The Readers-Writers Problem

Reader/writer problem

• when writers are writing, only one writer can access
• When readers are reading, as many readers can access as long as no writers are writing.

To implement:

• you can only write if no one is reading or writing
• when you read, make sure you grab the semaphore. (But if there’s another thread reading, then you don’t have to acquire anything).
• Use a read count counter to know who to up / down semaphore. Avoids race condition on read_count
  • Writers don’t need a mutex because they don’t care about read count.

Shared bounded queue or “bounded buffer: a set of buffered shared by producer (inserts into buffer) and consumer (removes resources)threads.

• producer and consumer execute at different rates
• nc is number consumed, np is number produced, N is the size of the buffer: $0 \leq np - nc \leq N$

Implementation:

• requires three semaphores
  • empty: counts the number of empty buffers or “slots”
    • Empty = (nc - np) + N
  • full: counts the number of full buffered or “items”
    • Full = np - nc
  • Mutex: mutual exclusion to shared set of buffer. Use a binary semaphore