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Atomic types

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 37 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.

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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 coherences that are guaranteed for all atomic operations:

-

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.

-

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

#include <stdio.h>
-#include <threads.h>
-#include <stdatomic.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

See also

- -
C documentation for thread (Concurrency support library)
C++ documentation for atomic
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/language/atomic -

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