK    1	 /* lock section for exclusive use */
	 #define   F_TLOCK   2	 /* test & lock section for exclusive use */
	 #define   F_TEST    3	 /* test section for other locks */

       All  other  values  of  function are reserved for future extensions and
       will result in an error 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

       The  size  argument  is	the number of contiguous bytes to be locked or
       unlocked. The resource to be locked or unlocked 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 (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 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.	 Locked sections will be unlocked starting at the point of the
       offset through size bytes or to the end of file if size is  (off_t)  0.
       When  this  situation  occurs,  or if this situation occurs in adjacent
       sections, 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 blocks the calling thread  until  the
       resource is available. F_TLOCK causes the function to return −1 and set
       errno to EAGAIN if the section is already locked by another process.

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

       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 errno
       is set to EDEADLK and the requested section is not released.

       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, will 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 will attempt to
       unlock only the requested section.

       A potential for deadlock occurs if the threads of a process controlling
       a  locked  resource  is	put  to	 sleep by requesting another process's
       locked resource. Thus calls to lockf() or fcntl(2) 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)
       function may be used to provide a timeout facility in applications that
       require this facility.

       Upon successful completion, 0 is returned.  Otherwise, −1  is  returned
       and errno is set to indicate the error.

       The lockf() function will fail if:

			   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

			   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

			   A signal was caught during execution of  the	 func‐

			   The	fildes argument is on a remote machine and the
			   link to that machine is no longer active.

			   The	function  argument  is	not  one  of   F_LOCK,
			   F_TLOCK,  F_TEST, or F_ULOCK; or size plus the cur‐
			   rent 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:

			       The function argument is F_LOCK or F_TLOCK  and
			       the file is mapped with mmap(2).

			       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  locking  of files of the type indicated by
			       the fildes argument is not supported.

       Record-locking should not be used in combination	 with  the  fopen(3C),
       fread(3C),  fwrite(3C)  and  other  stdio functions.  Instead, the more
       primitive, non-buffered functions (such as  open(2))  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

       The alarm(2) function may be used to  provide  a	 timeout  facility  in
       applications requiring it.

       The  lockf() function has a transitional interface for 64-bit file off‐
       sets.  See lf64(5).

       See attributes(5) for descriptions of the following attributes:

       │Interface Stability │ Standard	      │
       │MT-Level	    │ MT-Safe	      │

       Intro(2), alarm(2), chmod(2), close(2),	creat(2),  fcntl(2),  mmap(2),
       open(2), read(2), write(2), attributes(5), lf64(5), standards(5)

				 Apr 10, 2002			     LOCKF(3C)

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