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From: Markus Schoepflin (schoepflin_at_[hidden])
Date: 2001-09-20 03:27:04


--- In boost_at_y..., Beman Dawes <bdawes_at_a...> wrote:

See below...

> The definition of "thread-safe" given in the Boost.Threads docs
says that a
> program is thread-safe if it has no race-conditions, deadlocks, or
priority
> failures. A program is either thread-safe or it isn't. There are
no
> degrees of thread-safety. (I guess you might have a program that
is
> thread-safe on a single processor system but not on a multi-
processor
> system, but hardware isn't what I'm worried about in this message.)
>
> But when we get to libraries, classes, member functions, and non-
member
> functions, there seem to be degrees of thread-safety:
>
> 0) Not thread-safe.
>
> 1) Non-shared objects are thread-safe but only if all access is
> read-only. (Certain COW string implementations would fit
here.)
>
> 2) Non-shared objects are thread-safe for read/write access.
>
> class Foo {
> int x;
> public:
> void set( int v ) { x = v; }
> int get() const { return v; }
> };
>
> void thread_func() {
> Foo foo; // foo safe since not shared between threads
> ...
> }
>
> 3) Shared objects are thread-safe. (Either stateless or
> protected by some type multi-threading construct.)
>
> int foo; // foo is shared
>
> void thread_func() {
> ...
> {
> multi-threading-exclusion // exact form immaterial
> foo = ...;
> ... = foo;
> }
> }
>

I think 3 should actualle by 3a and 3b.

3a) Thread safe for shared/ro.

The SGI STL-Implementation conforms to this level of thread safety.
See http://www.sgi.com/tech/stl/thread_safety.html

3b) Thread safe for shared/rw.

> While I think the idea of differing degrees of thread-safety is
correct, my
> explanation above seems really lame. Can anyone come up with a
better
> explanation?
>
> What are good names for these degrees of thread-safety?
>
> Have these degrees of thread-safety already been define well
elsewhere?
>
> (We need better definitions since currently different people have a
> different degree in mind when they use the term "thread-safety".
The
> result is much confusion.)
>
> --Beman

Markus


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