C Concurrency: POSIX Threads (pthreads), Mutexes & Race Conditions Masterclass

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 4
#define ITERATIONS 100000

static long counter = 0;
static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

void *worker(void *arg) {
    for (int i = 0; i < ITERATIONS; i++) {
        pthread_mutex_lock(&lock);   // CRITICAL SECTION START
        counter++;                   // Safe increment
        pthread_mutex_unlock(&lock); // CRITICAL SECTION END
    }
    return NULL;
}

int main(void) {
    pthread_t threads[NUM_THREADS];
    for (int i = 0; i < NUM_THREADS; i++) {
        pthread_create(&threads[i], NULL, worker, NULL);
    }
    for (int i = 0; i < NUM_THREADS; i++) {
        pthread_join(threads[i], NULL);
    }
    printf("Final Synchronized Counter: %ld (Expected: %d)\n",
           counter, NUM_THREADS * ITERATIONS);
    pthread_mutex_destroy(&lock);
    return 0;
}