Submitter: UK C Panel
Submission Date: 2001-09-07
Source: Clive D.W. Feather <email@example.com>
Subject: multiple inclusion of headers
Consider the code:
#include <stdio.h> // Line 1 #undef FOPEN_MAX // Line 2, permitted by 7.1.3#3 #include <stdio.h> // Line 3 #ifdef FOPEN_MAX // Line 4
[#4] 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).
Does "with no effect different" mean:
Most current implementations wrap the contents of headers with an "idempotent guard", such as:
#ifndef _STDIO_H_INCLUDED_ #define _STDIO_H_INCLUDED_ // Real contents go here #endifThis will provide behaviour (2), which I would suggest is the most desirable.
Furthermore, the concept of scope doesn't apply here, both because includes happen during preprocessing and because there is a requirement in the same paragraph that:
If used, a header shall be included outside of any external declaration or definition,
If the wording is being altered, this would be a good opportunity to fix this as well.
Suggested Technical Corrigendum
Change the first sentence of 7.1.2#4 to:
[#4] Standard headers may be included in any order; each may be included any number of times in a preprocessing translation unit. The second and subsequent occurrences of a given header shall be ignored, except in the case of <assert.h> (where the behaviour is defined in subclause 7.2).
Proposed Committee Response
The Committee believe that both answer 1 and 2 are allowed, and does not see a compelling reason to change this.
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