Canonical Reader/Writer Example

• Thread Rendezvous
  • semaphore::wait() and semaphore::signal() can be exploited to provide a different form of thread communication: rendezvous.
  • Here's our first example (full program is right here):

  static const unsigned int kNumBuffers = 30;
  static const unsigned int kNumCycles = 4;
  
  static char buffer[kNumBuffers];
  static semaphore emptyBuffers(kNumBuffers);
  static semaphore fullBuffers(0);
  
  static void writer() {
    cout << oslock << "Writer: ready to write." << endl << osunlock;
    for (unsigned int i = 0; i < kNumCycles * kNumBuffers; i++) {
      char ch = prepareData();
      emptyBuffers.wait();   // don't try to write to a slot unless you know it's empty
      buffer[i % kNumBuffers] = ch;
      fullBuffers.signal();  // signal reader there's more stuff to read 
      cout << oslock << "Writer: published data packet with character '"
           << ch << "'." << endl << osunlock;
    }
  }
  
  static void reader() {
    cout << oslock << "\t\tReader: ready to read." << endl << osunlock;
    for (unsigned int i = 0; i < kNumCycles * kNumBuffers; i++) {
      fullBuffers.wait();    // don't try to read from a slot unless you know it's full
      char ch = buffer[i % kNumBuffers];
      emptyBuffers.signal(); // signal writer there's a slot that can receive data
      processData(ch);
      cout << oslock << "\t\tReader: consumed data packet "
           << "with character '" << ch << "'." << endl << osunlock;
    }
  }
  
  int main(int argc, const char *argv[]) {
    thread w(writer);
    thread r(reader);
    w.join();
    r.join();
    return 0;
  }

  • Think of the writer thread as one that serves data to a network connection, and think of the reader thread as one that consumes it.
  • Two semaphores are used to synchronize the two so that:
    • reader is never further along than writer, and
    • writer is never so far ahead of reader that it clobbers data that has yet to be consumed.