Subj : condvar wait / signal on arbitrary object
To   : comp.programming.threads
From : Michael B Allen
Date : Tue Jul 12 2005 03:46 pm

I just had a thought and I'd like your opinion.

Instead of embedding a condition variable in each object one would want
to wait and signal on, the object could be used as a key to lookup a
condvar by it's address modulo the size of an array of convars. This has
the benifit of permitting one to wait on an arbitrary object (like Java
except you can specify the mutex to unlock). For example:

  #define CONDVAR_ARRAY_SIZE 17

  struct ctx {
      pthread_cond_t array[CONDVAR_ARRAY_SIZE];
  };
  
  int
  ctx_pthread_cond_wait(struct ctx *ctx, void *obj, pthread_mutex_t *mutex)
  {
      pthread_cond_t *cond = ctx->array[(size_t)obj % CONDVAR_ARRAY_SIZE];
      return pthread_cond_wait(cond, mutex);
  }
  int
  ctx_pthread_cond_signal(struct ctx *ctx, void *obj, pthread_mutex_t *mutex)
  {
      pthread_cond_t *cond = ctx->array[(size_t)obj % CONDVAR_ARRAY_SIZE];
      return pthread_cond_broadcast(cond);
  }

Of course depending on the table size there could be many spurious wakeups.

What do you think? Good idea or should I not bother?

Thanks,
Mike

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