how to set include paths with autotools

Question

I\'m working on a C++ project that uses autoconf & automake, and I\'m struggling to correctly set up the include paths in *CPPFLAGS. I

Answer 1

I know it's difficult to get a straight answer from the autotools manuals. There are a couple of good start-to-finish tutorials here and here.

There isn't a standard variable for package-specific *CPPFLAGS in autoconf. configure can be invoked with CPPFLAGS=..., and automake will add this CPPFLAGS to the relevant makefile rules - search for CPPFLAGS in a Makefile.in file for examples. For that reason, I suggest that you not use this variable for anything else.

Add flags in Makefile.am to the AM_CPPFLAGS variable (the default for all preprocessor calls) or override individual preprocessor flags with target_CPPFLAGS. In the example of a 3rd party library, it's best to use a name like: FOO_CPPFLAGS to hold preprocessor options, e.g.,

FOO_CPPFLAGS="-I${FOO_DIR}/include -DFOO_BAR=1"
...
AC_SUBST(FOO_CPPFLAGS)

and in the Makefile.am :

AM_CPPFLAGS = -I$(top_srcdir) $(FOO_CPPFLAGS)
# or:
target_CPPFLAGS = -I$(top_srcdir) $(FOO_CPPFLAGS)

The top_srcdir variable is defined by configure - I use it to illustrate the 2nd case. Let's say you have file.h in another directory other, under the top-level directory. -I$(top_srcdir) allows you to include it as <other/file.h>. Alternatively, -I$(top_srcdir)/other would allow you to include it as <file.h>.

Another useful preset variable is srcdir - the current directory. -I$(srcdir) is added to AM_CPPFLAGS by default. So if file.h is in the current directory you can include it with <file.h> or even "file.h". If other was a 'sibling' directory, -I$(srcdir)/.. would allow you to include <other/file.h>, and -I$(srcdir)/../other would allow <file.h>.

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I'd also add that some packages install a pkg-config .pc file. Provided the installation of pkg-config is set up to search the right directories, you might find the PKG_CHECK_MODULES macro very useful.