<stdatomic.h> provides _Atomic type and lock-free atomic operations for thread synchronization (C11). Atomic operations complete without interruption and support compare-and-swap for building lock-free data structures.

Memory order control (relaxed, acquire, release, seq_cst) allows trading off ordering guarantees for performance.

This example uses atomic operations to safely increment a counter from multiple threads.

// compile: gcc -std=c11 -pthread -o atomicexample atomicexample.c
// run: ./atomicexample
// description: thread-safe counter using atomic operations
 
#include <stdatomic.h>
#include <pthread.h>
#include <stdio.h>
 
atomic_int counter = 0;
 
void* worker(void* arg) {
    for (int i = 0; i < 100000; i++) {
        atomic_fetch_add_explicit(&counter, 1, memory_order_relaxed);
    }
    return NULL;
}
 
int main() {
    pthread_t t1, t2;
    pthread_create(&t1, NULL, worker, NULL);
    pthread_create(&t2, NULL, worker, NULL);
 
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
 
    printf("counter = %d (expected 200000)\n", atomic_load(&counter));
    return 0;
}