What is the readers-writers problem?

The Readers-Writers problem synchronizes access to a shared resource where multiple readers can read simultaneously (safe — no modification) but writers need exclusive access (modifying means no readers or writers can access concurrently). Requirements: multiple readers can read simultaneously; only one writer at a time; readers and writers are mutually exclusive; no starvation. First readers-writers solution (readers priority): Semaphore mutex = 1; // Protect readCount Semaphore wrt = 1; // Writer lock int readCount = 0; // Reader: wait(mutex); readCount++; if (readCount == 1) wait(wrt); // First reader locks signal(mutex); READ_DATA(); wait(mutex); readCount--; if (readCount == 0) signal(wrt); // Last reader unlocks signal(mutex); // Writer: wait(wrt); // Exclusive access WRITE_DATA(); signal(wrt);. Problem: writers can starve if readers keep arriving (there's always a reader, so wrt is never released for the writer). Second readers-writers (writers priority): writers are given preference when waiting — complex implementation preventing new readers when a writer is waiting. Real-world implementations: Java ReadWriteLock: ReadWriteLock lock = new ReentrantReadWriteLock(); lock.readLock().lock(); try { readData(); } finally { lock.readLock().unlock(); } lock.writeLock().lock(); try { writeData(); } finally { lock.writeLock().unlock(); }. Used in: database systems (many readers, few writers), cache systems, configuration management.