Hi Philip,
On Mon, 2007 Jul 30 10:23:33 +0100, Philip Hazel wrote:
>
> I don't understand
>
> #ifdef HAVE_CONFIG_H
> # include <config.h>
> #endif
>
> Three issues: (1) I think it should be "config.h" and not <config.h>
> because it's a local header file, not a system header file. (2) What's
> with the ifdef? Isn't config.h always present? (3) Why have you moved it
> into many individual sources rather than left it in pcre_internal.h
> which all of them include?
(1) It is indeed a local file, but remember that the behavior is slightly
different for #include<> versus #include"". More on this in a moment.
(2) Not necessarily. The autotools configuration can easily be tweaked to
pass all the preprocessor symbols as -Dfoo=bar directives on the
compiler command line instead. Not that we would do that, but in cases
where PCRE is being built using an alternative build system, it's
helpful to have the flexibility of using a config header or not.
(3) This way, it's much easier to ensure that all the files are
consistently/correctly #including config.h. What about those files that
don't #include pcre_internal.h? Or that don't #include it as the first
file? You have to do a lot more mental gymnastics in such cases to
ensure that config.h is the first non-comment bit that the preprocessor
sees.
Points #1 and #3 are addressed in the Autoconf manual. From
http://www.gnu.org/software/autoconf/manual/html_node/Configuration-Headers.html
:
The package should `#include' the configuration header file before
any other header files, to prevent inconsistencies in declarations
(for example, if it redefines const).
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.
--Daniel
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