ATOMIC_VAR_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>
#define ATOMIC_VAR_INIT(value) /* unspecified */		(since C11) (deprecated in C17) (removed in C23)

Expands to an expression that can be used to initialize an atomic variable of the same type as value.

The initial value of atomic object of automatic storage duration that is not explicitly initialized is indeterminate. The default (zero) initialization of static and thread-local variables produces a valid value however.

When initializing an atomic variable, any concurrent access, even through an atomic operation, is a data race (it may happen if the address is immediately passed to another thread with a memory_order_relaxed operation).

Notes

This macro was a part of early draft design for C11 atomic types. It is not needed in C11, and is deprecated in C17 and removed in C23.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

DR	Applied to	Behavior as published	Correct behavior
DR 485	C11	the specification was redundant and contradictory to the core language	fixed

References

• C17 standard (ISO/IEC 9899:2018):

• 7.17.2.1 The ATOMIC_VAR_INIT macro (p: 201)

• C11 standard (ISO/IEC 9899:2011):

• 7.17.2.1 The ATOMIC_VAR_INIT macro (p: 274)

See also

ATOMIC_FLAG_INIT (C11)	initializes a new atomic_flag (macro constant)
C++ documentation for ATOMIC_VAR_INIT