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NAME    [Toc]    [Back]

     lockf - record locking on files

SYNOPSIS    [Toc]    [Back]

     #include <unistd.h>

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

DESCRIPTION    [Toc]    [Back]

     The lockf() function allows sections of a file to be  locked
with advisory-mode
  locks.  Calls to lockf() from other processes which
attempt to
     lock the locked file section will  either  return  an  error
value or block
     until  the  section  becomes  unlocked.  All the locks for a
process are removed
 when the process terminates.

     The argument filedes is an open file descriptor.   The  file
     must  have  been  opened either for write-only (O_WRONLY) or
     (O_RDWR) operation.

     The function argument is a control value which specifies the
action to be
     taken.  The permissible values for function are 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.

     The  F_ULOCK  function  removes  locks from a section of the
file; F_LOCK and
     F_TLOCK both lock a section of a  file  if  the  section  is
available; F_TEST
     detects if a lock by another process is present on the specified section.

     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).  However, it is not permitted  to  lock  a  section
that starts or
     extends before the beginning of the file.  If size is 0, the
section from
     the current offset through the largest possible file  offset
is locked
     (that is, from the current offset through the present or any
future endof-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
 are combined into a single locked section.  If the request would
     cause the number of locks to exceed a system-imposed  limit,
the request
     will fail.

     The  F_LOCK  and  F_TLOCK requests differ only by the action
taken if the
     section is not available.  F_LOCK blocks the calling process
until the
     section  is  available.   F_TLOCK makes the function fail 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 release (wholly or in part) of one or more
locked sections
 controlled by the process.  Locked  sections  will  be
unlocked starting
  at the current file offset through size bytes or to the
end of the
     file if size is 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  are
still locked
     by  the  process.   Releasing the center portion of a locked
section will
     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
will fail.

     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

     A  potential  for deadlock occurs if a process controlling a
locked region
     is put to sleep by attempting to lock the locked  region  of
another process.
   This  implementation  detects  that sleeping until a
locked region is
     unlocked would cause a deadlock and fails  with  an  EDEADLK

     lockf(), fcntl(2) and flock(2) locks may be safely used concurrently.

     Blocking on a section is interrupted by any signal.

RETURN VALUES    [Toc]    [Back]

     If successful, the lockf() function returns  0.   Otherwise,
it returns -1,
     sets the global variable errno to indicate an error, and existing locks
     are not changed.

ERRORS    [Toc]    [Back]

     lockf() will fail if:

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

     [EBADF]        The argument filedes is not a valid open file

                   The argument function is  F_LOCK  or  F_TLOCK,
and filedes is
                   not  a valid file descriptor open for writing.

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

     [EINTR]        The  argument  function is F_LOCK and lockf()
was interrupted
                   by the delivery of a signal.

     [EINVAL]      The argument function is not one  of  F_ULOCK,
                   F_TLOCK or F_TEST.

                   The  argument  filedes  refers  to a file that
does not support

     [ENOLCK]      The argument function is  F_ULOCK,  F_LOCK  or
F_TLOCK, and
                   satisfying  the  lock  or unlock request would
result in the
                   number of locked regions in the system exceeding a systemimposed

SEE ALSO    [Toc]    [Back]

     fcntl(2), flock(2)

STANDARDS    [Toc]    [Back]

     The  lockf()  function  conforms to X/Open Portability Guide
Issue 4.2

OpenBSD     3.6                        December     19,      1997
[ Back ]
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