Re: HANDLE
On Apr 9, 2:29 pm, Martin York <Martin.YorkAma...@gmail.com> wrote:
On Apr 9, 2:05 am, James Kanze <james.ka...@gmail.com> wrote:
In computer science terms a 'handle' is a pointer to a
pointer to a resource. The concept was used by many OS to
allow the easy movements of resources in memory.
Do you have a reference for that?
Not really. Its from my days at University. But the following
may be clearer than I
was.http://en.wikipedia.org/wiki/Handle_%28computing%29
You'll notice that the article doesn't provide any references
either.
By providing a layer of indirection it allows a system to
perform some form of resource management without having to
keep to track of all pointers to an object (as there is only
one pointer, all the handles point at this pointer). If the
resource is moved you just need to update one pointer and all
handles are automatically still referencing the correct
resource.
The concept of handle definitely involves a conceptual level of
indirection. It's something which allows the OS to find the
relevant object. It "might" be an actual pointer to the object,
or it might be anything else. (I seem to recall file handles in
MS-DOS being small integers. Just like file descripters in
Unix. And a file descripter in Unix would be a handle IF is
wasn't documented to be an int, with a few documented integral
values for predefined file descripters and the error case.)
Though handles can be opaque (thus forcing access to the data
via a functional API) this is not a requirement of handle.
That's the key to what I've always understood by the word
"handle". But I'll admit, I'm not aware of any formal
definition anywhere; that just corresponds to the use that seems
to be current among the programmers (and the OS's) I work with.
Most systems force you to use an API because accessing data
directly via the handle is dangerous (as the resource could
move during access). Using a handle directly requires that it
be locked before use and unlocked after. SIDE-NOTE: Not
locking a handle before use leads to very hard to find bugs
(as they can be non deterministic [especially in multithreaded
environments]). As a consequence most systems that use handles
at a high level (were there is a high proportion of developers
that are less experienced with CS techniques) now make the
handle opaque to force you to use the API so that the
lock/unlock can be done automatically. On systems were
performance is an issue handles are generally not opaque to
allow the developers to optimize usage but adds the complexity
of forcing them to manually lock/unlock the handle.
OK. That's what I remember, but as I can't provide a good
source for the definition (my CS text books are all in
storage) I am quite happy to stand corrected.
Could it be that different communities use the term differently?
I've never heard it used in the way you describe, but I don't
doubt that you have. And the "natural" meaning of the word in
English could lead to either or both senses.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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