On Wed, Jun 04, 2008 at 02:37:26PM -0700, Steven Yi wrote:

> Is pthread_t an object that is used by value or by reference?  If by
> value (i.e. if pthread_t maps to an int or something like that), then
> we can just mmalloc size(pthread_t), memcpy, and return the value on
> the heap.  Any caller using that method can mfree or allow csound to
> cleanup at end in worst case. Am I off?

No. You are touching on the real nature of this problem.

The pthread_t is never to be interpreted by any non-system user.
If it is part of an ABI, and you want that ABI to be independent
of the the actual representation, then you need to abstract it.

If the system interface always uses the type by reference, then
that abstraction is easy: you just use that pointer, cast to void
pointer if you want. It means that the system assumes responsability
for maintaining the value it provided a pointer to.

But that is not the case. What you get from pthread_self() is
a value, not a pointer. You have to preserve this value, even
if all you ever provide to the external world is a pointer to
it. This means that somewhere there has to be some code that
is responsible for the memory management of such copies. In
practice this means you have to explicitly maintain an array
(or whatever data structure you like) of them.

If you just provide a pointer to a static value, then either
that value will become invalid on the next call that overwrites
it, and from that point on any owners of a pointer to it will
read the wrong information, OR you have to impose the rule that
everyone who gets such a pointer has to dereference it ASAP and
make a private copy the value it points to.

The latter, apart from being a hack relying on the value being
valid until it is copied (bad idea), is also an implicit change 
of the ABI, and requires recompilation of all clients if they
are still expected to work correctly. An ABI is not defined
only by its data structures, but also by the semantics.

In summary: A pointer to a data value is *not* an abstraction
of that data value.

Ciao,

-- 
FA

Laboratorio di Acustica ed Elettroacustica
Parma, Italia

Lascia la spina, cogli la rosa.


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