Issue 0412: #elif

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

Authors: Project Editor (Larry Jones), Edward Diener (comp.std.c)
Date: 2012-01-18
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

It appears that #elif is not entirely equivalent to #else, #if, and #endif.

Consider the code:

#if 1
...
#else
#if this is not a valid expression
...
#endif
#endif

This is well-defined. Since the controlling expression of the #if evaluates to true, the #else group is skipped and thus the nested #if is only processed through the directive name (6.10.1p6).

However, if this is recast using #elif:

#if 1
...
#elif this is not a valid expression
...
#endif

the #elif is not part of a group that is skipped and thus must be processed completely, including evaluating the controlling condition (even though the resulting value is of no interest).

I do not believe this was the committee's intent.

Suggested Technical Corrigendum

In 6.10.1p6, change:

Only the first group whose control condition evaluates to true (nonzero) is processed.

to:

Only the first group whose control condition evaluates to true (nonzero) is processed; any following groups are skipped and their controlling directives are processed as if they were in a group that is skipped.

Comment from WG14 on 2017-11-03:

Feb 2012 meeting

Proposed Technical Corrigendum

In 6.10.1p6, change:

Only the first group whose control condition evaluates to true (nonzero) is processed.

to:

Only the first group whose control condition evaluates to true (nonzero) is processed; any following groups are skipped and their controlling directives are processed as if they were in a group that is skipped.