Document: WG14 N1400

Headers not idempotent

Submitter: Fred J. Tydeman (USA)
Submission Date: 2009-09-18
Related WG14 documents: N1362
Subject: Headers not idempotent

7.1.2 Standard headers, in paragraph 4, has:

Standard headers may be included in any order; each may be included more than once in a given scope, with no effect different from being included only once, except that the effect of including <assert.h> depends on the definition of NDEBUG (see 7.2).

7.8 <inttypes.h> and 7.18 <stdint.h> make reference to macros that affect which macros those headers define.

So, what should an implementation do for:

 #undef  __STDC_FORMAT_MACROS
 #include <inttypes.h>
 #define __STDC_FORMAT_MACROS
 #include <inttypes.h>

 #undef  __STDC_CONSTANT_MACROS
 #include <stdint.h>
 #define __STDC_CONSTANT_MACROS
 #include <stdint.h>

Should the extra macros be defined or not?

If a header sets a "guard" indicating that it has been included (and that nothing should happen the next time it is included), e.g.,

 #ifndef __Header_xxx_guard
 #define __Header_xxx_guard
 /* ... guts of header xxx */
 #endif

then, how are __STDC_FORMAT_MACROS and __STDC_CONSTANT_MACROS supposed to affect the header when it is included a 2nd time?

The same issue applies to many of the additions to headers done by the various technical reports over the years, e.g.,

 #undef  __STDC_WANT_DEC_FP__
 #include <float.h>
 #define __STDC_WANT_DEC_FP__
 #include <float.h>

Do we need to update 7.1.2 paragraph 4? If so, perhaps something along the lines of a footnote:

• [Footnote] Various C technical reports and C++ have identifiers that affect if additional macros are defined in some of the Standard C headers. That affect only happens on the first #include of the respective header.

Do we need to update the references to __STDC_FORMAT_MACROS and __STDC_CONSTANT_MACROS?

• Footnote 194: Change "is included" to "is first included"
• Footnote 229: Change "is included" to "is first included"
• Footnote 230: Change "is included" to "is first included"
• Footnote 233: Change "is included" to "is first included"