Locks a variable or a subroutine. A lock on a variable is maintained
until the lock goes out of scope. If the variable is already locked
by another thread, the
lock call blocks until the variable is
available. You can recursively lock the variable, which stays locked
until the outermost lock goes out of scope.
Note that locks on variables only affect other
lock calls; they
don't prevent normal access to a variable. Also, locking a container
object (e.g., an array) doesn't lock each element of the container.
Locking a subroutine blocks any calls to the subroutine until the lock goes out of scope; no other thread can access the subroutine while the lock is in effect.