Re: Problem with -gnatt

[Jeffrey Carter <spam@spam.com>]

Brian May wrote:
> 
> However, as I think I might have said before, I have read that
> creating a semaphore using a protected type is inefficient, because
> protected types where intended to be much more flexible then
> semaphores.

There are places where semaphores are required. One is when exclusive 
control of external resources is required, and different combinations of 
such resources are required in different circumstances. Special 
attention to the order of obtaining and releasing such resources is 
needed to avoid deadlock.

There are three ways to deal with mutual exclusion: semaphores, 
protected objects, and passive tasks. Semaphores should be avoided when 
possible. Passive tasks, the main idiom in Ada 83, have the advantages 
of protected objects without the disadvantages. Potentially blocking 
operations may be made from critical regions in passive tasks, for 
example. The disadvantage of passive tasks is that they are tasks, and 
each operation requires a context switch.

There are places where each approach is preferable.

Note that compilers implementing Annex D provide the package 
Ada.Synchronous_Task_Control, which defines type Suspension_Object, 
which is essentially a binary semaphore (D.10), probably implemented 
more efficiently than using a protected object.

-- 
Jeff Carter
"You've got the brain of a four-year-old boy,
and I bet he was glad to get rid of it."
Horse Feathers
47