[Prev][Next][Index][Thread]

Re: Passing parameters to threads

In article 
<Pine.GSO.3.96.1000413170453.4171D-100000@tardis.tardis.ed.ac.uk>, Hugh 
Greene <q@tardis.ed.ac.uk> wrote:

> On Thu, 13 Apr 2000, Dustin Voss wrote:
> > What is the best practice to pass a parameter to a thread?  The thread 
> > function passed to make(<thread>) looks like it takes no parameters.
> 
> That's correct, the thread function has to take no parameters; and,
> strictly speaking, you don't "pass parameters to" a thread, since it isn't
> called.  But, to be helpful rather than pedantic [:)], what you do is use
> closures; i.e., you mention inside a "method ... end" a binding which
> occurs outside its scope ("further up").


Huh...closures...Never wudda thunk it.  Thanks!


[not snipping for posterity's sake]


> So instead of
> 
>   make(<thread>,
>        function: do-something-with /* how do I pass 'my-state'?! */ );
> 
> you want to say
> 
>   let my-state = get-some-state();
>   make(<thread>,
>        function:
>          method ()
>            do-something-with(my-state)
>          end);
> 
> [And I'm opening myself up to pedantry, since I'm not sure I have the
> "function:" keyword right ;-)]
> 
> 
> Note that there's a real gotcha here if your state is closed over a
> thread-local variable.  If you say
> 
>   define thread variable *my-state* = #f;
> 
>   // Later ...
>   *my-state* := get-some-state();
>   make(<thread>,
>        function:
>          method ()
>            do-something-with(*my-state*)
>          end);
> 
> then do-something-with will be called with the value of *my-state* _in the
> new thread_, i.e., #f (or it might even be undefined, I can't remember)!
> The workaround is to make a new, non-thread-local binding to the value
> first, then "close over" that:
> 
>   define thread variable *my-state* = #f;
> 
>   // Later ...
>   *my-state* := get-some-state();
>   // This next binding isn't thread-local
>   let non-thread-local-my-state = *my-state*;
>   make(<thread>,
>        function:
>          method ()
>            do-something-with(non-thread-local-my-state)
>          end);
> 
> This is a pretty evil gotcha -- I'm sure it popped up in code in FD, back
> when it was HD, and it took us a while to spot -- but it very nicely
> illustrates closures and how "bindings are like pointers/references" (in
> that we have thread-local *bindings*, not thread-local state).
> 
> Hope that helps,
> Hugh
Follow-Ups: References: