Submitter: Jens Gustedt
Submission Date:
2012-10-08
Source: WG14
Reference Document:
N1649
Version: 1.1
Date: April 2013
Subject: initialization of atomic types
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;
Committee Discussion
Apr 2013 meeting
ATOMIC_VAR_INITis 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.
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.