Defect Report #421

Submitter: Jens Gustedt
Submission Date: 2012-10-08
Source: WG14
Reference Document: N1648
Version: 1.1
Date: April 2013
Subject: initialization of atomic_flag

Summary

C11 expresses the intention to have atomic_flag as a primitive that should allow to emulate all other atomic types and operations, 7.17.8 p3 in a note says:

The remaining types can be emulated with atomic_flag, though with less than ideal properties.

With the current semantic for the initialization of atomic_flag this goal cannot be achieved.

Details

This is a very good concept as far as I can see, but I have one problem to achieve this, initialization. The phrasing for atomic types in general and for atomic_flag are different. For atomic_flag we have:

An atomic_flag that is not explicitly initialized with ATOMIC_FLAG_INIT is initially in an indeterminate state.

The problem is how to emulate an atomic type with atomic_flag during initialization. Say we emulate with something like

struct atomic_int_struct {
  atomic_flag cat;
  int val;
};

Then the ATOMIC_VAR_INIT macro could simply look like:

#define ATOMIC_VAR_INIT(V) { .cat = ATOMIC_FLAG_INIT, .val = (V), }

But if I’d have a variable of atomic_int_struct with static storage duration

struct atomic_int_struct v;

is supposed to do the right thing, namely to guarantee that v has a valid state at startup, so it should contain a 0 for .val, and .cat must be in a determinate state. Since all atomic operations should work without problems on v, the state of .cat can’t be anything else than “clear”.

Now looking into the possible implementations of atomic_flag in assembler I didn’t yet meet a processor where the “clear” state would not be naturally represented by an all 0 value. So I guess in any reasonable implementation we would just have

#define ATOMIC_FLAG_INIT { 0 }

(or some equivalent formulation.)

If this is so, why ATOMIC_FLAG_INIT at all? Why not phrasing the same as for the other atomic types

Suggested Technical Corrigendum

Eliminate the mention of ATOMIC_FLAG_INIT in 7.17.1p3, B.16 and the index.

Proposed change for the initialization of atomic_flag, 7.17.8p4:

The default initializer { 0 } may be used to initialize an atomic_flag to the clear state. An atomic_flag object with automatic storage duration that is not explicitly initialized using { 0 } is initially in an indeterminate state; however, the default (zero) initialization for objects with static or thread-local storage duration initializes an atomic_flag to the clear state.
EXAMPLE
atomic_flag guard = { 0 };

If the committee would want to keep the macro ATOMIC_FLAG_INIT arround, a partial alternative to the above text would be to modify the text in 7.17.1

ATOMIC_FLAG_INIT
which expands to a default initializer ({ 0 } or equivalent) for an object of type atomic_flag.

Committee Discussion

• The standard deliberately does not specify values for the clear and set states of atomic_flag objects in order to support the widest possible set of architectures.
• The change suggested could be viewed as a proposal, but should be submitted as such, and since C11 shares the same memory model as C++11 a corresponding proposal should be submitted to WG21 as well.

Proposed Committee Response

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