C
Atomics
Learn atomic operations with stdatomic.h.
stdatomic.h atomic operations
The stdatomic.h header provides atomic operations introduced in C11. The atomic_int type offers indivisible increment and decrement. atomic_fetch_add(&counter, 1) atomically increments the counter.
Atomic operations are critical for thread safety. A plain counter++ is three machine instructions (load, add, store); two threads doing it at once can lose an increment. atomic_fetch_add is a single indivisible instruction, preventing such data races. Memory ordering is also controlled in this header; memory_order_relaxed is the weakest, memory_order_seq_cst the strongest.
Memory ordering
The behavior of atomic operations is controlled by memory ordering. The default memory_order_seq_cst is the strongest and safest; all threads see the same order.
memory_order_relaxed is the weakest; only atomicity is guaranteed, not ordering. It is used for performance but requires care. You can specify it explicitly with atomic_fetch_add_explicit(&counter, 1, memory_order_relaxed). In most cases the default ordering suffices; weaker ordering is used only after profiling.
Exercise Write an atomic counter
Add #include <stdatomic.h>. In int main(void) declare atomic_int counter = 0;. Call atomic_fetch_add(&counter, 1) twice. Print the result with printf("%d\n", counter).
Show example solution
#include <stdio.h>
#include <stdatomic.h>
int main(void) {
atomic_int counter = 0;
atomic_fetch_add(&counter, 1);
atomic_fetch_add(&counter, 1);
printf("%d\n", counter);
return 0;
}
Test yourself
-
Which header provides atomic operations?
- <atomic.h>
- <sync.h>
- <thread.h>
- <stdatomic.h>
Show answer
<stdatomic.h> — The stdatomic.h header holds C11 atomic operations.
-
Complete the atomic_fetch_add call
atomic_int counter = 0;
atomic_fetch_add(&counter, ____);
Show answer
atomic_int counter = 0;
atomic_fetch_add(&counter, 1);
-
Match the atomic function
- atomic_fetch_add
- Atomic increment
- atomic_fetch_sub
- Atomic decrement
- atomic_load
- Atomic read
- atomic_store
- Atomic write
Next lesson: threads.h
