makefile: implicit rules: same target different prerequisites

Question

I have the following makefile:

all: DIR/0/a.txt DIR/1/b.txt DIR/2/c.txt DIR/3/abc.txt

DIR/%/abc.txt: DIR/%/def.xtx # rule #1
    mkdir -p $(@D)
    touch $@

DIR/%.txt: # rule #2
    mkdir -p $(@D)
    touch $@

DIR/%.xtx:
    touch $@

I want to generate DIR/%/def.xtx whenever DIR/%/abc.txt is generated; otherwise generate only DIR/%.txt.

Using the makefile above with GNU Make 3.81 however, only DIR/3/abc.txt gets generated and DIR/%/def.xtx is not generated.

Following the "Implicit rule search algorithm" in the GNU make user manual we get:

(1) Split t into a directory part, called d, and the rest, called n. For example, if t is ‘src/foo.o’, then d is ‘src/’ and n is ‘foo.o’.

For DIR/3/abc.txt, d = DIR/3 and n = abc.txt.

(2) Make a list of all the pattern rules one of whose targets matches t or n. If the target pattern contains a slash, it is matched against t; otherwise, against n.

Rules #1 and #2 match.

(3) If any rule in that list is not a match-anything rule, then remove all nonterminal match-anything rules from the list.

NOT SURE: no rule is removed from the list.

(4) Remove from the list all rules with no recipe.

No rule is removed.

(5) For each pattern rule in the list:

(5.a) Find the stem s, which is the nonempty part of t or n matched by the ‘%’ in the target pattern.

For rule #1, s = 3. For rule #2, s = 3/abc

(5.b) Compute the prerequisite names by substituting s for ‘%’; if the target pattern does not contain a slash, append d to the front of each prerequisite name.

For rule #1, it becomes: DIR/3/abc.txt: DIR/3/def.xtx Rule #2 has no prerequisites.

(5.c) Test whether all the prerequisites exist or ought to exist. (If a file name is mentioned in the makefile as a target or as an explicit prerequisite, then we say it ought to exist.)

NOT SURE: DIR/3/def.xtx is mentioned by the rule DIR/%.xtx:.

After this long long long explanation, my opinion is that I am probably wrong in (5.c).

I may be wrong, but I don't think % will ever match something with a / in it. Thus, assuming the above is the entirety of your Makefile, you don't have any rule that will match DIR/3/def.txt and make should complain about "don't know how to build DIR/3/def.txt" or something along those lines. — twalberg
It [%] does match things with /; otherwise DIR/0/a.txt, DIR/1/b.txt and DIR/2/c would not get touched; but they do. — Adriano Carvalho
Learned something new... it does seem to work. So, perhaps DIR/3/def.xtx doesn't get created because the third rule lacks the mkdir part? Since it's a prerequisite for DIR/3/abc.txt it needs to be built first, before the directory exists, so the mkdir in rule 1 won't happen before it tries to run rule 3. — twalberg
Yes, that's an error but, make doesn't look at the recipes to find which ones it has to execute. If it worked touch would at least complain that the directory didn't exist. — Adriano Carvalho

Beta Beta · Accepted Answer · 2012-10-23T16:47:35

From the manual,

A rule whose prerequisites actually exist or are mentioned always takes priority over a rule with prerequisites that must be made by chaining other implicit rules.

Your rule #1 requires chaining, your rule #2 has no prerequisites, so Make will choose #2 over #1 when trying to build DIR/3/abc.txt.

EDIT:

If you can stand having a list of directories:

DIRS := DIR/0 DIR/1 DIR/2 DIR/3

then you can do it with a static pattern rule:

all: DIR/0/a.txt DIR/1/b.txt DIR/2/c.txt DIR/3/abc.txt

ABC_TXT := $(addsuffix /abc.txt, $(DIRS))

$(ABC_TXT): DIR/%/abc.txt : DIR/%/def.xtx

DIR/%.txt:
    mkdir -p $(@D)
    touch $@

DIR/%.xtx:
    touch $@

(And it avoids duplication of the .txt rule too!)

makefile: implicit rules: same target different prerequisites

1 Answers