GNU Make: Ensure existence of prerequisite and disable implicit rule search

Question

Question:

How can I disable implicit rule searches on a prerequisite while still ensuring that the prerequisite actually exists?

Background:

Consider the following initial Makefile:

b: a
    @echo MAKING B
    cp a b

a is a file which is required in order to make b. If the file a exists, make b runs successfully. If it doesn't exist, we obtain the following error:

make: *** No rule to make target `a', needed by `b'.  Stop.`

This is exactly what we expected, however on inspecting the output of make --debug=a b, we see that even when a exists, make is searching through pre-defined implicit rules fitting a in order to see whether it can be re-made. For example, if the file a.c happened to exist, then make would try to compile a.c to produce the file a. To prevent this, we define an explicit rule for a with an empty recipe. This gives us the updated Makefile:

a: ;

b: a
    @echo MAKING B
    cp a b

The problem now is that the recipe for make b runs even if a does not exist, which results in a failure. Is there any other way to indicate that a should exist, while not searching for implicit rules to build a? I would like to do this without giving a recipe for a which checks its existence.

Would this be sufficient for you? a: ; [ -f $@ ] # a should exist I.e. fail make b with a mild hint. I've tried few ways to suppress implicit pattern rules or override them for a with static patter, but I'd then end up seeing same thing as your empty a, i.e. it'd just dive into making b without a being present. — Ondrej K.
Thanks for the suggestion. That would certainly work, but I would like to know whether this is possible without defining a recipe to check for the target's existence. I put this clarification at the end. I feel like this should be possible without any explicit commands. — justinpc
Not sure if this would not mess with the rest of your Makefile, but to get desired behavior for this specific case, you could run make with -r or add empty .SUFFIXES: to your Makefile. Is that acceptable? Or too far reaching? — Ondrej K.
I don't fully understand how .SUFFIXES would work, but -r is too far reaching. I might want to retain built-in rules for other targets. — justinpc
I had a look into .SUFFIXES and would similarly like to keep it as it is. Let's say I did set .SUFFIXES to empty or used -r, I might still have defined an implicit rule myself whose target matches a and which in this case I would not want to match. — justinpc

Ondrej K. Ondrej K. · Accepted Answer · 2019-01-13T22:50:44

I'll try to sum up state of our discussion so far. Perhaps someone still pop's up with another/better insight.

Besides the option also mentioned in the question itself (see bellow for explainer on latest iteration for this approach):

a:
        $(error missing file "$@")
b: a
        @echo MAKING B
        cp a b

In theory it should be possible to disable implicit pattern rule altogether or for specific (set) of target(s) by either defining a no recipe target rule (% : %.c) or defining a static pattern rule (a: % : %.c). Nonetheless the resulting behavior, in case there is an a.c file, seems to be the same as with an empty rule for a:. I.e. make b just proceeds without file a being present (and we'd later fail trying to access it).

Since at least some of the implicit rule seem to be implemented as suffix rules, it's possible to disable consideration of inputs like a.c by purging list of default suffices:

.SUFFIXES:

Or inhibit use of implicit built-in rules altogether by invoking make with -r (or --no-builtin-rules) option. These however are rather heavy handed as they impact processing of all the rules across the Makefile.

To work the comment in:

as mentioned disabling couple of the built in rules for C compilation would appear to yield the desired result, namely:

% : %.c
% : %.o

Would result with a.c present and no a in make: *** No rule to make target 'a', needed by 'b'. Stop.

However (like -r) it's rather intrusive as in all other targets relying on the implicit rule would be impacted. While at the same time it's not as very far reaching, because it does not cover other cases like a.C, a.cpp, a,v,...

Static rule should be able to replace pattern rules where applicable (a more specific rule being applied over the more generic one when matching). But indeed limiting its to a single target does basically put it on par with just a specific a: rule.

I am actually not sure what the rest of the tree looks like and what all possible build steps could occur. With current understanding I would still gravitate to explicit target with file existence check should files with colliding names be a possibility and concern.

Explanation for the latest version of simple failing rule:

As @Stein followed up on the topic, he actually very helpfully pointed out: Simple (always) failing rule for "building" a is perfectly sufficient. If a file of that name (a) exists, the rule for target a never gets to run its recipe. For the case the file is not there, we can just have a recipe that fails with an error message.

GNU Make: Ensure existence of prerequisite and disable implicit rule search

1 Answers