These are configuration files for various features as detected by configure. Include only those files you need (in order to reduce rebuilds when a setting changes). Settings here are only C/C++ #define directives, so they apply only to C/C++ source, not to Makefiles. Adding a new setting: ===================== - do AC_DEFINE(HAVE_FOO) in configure.ac when a setting should be set - choose the proper config_host/config_XXX.h file to use - if it is a global setting (such as availability of a compiler feature), use config_host/config_global.h - otherwise check if there is a matching config_host/config_XXX.h file - if none matches, add a new one: - add config_host/config_XXX.h.in here, with just #ifndef include guard - add AC_CONFIG_HEADERS([config_host/config_XXX.h]) next to the others in configure.ac - add #define HAVE_FOO 0 to the config_host/config_XXX.h , possibly with a comment (do not use #undef HAVE_FOO, unless the setting has more values than on/off) - add #include <config_XXX.h> before any #if HAVE_FOO in a source file - make sure you use #if HAVE_FOO for on/off settings, do not use #ifdef