Summary

Section 7.17.1 Introduction (to section 7.17 Atomics ) specifies that the header define a number of macros having the form ATOMIC_XXX_LOCK_FREE that indicate the lock-free property of the corresponding atomic types. No further description of the macros is provided here.

Section 7.17.5 Lock-free property, then goes on to specify that the atomic lock-free macros (presumably the same ones as those listed in 7.17.1) expand to one of three values: 0, 1, or 2.

Neither of the two sections above, nor any other in the standard, specifies whether or not the macros are required to expand to constant expressions usable in preprocessor #if directives. This is in contrast to some other standard macros such as those defined in <limits.h> which are typically so specified using language such as:

The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives.

As discussed in the thread starting with SC22WG14.13216, the only purpose for the existence of the ATOMIC_XXX_LOCK_FREE macros is to be able to write more efficient code by relying on their use in preprocessor #if conditionals. Thus, the absence of the requirement that they expand to constant expressions makes the macros unsuitable for that purpose.

Suggested Technical Corrigendum

In section 7.17.1, modify paragraph 3 as indicated below:

...which expand to constant expressions suitable for use in #if preprocessing directives and which indicate the lock-free property of the corresponding atomic types (both signed and unsigned); and