Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!eecae!netnews.upenn.edu!rutgers!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: Include files
Message-ID: <9526@smoke.BRL.MIL>
Date: 28 Jan 89 04:18:11 GMT
References: <10991@umn-cs.CS.UMN.EDU>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 35

In article <10991@umn-cs.CS.UMN.EDU> raghavan@umn-cs.cs.umn.edu (Vijay Raghavan) writes:
- Is the Sun style of surrounding the text of standard "include" files 
-with a #ifndef-#endif pair really okay? Specifically, does the standard
-condone/permit/require stdio.h (say) to have the following structure: 
-#ifndef FILE 
- ...
-#define FILE struct _iobuf
- ...
-#endif !__FILE

Well, more or less.  There are several detailed points involved here.

- If it does, programs like the following should not compile: 
-main() {
-#include <stdio.h>
-FILE *xx;
-... 
-}
-proc1() {
-#include <stdio.h>
-FILE *yy;
-...
-}

The first point is that the standard headers may be included only
outside all function bodies (i.e. at file scope level).

The next point is that a #defined macro's scope extends to the end
of the translation unit, not just to the end of a block.

The third point is that FILE should be a typedef, not a #define.

The final point is that the standard headers may be included more
than once with no effect different from being included exactly once.
This is what the "#ifndef FILE" was attempting to accomplish.