Doing Two Things at Once

By default, a C program is Single-Threaded. It executes one line of code at a time. In the modern world of 8-core and 16-core CPUs, this is a waste of power!

What is a Thread?

A thread is a "lightweight process." You can have multiple threads running inside one program, all sharing the same memory.

POSIX Threads (pthreads)

On Linux and Mac, we use the <pthread.h> library.

• pthread_create: Spawns a new thread to run a specific function.
• pthread_join: Waits for a thread to finish.

Why is it hard?

Race Conditions: When two threads try to change the same variable at the exact same time. The result is unpredictable and causes "Heisenbugs" (bugs that disappear when you try to find them).

Synchronization (The Solution)

To keep threads safe, we use:

1. Mutex (Mutual Exclusion): Like a key to a bathroom. Only the thread with the key can enter the "critical section" and change a variable.
2. Semaphores: Signals used to coordinate complex tasks.

Multithreading is one of the most difficult topics in computer science. It requires a completely different way of thinking. Master the basics of C first before trying to build a multi-threaded web server!