system-programming/07_Threading

🧵 Chapter 7: Threads – Summary

📌 Key Concepts

🔹 What is a Thread?

• A thread is a lightweight process that shares the same address space (memory) with other threads of the same process.
• Threads allow concurrent execution within a single process, enabling parallelism and responsiveness.

🔹 Benefits of Threads

• More efficient than processes in terms of resource usage and context switching.
• Ideal for applications that perform blocking operations (e.g., I/O-bound tasks).
• Useful for multicore scalability.

---

🛠️ POSIX Threads API (pthreads)

➕ Creating Threads

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine)(void *), void *arg);

• Spawns a new thread running start_routine(arg).
• Returns a pthread_t handle.

🛑 Joining Threads

int pthread_join(pthread_t thread, void **retval);

• Waits for the specified thread to terminate.
• Returns its return value if retval is not NULL.

❌ Detaching Threads

int pthread_detach(pthread_t thread);

• Allows a thread to run independently.
• Its resources are automatically reclaimed on exit (cannot be joined).

---

🧩 Thread Functions

Returning from a Thread

• Use return or pthread_exit() to terminate a thread and optionally pass back a result.

Thread IDs

• Use pthread_self() to get the calling thread's ID.

---

🔒 Synchronization

Mutexes

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_lock(&lock);
/* critical section */
pthread_mutex_unlock(&lock);

• Used to prevent data races in shared memory.
• Must be used carefully to avoid deadlocks.

Condition Variables

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

pthread_cond_wait(&cond, &mutex);
pthread_cond_signal(&cond);

• Used for thread coordination, e.g., waiting for a condition to become true.
• Must be used with an associated mutex.

---

🚧 Common Pitfalls

• Forgetting to initialize/destroy mutexes or condition variables.
• Accessing shared data without synchronization.
• Deadlocks from improper mutex locking order.
• Detached threads cannot be joined.
• Stack size: default per-thread stack may be small (configurable).

---

🧪 Thread Safety

• Not all C library functions are thread-safe (e.g., strtok is not, strtok_r is).
• Thread-local storage (TLS) can help isolate data per thread.

---

📋 Summary Table

Function	Purpose
pthread_create	Create a thread
pthread_join	Wait for a thread to finish
pthread_exit	End a thread
pthread_detach	Run thread without joinable state
pthread_mutex_*	Locking primitives
pthread_cond_*	Waiting/signaling conditions

---

✅ Best Practices

• Always check return values of pthread functions.
• Protect all shared data with mutexes or atomic operations.
• Use condition variables for coordination (not busy waiting).
• Prefer modular, short-lived threads when possible.
• Clean up thread resources (pthread_join, pthread_detach).

---

Last updated: 2025-07-28