Make: Redo some targets if configuration changes

Question

I want to reexecute some targets when the configuration changes.

Consider this example:

I have a configuration variable (that is either read from environment variables or a config.local file):

CONF:=...

Based on this variable CONF, I assemble a header file conf.hpp like this:

conf.hpp:
    buildConfHeader $(CONF)

Now, of course, I want to rebuild this header if the configuration variable changes, because otherwise the header would not reflect the new configuration. But how can I track this with make? The configuration variable is not tied to a file, as it may be read from environment variables.

Is there any way to achieve this?

Answer 1

There is no way for make to know what to rebuild if the configuration changed via a macro or environment variable.

You can, however, use a target that simply updates the timestamp of conf.hpp, which will force it to always be rebuilt:

conf.hpp: confupdate
    buildConfHeader $(CONF)

confupdate:
    @touch conf.hpp

However, as I said, conf.hpp will always be built, meaning any targets that depend upon it will need rebuilt as well. A much more friendly solution is to generate the makefile itself. CMake or the GNU Autotools are good for this, except you sacrifice a lot of control over the makefile. You could also use a build script that creates the makefile, but I'd advise against this since there exist tools that will allow you to build one much more easily.