atomic_init

 

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Concurrency support library

Threads thrd_create thrd_equal thrd_current    thrd_sleep thrd_yield thrd_exit thrd_detach thrd_join thrd_successthrd_timedoutthrd_busythrd_nomemthrd_error    Atomic operations atomic_init ATOMIC_VAR_INIT (until C23) ATOMIC_***_LOCK_FREE atomic_is_lock_free atomic_store atomic_load atomic_exchange atomic_compare_exchange atomic_fetch_add atomic_fetch_sub atomic_fetch_or atomic_fetch_xor atomic_fetch_and

Atomic flags atomic_flag ATOMIC_FLAG_INIT atomic_flag_test_and_set atomic_flag_clear Memory ordering memory_order kill_dependency atomic_thread_fence atomic_signal_fence Mutual exclusion mtx_init mtx_lock mtx_timedlock mtx_trylock call_once mtx_unlock mtx_destroy mtx_plainmtx_recursivemtx_timed Condition variables cnd_init cnd_signal cnd_broadcast cnd_wait cnd_timedwait cnd_destroy Thread-local storage thread_local TSS_DTOR_ITERATIONS    tss_create tss_get tss_set tss_delete

 

Defined in header <stdatomic.h>
void atomic_init( volatile A* obj, C desired );		(since C11)

Initializes the default-constructed atomic object obj with the value desired. The function is not atomic: concurrent access from another thread, even through an atomic operation, is a data race.

This is a generic function defined for all atomic object types A. The argument is pointer to a volatile atomic type to accept addresses of both non-volatile and volatile (e.g. memory-mapped I/O) atomic objects, and volatile semantic is preserved when applying this operation to volatile atomic objects. C is the non-atomic type corresponding to A.

It is unspecified whether the name of a generic function is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function (e.g. parenthesized like (atomic_init)(...)), or a program defines an external identifier with the name of a generic function, the behavior is undefined.

Parameters

obj	-	pointer to an atomic object to initialize
desired	-	the value to initialize atomic object with

Return value

(none)

Notes

atomic_init is the only way to initialize dynamically-allocated atomic objects. For example:

_Atomic int *p = malloc(sizeof(_Atomic int));
atomic_init(p, 42);

References

• C23 standard (ISO/IEC 9899:2024):

• 7.17.2.2 The atomic_init generic function (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.17.2.2 The atomic_init generic function (p: 201)

• C11 standard (ISO/IEC 9899:2011):

• 7.17.2.2 The atomic_init generic function (p: 274-275)

See also

ATOMIC_VAR_INIT (C11)(deprecated in C17)(removed in C23)	initializes a new atomic object (function macro)
C++ documentation for atomic_init