Subtle MT problem with __gnu_cxx::hash_map

[Lothar Werzinger]

On Friday 19 November 2004 12:30, Matt Austern wrote:
> On Nov 19, 2004, at 12:21 PM, Paul Dubuc wrote:
> > There's a subtle thread safety problem with hash_map that was found in
> > our testing recently.  It's understood that operator[] is a non-const
> > method since it can insert an element into a container if one is not
> > found with the given index.
> >
> > In our case we were using operator[] to access a hash_map that had
> > been fully populated.  Each index we were using did have an entry in
> > the hash_map.  So we were accessing elements in the map with
> > operator[] using multiple threads thinking that this would be a thread
> > safe, const operation.  This is implied by the statement "simultaneous
> > read accesses to to shared containers are safe" in the SGI STL user's
> > guide (http://www.sgi.com/tech/stl/thread_safety.html).
>
> But operator[] isn't read access.  It's defined to be equivalent to a
> certain form of insert().

If one can be sure that the elements that are accessed using operator[] do 
exist, I would also tend to assume it should behave like a read only access.

I therefore support the original authors suggestion, as the "fix" he discribes 
does not change the observable behaviour, but removes a twitch in MT 
programming.

Lothar
-- 
Lothar Werzinger Dipl.-Ing. Univ.
framework & platform architect
Xcerla Corporation
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