[Tutor] Locking a specific variable (fwd)

[Kent Johnson]

> Yup, I'm familiar with those. In all of the examples, I have seen, the
> critical region happens in one specific area eg a part of a function with a
> lock and release around it. My question is there a way to lock a specific
> variable so that even though other functions are attempting to access it,
> they will not until the first function has released it. Examples below
> 
> # This example shows the way I have seen it in python tutorials
> def function1():
>       blah blah
>       function3()
>       blah blah
> 
> def function2():
>       blah blah
>       function3()
>       blah blah
> 
> def function3():
>      acquire lock
>      does some stuff
>      release lock
> 
> t = Thread.thread()
> t.start(function1())
> t.start(function2())

What is wrong with doing exactly this?
> 
> # The way I would like to do it, so we have two different instances of the
> same variable, and only # one function can access it at a time. If a

I don't understand what you mean by "two different instances of the same 
variable." Can you give an example?

> function comes to that variable and finds a lock on it, # it will wait until
> the lock is released.  And the variable happens to be a queue

Do you know that Queues are already thread safe? They use locks 
internally and are intended for multi-threaded use. You shouldn't have 
to use any additional locks to use a Queue.

> def function1():
>       blah blah
>       acquire lock of variable only so that only function1 can access this at
> a given time
>       ....increment some variable.....
>           release the function2 lock on this variable
>       blah blah
> 
> def function2():
>       blah blah
>       acquire lock of variable only so that only function2 can access this at
> a given time
>       ....decrement some variable......
>       release the function2 lock on this variable
>       blah blah
> 
> t = Thread.thread()
> t.start(function1())
> t.start(function2())

This code is the same as the first version except function3() has been 
inlined. I don't see the benefit of this style, it just causes code 
duplication.

Kent