Re: CONDITINAL DEFINE

[Eric Blake]

On 06/11/2011 03:32 AM, Luca Formaggia wrote:
> Dear all,
> 
> I am rather new to Autoconf and I could not find an answer to this
> question in the manual. I would like to know if there is a command to
> produce
> conditional defines. Something that from
> 
> AC_CONDEFINE(FOO,  "1")
> 
> would produce
> 
> #ifndef FOO
> #define FOO 1
> #endif

According to the manual, doing this directly in config.h is discouraged;
you should instead strive to make config.h self-contained (perhaps by
having your separate libraries define separate macro names, then in a
non-config.h header you can probe all of those macro names to determine
whether to define FOO).

http://www.gnu.org/software/autoconf/manual/autoconf.html#Header-Templates

However, it is indeed possible to use AH_VERBATIM to produce a template
that looks like:

#ifndef FOO
# undef FOO
#endif

for inclusion into config.h.in; at which point the #undef FOO line will
be replaced with either a comment or a #define FOO value line by
config.status.

Gnulib's size_max.m4 is an example of this idiom:

http://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/size_max.m4

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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