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LOCK(9)                FreeBSD Kernel Developer's Manual               LOCK(9)

     lockinit, lockdestroy, lockcount, lockmgr, lockstatus, lockmgr_printinfo
     - lockmgr family of functions

     #include <sys/types.h>
     #include <sys/lockmgr.h>

     lockinit(struct lock *lkp, int prio, const char *wmesg, int timo,
         int flags);

     lockdestroy(struct lock *lkp);

     lockcount(struct lock *lkp);

     lockmgr(struct lock *lkp, u_int flags, struct mtx *interlkp,
         struct thread *td);

     lockstatus(struct lock *lkp, struct thread *td);

     lockmgr_printinfo(struct lock *lkp);

     The lockinit() function is used to initialize a lock.  It must be called
     before any operation can be performed on a lock.  Its arguments are:

     lkp        A pointer to the lock to initialize.

     prio       The priority passed to sleep(9).

     wmesg      The lock message.  This is used for both debugging output and

     timo       The timeout value passed to sleep(9).

     flags      The flags the lock is to be initialized with.

                LK_NOWAIT          Do not sleep while acquiring the lock.

                LK_SLEEPFAIL       Fail after a sleep.

                LK_CANRECURSE      Allow recursive exclusive locks.

                LK_NOSHARE         Allow exclusive locks only.

                LK_TIMELOCK        Use timo during a sleep; otherwise, 0 is

     The lockdestroy() function is used to destroy a lock, and while it is
     called in a number of places in the kernel, it currently does nothing.

     The lockcount() function returns a count of the number of exclusive locks
     and shared locks held against the lock lkp.

     The lockmgr() function handles general locking functionality within the
     kernel, including support for shared and exclusive locks, and recursion.
     lockmgr() is also able to upgrade and downgrade locks.

     Its arguments are:

     lkp           A pointer to the lock to manipulate.

     flags         Flags indicating what action is to be taken.

                   LK_SHARED           Acquire a shared lock.  If an exclusive
                                       lock is currently held, it will be

                   LK_EXCLUSIVE        Acquire an exclusive lock.  If an
                                       exclusive lock is already held, and
                                       LK_CANRECURSE is not set, the system
                                       will panic(9).

                   LK_DOWNGRADE        Downgrade exclusive lock to a shared
                                       lock.  Downgrading a shared lock is not
                                       permitted.  If an exclusive lock has
                                       been recursed, all references will be

                   LK_EXCLUPGRADE      Upgrade a shared lock to an exclusive
                                       lock.  Fails with EBUSY if there is
                                       someone ahead of you in line waiting
                                       for an upgrade.  If this call fails,
                                       the shared lock is lost.  Attempts to
                                       upgrade an exclusive lock will cause a

                   LK_UPGRADE          Upgrade a shared lock to an exclusive
                                       lock.  If this call fails, the shared
                                       lock is lost.  During the upgrade, the
                                       shared lock could be temporarily
                                       dropped.  Attempts to upgrade an
                                       exclusive lock will cause a panic(9).

                   LK_RELEASE          Release the lock.  Releasing a lock
                                       that is not held can cause a panic(9).

                   LK_DRAIN            Wait for all activity on the lock to
                                       end, then mark it decommissioned.  This
                                       is used before freeing a lock that is
                                       part of a piece of memory that is about
                                       to be freed.  (As documented in

                   LK_SLEEPFAIL        Fail if operation has slept.

                   LK_NOWAIT           Do not allow the call to sleep.  This
                                       can be used to test the lock.

                   LK_CANRECURSE       Allow recursion on an exclusive lock.
                                       For every lock there must be a release.

                   LK_INTERLOCK        Unlock the interlock (which should be
                                       locked already).

     interlkp      An interlock mutex for controlling group access to the
                   lock.  If LK_INTERLOCK is specified, lockmgr() assumes
                   interlkp is currently owned and not recursed, and will
                   return it unlocked.  See mtx_assert(9).

     td            A thread responsible for this call.  NULL becomes

     The lockstatus() function returns the status of the lock in relation to
     the thread passed to it.  Note that if td is NULL and an exclusive lock
     is held, LK_EXCLUSIVE will be returned.

     The lockmgr_printinfo() function prints debugging information about the
     lock.  It is used primarily by VOP_PRINT(9) functions.

     The lockcount() function returns an integer greater than or equal to

     The lockmgr() function returns 0 on success and non-zero on failure.

     The lockstatus() function returns:

     LK_EXCLUSIVE      An exclusive lock is held by the thread td.

     LK_EXCLOTHER      An exclusive lock is held by someone other than the
                       thread td.

     LK_SHARED         A shared lock is held.

     0                 The lock is not held by anyone.

     lockmgr() fails if:

     [EBUSY]            LK_FORCEUPGRADE was requested and another thread had
                        already requested a lock upgrade.

     [EBUSY]            LK_NOWAIT was set, and a sleep would have been

     [ENOLCK]           LK_SLEEPFAIL was set and lockmgr() did sleep.

     [EINTR]            PCATCH was set in the lock priority, and a signal was
                        delivered during a sleep.  Note the ERESTART error

     [ERESTART]         PCATCH was set in the lock priority, a signal was
                        delivered during a sleep, and the system call is to be

     [EWOULDBLOCK]      a non-zero timeout was given, and the timeout expired.

     If LK_INTERLOCK is passed in the flags argument to lockmgr(), the
     interlkp must be held prior to calling lockmgr(), and will be returned

     Upgrade attempts that fail result in the loss of the lock that is
     currently held.  Also, it is invalid to upgrade an exclusive lock, and a
     panic(9) will be the result of trying.

     condvar(9), locking(9), mutex(9), rwlock(9), sleep(9), sx(9),
     mtx_assert(9), panic(9), VOP_PRINT(9)

     This manual page was written by Chad David <>.

FreeBSD 11.0-PRERELEASE          June 20, 2006         FreeBSD 11.0-PRERELEASE


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