Document: N1675
Date: 2013-02-21

Defect Report #4nn

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Submitter: Douglas Walls
Submission Date: 2013-02-21
Reference Document: N1675
Version: 1.0
Subject: atomic_compare_exchange: What does it mean to say two structs compare equal?

Summary The atomic_compare_exchange generic functions Description

  Atomically, compares the value pointed to by object for equality with
  that in expected, and if true, replaces the value pointed to by object
  with desired, and if false, updates the value in expected with the
  value pointed to by object.

When object is an atomic struct type and expected is the corresponding
non-atomic struct type.  What does it mean to compare two struct types
as equal?

Where does the C standard define what it means for two objects of struct
type to be equal? NOTE 1 gives an example using memcmp on how the test for
equality might be defined.  But that is non-normative.

But the padding bytes in a struct have unspecified values ( The memcmp function, footnote 310 reminds us that the contents
of padding in a struct is indeterminate.

Even integers can have padding bits, whose values are unspecified (

A similar issue probably occurs for Atomic union types.

Suggested Technical Corrigendum

Either define equality of objects of struct type, add a restriction disallowing
use of atomic structs as arguments for the atomic_compare_exchange generic functions,
or note that atomic_compare_exchange generic functions for objects of atomic
struct type results in undefined behavior.

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