Re: Variably determined prerequisites

[Paul Smith]

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.