Re: how to specify arbitrary macro variable for config.h

[Ralf Wildenhues]

Hello John,

* John Lumby wrote on Sat, Jul 17, 2010 at 08:09:32PM CEST:
> 
> This is a beginner question.    I am modifying a package and would
> like to introduce a new configure option to set a value which had been
> previously hardcoded.
> 
> So  -  for example  -     I want to provide an optional configure option
> 
>     --foo-barian-characteristic=VALUE
> 
> and place a default definition in config.h.in
> 
> #undef FOO_BARIAN_CHARACTERISTIC
> 
> and then write something in configure.ac which would respond to the
> user specifying the option by replacing #undef by
> 
> #define FOO_BARIAN_CHARACTERISTIC=VALUE

you mean a space not = here.

> in config.h
> 
> How can I do this?     I can see how by using AC_ARG_ENABLE and
> AC_DEFINE_UNQUOTED, but that would require that my option appear to be
> an --enable-xxx kind -  the user would have to specify
>    configure ... --enable-foo-barian-characteristic=VALUE
> which is slightly misleading since this is a setting for a default feature.

But your description in the second argument of AC_ARG_ENABLE can help
clarify that, right?

There is a reason configure does not accept arbitrary arguments: API
consistency.  For example, it should be possible to configure a number
of (even unrelated) packages with the same command line, and developers
should be able to stack packages using AC_CONFIG_SUBDIRS.

The extension mechanisms are provided by --enable-* and --with-*
arguments, and you shouldn't try to change that.

Hope that helps.

Cheers,
Ralf