Atomic types

Syntax

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:

1. 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 .
2. 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 .
3. 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 .
4. 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.

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 <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);
}

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

References

See also