Solution to the Readers and Writers problem using Semaphores

Semaphores can be used to restrict access to the database under certain conditions. In this example, semaphores are used to prevent any writing processes from changing information in the database while other processes are reading from the database.

semaphore mutex = 1;                 // Controls access to the reader count
semaphore db = 1;                    // Controls access to the database
int reader_count;                    // The number of reading processes accessing the data

Reader()
{
  while (TRUE) {                     // loop forever
     down(&mutex);                          // gain access to reader_count
     reader_count = reader_count + 1;       // increment the reader_count
     if (reader_count == 1)
         down(&db);                         // if this is the first process to read the database,
                                            // a down on db is executed to prevent access to the 
                                            // database by a writing process
     up(&mutex);                            // allow other processes to access reader_count
     read_db();                             // read the database
     down(&mutex);                          // gain access to reader_count
     reader_count = reader_count - 1;       // decrement reader_count
     if (reader_count == 0)
         up(&db);                           // if there are no more processes reading from the 
                                            // database, allow writing process to access the data
     up(&mutex);                            // allow other processes to access reader_countuse_data();
                                            // use the data read from the database (non-critical)
}

Writer()
{
  while (TRUE) {                     // loop forever
     create_data();                         // create data to enter into database (non-critical)
     down(&db);                             // gain access to the database
     write_db();                            // write information to the database
     up(&db);                               // release exclusive access to the database
}


               
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