Atomic types

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Syntax

_Atomic ( type-name ) (1) (since C11)
_Atomic type-name (2) (since C11)
1) Use as a type specifier; this designates a new atomic type
2) Use as a type qualifier; this designates the atomic version of type-name. In this role, it may be mixed with const, volatile, and restrict, although unlike other qualifiers, the atomic version of type-name may have a different size, alignment, and object representation.
type-name - any type other than array or function. For (1), type-name also cannot be atomic or cvr-qualified

The header <stdatomic.h> defines many convenience type aliases, from atomic_bool to atomic_uintmax_t, which simplify the use of this keyword with built-in and library types. 

_Atomic const int* p1;  // p is a pointer to an atomic const int
const atomic_int* p2;   // same
const _Atomic(int)* p3; // same

If the macro constant __STDC_NO_ATOMICS__ is defined by the compiler, the keyword _Atomic is not provided.

Explanation

Objects of atomic types are the only objects that are free from data races; that is, they may be modified by two threads concurrently or modified by one and read by another.

Each atomic object has its own associated modification order, which is a total order of modifications made to that object. If, from some thread's point of view, modification A of some atomic M happens-before modification B of the same atomic M, then in the modification order of M, A occurs before B.

Note that although each atomic object has its own modification order, there is no single total order; different threads may observe modifications to different atomic objects in different orders.

There are four coherence kinds that are guaranteed for all atomic operations:

  • write-write coherence: If an operation A that modifies an atomic object M happens-before an operation B that modifies M, then A appears earlier than B in the modification order of M.
  • read-read coherence: If a value computation A of an atomic object M happens before a value computation B of M, and A takes its value from a side effect X on M, then the value computed by B is either the value stored by X or is the value stored by a side effect Y on M, where Y appears later than X in the modification order of M.
  • read-write coherence: If a value computation A of an atomic object M happens-before an operation B on M, then A takes its value from a side effect X on M, where X appears before B in the modification order of M.
  • write-read coherence: If a side effect X on an atomic object M happens-before a value computation B of M, then the evaluation B takes its value from X or from a side effect Y that appears after X in the modification order of M.

Some atomic operations are also synchronization operations; they may have additional release semantics, acquire semantics, or sequentially-consistent semantics. See memory_order.

Built-in increment and decrement operators and compound assignment are read-modify-write atomic operations with total sequentially consistent ordering (as if using memory_order_seq_cst). If less strict synchronization semantics are desired, the standard library functions may be used instead.

Atomic properties are only meaningful for lvalue expressions. Lvalue-to-rvalue conversion (which models a memory read from an atomic location to a CPU register) strips atomicity along with other qualifiers.

This section is incomplete
Reason: more, review interaction with memory_order and atomic library pages

Notes

Accessing a member of an atomic struct/union is undefined behavior.

The library type sig_atomic_t does not provide inter-thread synchronization or memory ordering, only atomicity.

The volatile types do not provide inter-thread synchronization, memory ordering, or atomicity.

Implementations are recommended to ensure that the representation of _Atomic(T) in C is same as that of std::atomic<T> in C++ for every possible type T. The mechanisms used to ensure atomicity and memory ordering should be compatible.

Keywords

_Atomic

Example

Run this code
#include <stdatomic.h>
#include <stdio.h>
#include <threads.h>
 
atomic_int acnt;
int cnt;
 
int f(void* thr_data)
{
    for (int n = 0; n < 1000; ++n)
    {
        ++cnt;
        ++acnt;
        // for this example, relaxed memory order is sufficient, e.g.
        // atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed);
    }
    return 0;
}
 
int main(void)
{
    thrd_t thr[10];
    for (int n = 0; n < 10; ++n)
        thrd_create(&thr[n], f, NULL);
    for (int n = 0; n < 10; ++n)
        thrd_join(thr[n], NULL);
 
    printf("The atomic counter is %u\n", acnt);
    printf("The non-atomic counter is %u\n", cnt);
}

Possible output: 

The atomic counter is 10000
The non-atomic counter is 8644

References

  • C23 standard (ISO/IEC 9899:2024):
  • 6.7.2.4 Atomic type specifiers (p: TBD)
  • 7.17 Atomics <stdatomic.h> (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 6.7.2.4 Atomic type specifiers (p: 87)
  • 7.17 Atomics <stdatomic.h> (p: 200-209)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.7.2.4 Atomic type specifiers (p: 121)
  • 7.17 Atomics <stdatomic.h> (p: 273-286)

See also

Concurrency support library
C++ documentation for atomic
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