Re: [boost] [thread] Interest in a locking pointer and lockable concept

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2008-03-24 11:27:09

Frank Mori Hess wrote:
> On Sunday 23 March 2008 18:47 pm, Phil Endecott wrote:
>> The point is that all of the values share a mutex.  Our Lockable<>
>> classes could be used here if you wanted one mutex per value, but that
>> could be wasteful and perhaps wrong.  How can we adapt Lockable<> to
>> work in this sort of situation?  I'm wondering about something like this:
>>
>> Lockable< array<int,100 > values;
>>
>> for (int i=0; i<100; ++i) {
>>    start_updater(LockableRef<int>(values,values[i]);
>> }
>>
>> i.e. LockableRef contains a reference to the mutex and one element of
>> its "parent" Lockable<>, and presents the same interface as a data-full
>> Lockable<>.
>
> I think what you would want to do is just create one scoped lock, perhaps 
> outside of the loop (a lock_acquirer in the lockable lib syntax) and use that 
> to access the values in the array.  It could just pass an ordinary reference 
> to the start_updater function, as long as start_updater doesn't keep any 
> persistent copies of the reference around.

No, I don't think so.  You mean something equivalent to this:

mutex m;
array<int,100> values;

{
   scoped_lock l(m);
   for (int i=0; i<100; ++i) {
     start_updater(LockableRef<int>(values,values[i]);
   }
}

Right?  That's not the scenario that I'm trying to describe.

> It could just pass an ordinary reference 
> to the start_updater function, as long as start_updater doesn't keep any 
> persistent copies of the reference around.

My point is that start_updater does keep a copy of the reference; from 
my original post:

void start_updater(mutex& m, int& val)
{
   // spawn a thread that periodically locks m and changes val
}

Or to be more explicit:

void run_updater(mutex& m, int& val)
{
   while (1) {
     sleep(1000);
     scoped_lock l(m);
     val = read_sensor();
   }
}

void start_updater(mutex&m, int& val)
{
   thread t(bind(run_updater,m,val));
}

Phil.