Subj : really simple portable eventcount...
To   : comp.programming.threads
From : SenderX
Date : Sat Mar 05 2005 10:54 pm

This is real fast in the signals without waiters case:


// pseudo-code
class ec
{

  pthread_mutex_t m_mutex;
  pthread_cond_t m_cond;
  int m_waiters;
  int m_ec;


public:

  ec() : m_waiters( 0 ), m_ec( 0 )
  {
    // init mutex and cond
  }


public:

  int get()
  {
    int local = m_ec;
    atomic_bitset( &m_ec, 0x80000000 );
    return local & 0x7FFFFFFF;
  }


  int signal()
  {
    int local = m_ec;

    if ( local & 0x80000000 )
    {
      pthread_mutex_lock( &m_mutex );

      while ( ! cas( &m_ec,
                     local,
                     ( local + 1 ) & 0x7FFFFFFF )
      {
        local = m_ec
      }

      local = m_waiters
      m_waiters = 0;

      pthread_mutex_unlock( &m_mutex );

      if ( local )
      {
        pthread_cond_broadcast( &m_cond );
      }
    }
  }


  int wait( int cmp )
  {
    int local = m_ec & 0x7FFFFFFF;

    if ( local == cmp )
    {
      pthread_mutex_lock( &m_mutex );

      local = m_ec & 0x7FFFFFFF;

      if ( local == cmp )
      {
        ++m_waiters;

        pthread_cond_wait( &m_cond, &m_mutex );
      }

      pthread_mutex_unlock( &m_mutex );
    }
  }

};


Can be used as a simple signal for lock-free algorithms.

.