Re: Why conditionally include config.h?

[Kip Warner]

On Thu, 2012-09-13 at 17:17 -0700, Russ Allbery wrote:
> I believe the #ifdef wrapper used to be recommended by the Autoconf manual
> way back, many moons ago (2.13 days at the latest), because it was how the
> transition from defining things via -D on the command line to using a
> header was handled.  It goes along with the definition of @DEFS@, which
> previously (and by previously I mean a long time ago) used to contain all
> the results of configure as -D flags to the compiler, but which was
> replaced by just "-DHAVE_CONFIG_H" if you used the AC_CONFIG_HEADERS (or
> its earlier versions) macro.
> 
> So, in other words, you could transition your package that was assuming -D
> flags on the compiler command line to using a config.h header by adding
> that #ifdef code to the source files, and then it would work with either
> Autoconf method: either a config.h file or direct -D flags on the compiler
> command line.
> 
> I suspect the above is what's happening when you see this in really old
> projects, and in newer projects it's copy and paste from older projects.

Thanks Russ. That was very helpful. Is there a general rule on which
source files to #include <config.h>, or is it as simple as any files
that needs now or may need in the future the information contained in
it. One exception as previously pointed out would be of course to never
#include it in non-local / public headers.

-- 
Kip Warner -- Software Engineer
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http://www.thevertigo.com

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