Re: finding the value of a preprocessor symbol

[Ralf Wildenhues]

Hello David,

* David Byron wrote on Fri, Jan 22, 2010 at 12:38:03AM CET:
> Let's say I have a header file named foo.h that contains:
> 
> #define FOO an_interesting_value
> 
> (I think) I'm looking for a macro like this:
> 
> AC_GET_VALUE([foo.h],[FOO])
> 
> that populates ac_cv_value_of_FOO with an_interesting_value
> 
> Does such a macro exist?  Apologies if this is a FAQ.  I can think of a way
> to do this by compiling a program that writes the result to a file, but that
> seems complicated and a struggle when cross-compiling.

There is AC_COMPUTE_INT which allows you to compute the integer
(signed long) value of an arbitrary compile-time expression.  It is
a bit overkill for your case (in a cross-compile situation, it may
need up to log_2(value) compilations to find the value), but should
work nonetheless.

Autoconf provides AC_EGREP_CPP but that doesn't let you get at the
matched string.  I think we should add something like this, or allow
AC_PREPROC_IFELSE to store its output for inspection in the IF-TRUE
branch.

cpp -dM is not portable to non-GCC compilers.

Thanks for the report.

Cheers,
Ralf