reader/writer

from http://cne.gmu.edu/modules/ipc/orange/readmon.html

  monitor ReadersWriters
   condition OKtoWrite, OKtoRead; 
   int ReaderCount = 0;
   Boolean busy = false;
  

 procedure StartRead()
 {
   if (busy)   // if database is not free, block
     OKtoRead.wait;
   ReaderCount++;                      // increment reader ReaderCount
   OKtoRead.signal();

 }

 procedure EndRead()
 {
    ReaderCount-- ;   // decrement reader ReaderCount
    if ( ReaderCount == 0 )
       OKtoWrite.signal();
 }


 procedure StartWrite()
 {
   if ( busy || ReaderCount != 0 )
      OKtoWrite.wait();
   busy = true;
 }

 procedure EndWrite()
 {
   busy = false;
   If (OKtoRead.Queue)
     OKtoRead.signal();
   else
     OKtoWrite.signal();
  }

 Reader()
 {
    while (TRUE)  // loop forever     
    {
       ReadersWriters.StartRead();
  readDatabase();   // call readDatabase function in monitor
       ReadersWriters.EndRead();
    }
 }

 Writer()
 {
    while (TRUE)  // loop forever
    {
 make_data(&info);  // create data to write
       ReaderWriters.StartWrite();
 writeDatabase();  // call writeDatabase function in monitor
       ReadersWriters.EndWrite();
    }
 }