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Re: why not #include "config.h"?

From: Steffen Dettmer
Subject: Re: why not #include "config.h"?
Date: Tue, 22 Sep 2009 17:06:41 +0200

On Tue, Sep 22, 2009 at 3:02 PM, Patrick Welche <address@hidden> wrote:
> On Tue, Sep 22, 2009 at 10:49:58AM +0200, Steffen Dettmer wrote:
>> I've read here that someone could (should?) write
>>   #include <config.h>
>> specifying a system header instead of the IMHO correct
>>   #include "config.h"
> I think this is the relevant part of the autoconf info file:
> 4.9 Configuration Header Files
> ==============================

yeah, I knew this text (actually google did :)), but thought it
would be some mistake?

> ...
>   To provide for VPATH builds, remember to pass the C compiler a `-I.'
> option (or `-I..'; whichever directory contains `config.h').  Even if
> you use `#include "config.h"', the preprocessor searches only the
> directory of the currently read file, i.e., the source directory, not
> the build directory.
>   With the appropriate `-I' option, you can use `#include <config.h>'.
> Actually, it's a good habit to use it, because in the rare case when
> the source directory contains another `config.h', the build directory
> should be searched first.

I though this was a kind of lazyness; just used "" and <>
interchangingly without deeper meaning?

I tought correct is:

- with -I (and gcc), #include "config.h" is correct and works

- with -I (and gcc), #include <config.h> is not correct but works
  anyway, because gcc searches system headers in user directories
  if it was not found in system directories

- if #include <config.h> is really desired, I think for gcc it
  would be correct to use -isystem $(top_builddir)

- for other compilers, -I often is -I but -isystem can be -j

- other compilers may not distinguish that much or that flexible
  or not configurable where <file.h> is searched

- on other compilers, #include <config.h> may not even work
  (when they use a fixed system directory and support no fallback
  search to user directories)

Could * please correct my assumptions?



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