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From: Chuck Messenger (chuckm_at_[hidden])
Date: 2003-05-24 09:14:08

I've been experimenting with the thread lib. I found a bug related in
some way to thread_specific_storage (tss). In particular, I #ifdef'd
all the tss stuff out, using native pthread calls instead, and
everything worked as expected.

So, I went back through, trying to determine what could be going on.
I'm using tss to store a pointer to a thread-wide structure.

     thread_specific_ptr<mythread*> current_thread;

     mythread *mythread_get() {
         return *(current_thread.get());

     static int init() {
         // Need to initialize thread local storage for main thread

         // XXX I'd like to put something here to ensure that
         // current_thread has been constructed. But what?

         current_thread.reset(new mythread *);
         *(current_thread.get()) = new mythread;

         return 0;

     static int val = init();

The problem is that I can't be sure, during init(), that current_thread
has been constructed. I believe that's at the root of the bug I'm tracking.

By contrast, in my pthread-specific code, I simply put the
pthread_key_create() call at the start of init(), thus ensuring proper
initialization order. But there's no analogous call I can make for a
thread_specific_ptr -- that call is done during construction time.

So, what to do about it? Well, one obvious solution is to indirect

     thread_specific_ptr<mythread*> *current_thread;

then construct current_thread during init(). But that gives me yet one
more indirection I have to do during mythread_get(). It's already slow
as it is.

Any suggestions?

Which brings me to another source of slowness: because
thread_specific_ptr automatically invokes delete on a thread's tss
pointers when the thread goes away, I can't put a pointer to my real
object, like this:

     thread_specific_ptr<mythread> current_thread;

because mythread can't be deleted (for arcane reasons I won't get into
here). It would be nice if there were a version of thread_specific_ptr
which didn't delete, e.g.:

     thread_specific_ptr<mythread, non_deleting> current_thread;

Just a suggestion...

     - Chuck

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