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Atomics as part of the C language are an optional feature that is available since C11.

Their purpose is to ensure race-free access to variables that are shared between different threads. Without atomic qualification, the state of a shared variable would be undefined if two threads access it concurrently. Eg an increment operation (++) could be split into several assembler instructions, a read, the addition itself and a store instruction. If another thread would be doing the same operation their two instruction sequences could be intertwined and lead to an inconsistent result.

Other means to avoid race conditions are available in C11’s thread interface, in particular a mutex type mtx_t to mutually exclude threads from accessing critical data or critical sections of code. If atomics are not available, these must be used to prevent races.

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