Issue 0422: initialization of atomic types

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG14, Jens Gustedt
Date: 2012-10-08
Reference document: N1649
Submitted against: C11 / C17
Status: Closed
Cross-references: 0454
Converted from: n2396.htm

Summary

The current version of the standard doesn't specify to which value an atomic object should be initialized if it is initialized by default.

An atomic object with automatic storage duration that is not explicitly initialized using ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state.

The mentioned valid state (in contrast to the indeterminate state mentioned before) is thus a determinate state, but the value that is stored is not mentioned explicitly. In the introduced language of the standard it is no definition of a "determinate state". It could be an "implementation-defined value", just an "unspecified value" or a default (zero) initialization. Everything suggests the later, that this would be the same value as for initializing a variable of the underlying base type by { 0 }. But I think it would have helped to make that explicit.

Suggested Technical Corrigendum

Proposed change for the initialization of atomic objects, 7.17.2.1p2:

An atomic object with automatic storage duration that is not explicitly initialized using ATOMIC_VAR_INIT is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration is guaranteed to produce a valid state that corresponds to the value of a zero initialized object of the unqualified base type.
EXAMPLE All three of the following objects initially have an observable value of 0.

_Atomic(unsigned) A = { 0 };
_Atomic(unsigned) B = ATOMIC_VAR_INIT(0u);
static _Atomic(unsigned) C;

Comment from WG14 on 2014-04-11:

Oct 2012 meeting

Committee Discussion

• ATOMIC_VAR_INIT is required to initialize an atomic object to a known value.
• The default value for an atomic object is defined to be valid but is unspecified.
• The committee does not see this as a defect.
• N1649 proposes that the default value be specified to be the same as the non-atomic type’s default value.
• Such a proposal should be submitted as such and may also need to be submitted to and addressed by WG21 (C++) as well.

Apr 2013 meeting

Proposed Committee Response

ATOMIC_VAR_INIT is required to initialize an atomic object to a known value. This value is defined to be valid but is unspecified in order to support the widest possible set of architectures. This is not a defect.