On Wed, Apr 1, 2020 at 4:23 PM Paul Smith <psm...@gnu.org> wrote: > > On Wed, 2020-04-01 at 16:03 -0400, NightStrike wrote: > > I am trying to use a pattern rule to pick which variable gets added > > to a target's prereq list. > > All of the confusion here is due to not quite grasping make's rules for > expanding variables. > > Make operates in two stages: in the first stage all makefiles are > parsed and an internal graph of dependency relationships are created. > This stage is described here: > https://www.gnu.org/software/make/manual/html_node/Parsing-Makefiles.html > > In the second stage, make walks that internal graph and actually runs > recipes to try to bring targets up to date. > > All variables that are needed to create the graph are expanded during > the first stage, when make is parsing makefiles. This expansion > happens immediately, as the line is read in from the file. > > This means all variables that appear in either target or prerequisite > ists are expanded immediately in the first stage. > > Variables that are not needed until the second stage are not expanded > until make tries to actually run a recipe. > > This is discussed in the manual, here: > https://www.gnu.org/software/make/manual/html_node/Reading-Makefiles.html > > This explains your behavior. When your variables appear in a target or > recipe they are expanded right then, as the line is parsed. It doesn't > matter whether they are defined with "=" or ":=". For pattern rules, > since we're not actually trying to build any targets (that happens in > the second phase), there is no way to expand the pattern value so it > just expands the literal string "extra%" as a variable. > > > You should be able to use secondary expansion (see the manual) to do > what you want: > > .SECONDEXPANSION: > xx%: xx%.o $$(extra$$*) > echo $@ $^ > > I didn't test this.
Thank you for such a swift, detailed, and correct response! Certainly this is what I want, it works perfectly, and the example in the manual is even exactly what I want to do: https://www.gnu.org/software/make/manual/make.html#Secondary-Expansion Thank you for helping to explain this without making me feel stupid :)
