locking(2)                                             locking(2)



NAME
     locking - provides exclusive file  regions  for  reading  or
     writing

SYNOPSIS
     int locking(fildes, mode, size)
     int fildes;
     int mode;
     int size;

DESCRIPTION
     locking will allow a specified number of bytes to be
     accessed only by the locking process (mandatory locking).
     Other processes which attempt to lock, read, or write the
     locked area will sleep until the area becomes unlocked.
     (Advisory locking is available via lockf(3C)).

     fildes is the word returned from a successful open, creat,
     dup, or pipe system call.

     mode is zero for unlocking the area.  mode is one or two for
     making the area locked. If the mode is one and the area has
     some other lock on it, then the process will sleep until the
     entire area is available. If the mode is two and the area is
     locked, an error will be returned.

     size is the number of contiguous bytes to be locked or
     unlocked.  The area to be locked starts at the current
     offset in the file.  If size is zero, the area to the end of
     file is locked.

     The potential for a deadlock occurs when a process
     controlling a locked area is put to sleep by accessing
     another process's locked area.  Thus calls to locking, read,
     or write scan for a deadlock prior to sleeping on a locked
     area.  An error return is made if sleeping on the locked
     area would cause a deadlock.

     Lock requests may, in whole or part, contain or be contained
     by a previously locked area for the same process.  When this
     or adjacent areas occur, the areas are combined into a
     single area.  If the request requires a new lock element
     with the lock table full, an error is returned, and the area
     is not locked.

     Unlock requests may, in whole or part, release one or more
     locked regions controlled by the process.  When regions are
     not fully released, the remaining areas are still locked by
     the process.  Release of the center section of a locked area
     requires an additional lock element to hold the cut off
     section. If the lock table is full, an error is returned,
     and the requested area is not released.



January 1992                                                    1

locking(2)                                             locking(2)



     While locks may be applied to special files or pipes,
     read/write operations will not be blocked.  Locks may not be
     applied to a directory.

     Note that close(2) automatically removes any locks that were
     associated with the closed file descriptor.

STATUS MESSAGES AND VALUES
     The value -1 is returned if the file does not exist, or if a
     deadlock using file locks would occur.

     The locking program will fail if the following are true:

     EACCES
          The area is already locked by another process.

     EDEADLOCK
          Returned by read, write, or locking if a deadlock would
          occur.

     EDEADLOCK
          Locktable overflow.

     EREMOTE
          fildes is a file descriptor that refers to a file on a
          remotely mounted file system.

SEE ALSO
     close(2), creat(2), dup(2), open(2), read(2), write(2),
     lockf(3C)

























2                                                    January 1992