Correct installation of config.h for shared library using autotools

Question

I am converting a C++ program which uses the autotools build system to use a shared library, introducing the use of libtool. Most of the program functionality is being placed in the shared library, which is loaded by the main program, so that the common code can be accessed by other programs in future.

Throughout the program and library sources the autoheader generated config.h is used with the usual macro:

#if HAVE_CONFIG_H
# include <config.h>
#endif

In configure.ac I use the macro to generate it:

AC_CONFIG_HEADERS([config.h])

My question is, do I need to install config.h for others to be able to use my library, and if so, what is the appropriate way to do it, and should it be renamed to avoid conflicts etc?

The most information I have found on this is here:

http://www.openismus.com/documents/linux/building_libraries/building_libraries#installingheaders

But this is hardly an official source.

Answer 1

You will need to install config.h if it affects the interface. In practical terms, if the #define's are required by the header(s), not just the .cc implementation / compilation units.

If config.h is a problem, you can specify another name in the AC_CONFIG_HEADERS macro. e.g., AC_CONFIG_HEADERS([foo_config.h]).

The easiest way to install the header, assuming automake, is with:

nodist_include_HEADERS = foo_config.h

in the top-level Makefile.am. the nodist prefix tells automake that foo_config.h is generated rather than distributed with the package.

If not using automake, install foo_config.h in $includedir. $exec_prefix/include is arguably a more correct location for a generated header, but in practice the former location is fine.

---

I avoid using config.h, by passing relevant definitions in CPPFLAGS or foo_CPPFLAGS along with AC_SUBST to Makefile.am for source builds, or AC_SUBST to foo.h.in to generate headers at configure-time. A lot of config.h is test-generated noise. It requires more infrastructure, but it's what I prefer. I wouldn't recommend this approach unless you're comfortable with the autotools.

Answer 2

Never ever install autoheader's config.h.

The last thing the users of your library need is interference from the macros leaking out of your config.h. Your library may have HAVE_FOOBAR, but my software might be compiled in a way that foobar is disabled, so that HAVE_FOOBAR will break my compilation.

The AX_PREFIX_CONFIG macro from the archive is a workaround, where everything gets prefixed.

A better approach is to create a template file (e.g. blargconfig.h.in) with lines like:

typedef @BLARG_TYPE@ blarg_int_t;

@BLARG_RANDOM_INCLUDE@

And then AC_SUBST() those variables in configure.ac:

AC_SUBST(BLARG_TYPE, ["unsigned short"])
AC_SUBST(BLARG_RANDOM_INCLUDE, ["#include <somerandomheader.h>"])

Then list it as an output file:

AC_CONFIG_FILES([Makefile
                 src/Makefile
                 ...
                 include/blargconfig.h])

The .h file should be listed with nodist_include_HEADERS; the .h.in file will be automatically distributed because it's listed in AC_CONFIG_FILES.

Destination for such files is commonly $libdir/packagename/include. See GLib for example, although they generate glibconfig.h without a template (by writing the whole creation code inline in configure.ac, as the autobook suggests). I find this approach less maintainable than using AC_SUBST, but it's more flexible.

Of course, to help the compiler find the platform-dependent header you'll probably also want to write a pkgconfig script, like GLib does.