Defined in header <stdatomic.h>
#define ATOMIC_VAR_INIT(value) /* unspecified */
(since C11)
(deprecated in C17)

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 using this macro (until C17) 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).


This macro was a part of early draft design for C11 atomic types. It is not needed in C11, and is deprecated in C17 with plans to be removed in C2x.


  • C11 standard (ISO/IEC 9899:2011):
    • The ATOMIC_VAR_INIT macro (p: 274)

See also

initializes a new atomic_flag
(macro constant)

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