Makefile generator creates two files

Question

I have a generator program that creates two version files, say ver.h and ver.cpp. My ultimate build target depends on both of these files, and the rule

Answer 1

Using the phony target as the prerequisite is a bad idea. program will be run even if ver.* files exist, which is a false positive error.

More subtly, GNU Make is only guaranteed to update its file timestamp, if that file is a target of a rule with a recipe. So here, even though program is always run, anything that in turn depends on ver.* files might not get updated at all!

In my opinion it is best to not make up unnatural patterns for each target, but instead, go explicit:

There is a "main" file that you are generating, that is ver.cpp. Use the "no-op" recipe ; for the other one, which can be put on the same line like this:

ver.h: ver.cpp ;
ver.cpp: Makefile
    ./gen/version/program

This method starts with what you wrote, but adds the very important ;.

If you did not have a natural candidate for the "main" file, then in my opinion it is best to use a "sentinel":

ver.h ver.cpp: sentinel ;

sentinel: Makefile
    ./gen/version/program
    touch $@

Again, this method is similar to one of your methods, but very importantly, does not use a phony, but a real file.