mutex semaphore

Mutex and Semaphore — synchronization primitives that control access to shared resources in concurrent programs.

Mutex (Mutual Exclusion Lock)

• Binary: locked or unlocked
• Only the owner thread can unlock it
• Protects a critical section — one thread at a time
• Use when: Exactly one thread should access the resource

mutex.lock()
// critical section — only one thread here
mutex.unlock()

Semaphore

• Counter-based: initialized to N (number of concurrent accesses allowed)
• `wait()` / `P()`: Decrement counter; block if counter = 0
• `signal()` / `V()`: Increment counter; wake one waiting thread
• Use when: Multiple threads can share (e.g., connection pool of size N)

Other Synchronization Primitives

• Spinlock: Busy-wait loop instead of sleeping (fast for very short critical sections, wastes CPU otherwise)
• Read-Write Lock (RWLock): Multiple readers OR one writer. Great for read-heavy workloads
• Condition Variable: Thread waits until a condition becomes true (paired with mutex)
• Barrier: All threads must arrive before any can proceed

Performance Tip

• Minimize time spent inside critical sections
• Use lock-free data structures when possible (atomic CAS operations)

Choosing the right synchronization primitive is critical for both correctness and performance.