kernel lock functions
lock *lock, int
prio, const char
*wmesg, int timo,
lock *lock, u_int
flags, struct simplelock
functions provide synchronisation in the kernel
by preventing multiple processes from simultaneously executing critical
sections of code accessing shared data.
struct lock supports sleeping until the lock can be acquired. The
lock manager supplies both exclusive-access and shared-access locks, with
recursive exclusive-access locks within a single process. It also allows
upgrading a shared-access lock to an exclusive-access lock, as well as
downgrading an exclusive-access lock to a shared-access lock.
The functions which operate on locks are:
Successfully acquired locks return 0. A failed lock attempt always returns a
non-zero error value. No lock is held after an error return. Locks will always
succeed unless one of the following is true:
- The lock lock is initialised according to the
parameters provided. Arguments are as follows:
- The lock.
- The process priority when it is woken up after sleeping on the
- A sleep message used when a process goes to sleep waiting for the
lock, so that the exact reason it is sleeping can easily be
- The maximum sleep time. Used by
- Flags to specify the lock behaviour permanently over the lifetime of
the lock. Valid lock flags are:
- Processes should not sleep when attempting to acquire the
- Processes can acquire the lock recursively.
- Set, change or release a lock according to the parameters provided.
Arguments are as follows:
- The lock.
- Flags to specify the lock request type. In addition to the flags
specified above, the following flags are valid:
- Get one of many possible shared-access locks. If a process holding
an exclusive-access lock requests a shared-access lock, the
exclusive-access lock is downgraded to a shared-access lock.
- Stop further shared-access locks, when they are cleared, grant a
pending upgrade if it exists, then grant an exclusive-access lock.
Only one exclusive-access lock may exist at a time, except that a
process holding an exclusive-access lock may get additional
exclusive-access locks if it explicitly sets the LK_CANRECURSE
flag in the lock request, or if the LK_CANRECURSE flag was set
when the lock was initialised.
- Release one instance of a lock.
- Wait for all activity on the lock to end, then mark it
decommissioned. This feature is used before freeing a lock that is
part of a piece of memory that is about to be freed.
- Attempt at recursive lock fails.
- This argument exists for legacy reasons, it is now ignored.
- Returns the current state of lock lock.
- Lock is locked for exclusive-access by the calling thread.
- Lock is locked for exclusive-access by a different thread.
- Lock is locked for shared-access. The current thread may be one of the
threads that has it locked.
- Lock is not locked.
The kernel locking API first appeared in 4.4BSD-Lite2.
It was progressively deprecated in favor of
- LK_NOWAIT is set and a sleep would be required.
- PCATCH is set in lock priority and a signal arrives to interrupt a system
- PCATCH is set in lock priority and a signal arrives so that the system
call is restarted.
- Non-null lock timeout and timeout expires.