Question

I have a little problem with autoconf, I know that you can use configure.ac to add some defines to configure.h, but is there a way to do something like this:

in one of my headers I have

#ifndef SIZE
#define SIZE 4
#endif

now I want to have an option that if I invoke

./configure

it creates makefile and the size is 4, but when someone does

./configure --block-size=num

the SIZE will be set to num, preferably I want to do this without config.h, I just want him to add something to makefile, so the compilation will be invoked with

-DSIZE=num

Answer 1

# configure.ac
AC_ARG_WITH([blocksize],
            AS_HELP_STRING([The desired blocksize [[default: 4]]]),
            [blocksize="$withval"], [blocksize=4])
my_CPPFLAGS="-DSIZE=$blocksize"
AC_SUBST([my_CPPFLAGS])

Quite simple.

# Makefile.am
AM_CPPFLAGS = ${my_CPPFLAGS}