atomic_init is not thread safe in C, so why does it exist?-CodePudding

The C17 standard, section 7.17.2.2, states the below regardingvoid atomic_init(volatile A *obj, C value) defined in <stdatomic.h>:

Although this function initializes an atomic object, it does not avoid data races; concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race.

Since the whole point of having atomic objects and atomic operations is to avoid data races, why does the atomic_init function exist? For example, why not do the below?

_Atomic int x = 7;

Instead of:

_Atomic int x; 
atomic_init(&x, 7);

Also, why is it called atomic_init when it really is an assignment?

CodePudding user response：

Atomic values may have hidden fields used to achieve the desired semantics, and those need to be initialized somehow. This can be achieved using an C variable initializer or using atomic_init.

For an static or automatic variable, that provides two options:

_Atomic int x = 7;

_Atomic int x; 
atomic_init( &x, 7 );

But that only leaves one option for dynamically-allocated variables.

_Atomic int *p = malloc( sizeof( _Atomic int ) );
atomic_init( p, 7 );