thread static

Is there a technical reason why most languages don't support threads? I would
like to be able to declare in 'C' a variable as "thread static", there would
be a copy of the variable for each thread. A thread can not modify or read
another thread's variable.


This would allow me to write re-entrant routines like this:


typedef struct {...} cache_t;


int foo(....) {
    thread static cache_t cache;


    if (value is in cache)
          return value;
    else {
          value = Do_it_the_hard_way();
          Put_value_in_cache();
          return value;
      }
}


I'm aware that there are other, more general, ways of writing
reentrant code in a multi-threaded system -- but this way seems kinda
elegant and easy to use. I suppose you could get into serious
trouble if you declare pointers as thread static, but this may have
more to do with 'C' than this concept.


Most multi-threaded extensions to 'C' and languages that directly
support threads (JAVA is the only one I know) concentrate on locking
resources. This is good. But why not go one step further and allow
thread specific variables?


Thanks.
[I'd guess that it's mostly historical -- languages like C weren't designed
with multiprocessors in mind. It'd also take some linker hackery, but that
shouldn't be too hard. -John]
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