lock(lockname)
unlock(lockname)lock() on lockname, lockname is locked. If the lock is unlocked,
lock() will immediately return. If on
the other hand the variable is locked, this client will join the queue
for the lock, and its call to lock() will block.When a client calls unlock(), the call will immediately return.
lock() made previously by the new head of the queue will now
return.
One does not use newdsm() to create a lock variable. Instead, the
variable is automatically created the first time lock() is called
on it.
barr,
wait,
signal,
fa