How to specify a makefile target in a different directory?

Question

I am trying to figure out why I can't specify a target as being in different directory.

I am trying to generate some php classes from json files, but I've changed the line that actually generates the php to a dummy echo statement, so somebody else can just copy paste and test if they feel very generous.

There are two other things to add to the directory containing the makefile:

• A dummy dependency file called PatientDbPath.json
• A directory called out

If I have a makefile with the following:

.SUFFIXES: .php .json
.json.php:
    echo "HelloMe" > PatientDbPath.php

PatientDbPath.php: PatientDbPath.json

clean:
    $(RM) PatientDbPath.php

Then everything works when I run make; PatientDbPath.php is correctly created and the next time I run make, I get the message make: 'PatientDbPath.php' is up to date.

However, I want to generate the php file in a separate directory, so I updated my makefile to the following:

.SUFFIXES: .php .json
.json.php:
    echo "HelloMe" > out/PatientDbPath.php

out/PatientDbPath.php: PatientDbPath.json

clean:
    $(RM) out/PatientDbPath.php

As soon as I do that, Make tells me make: Nothing to be done for 'out/PatientDbPath.php' even though there is no file PatientDbPath.php in the out directory.

So I thought maybe it was something with the suffix rules and I created a third makefile.

out/PatientDbPath.php: PatientDbPath.json
    echo "Whatever" > out/PatientDbPath.php

clean:
    rm out/PatientDbPath.php

This one works well, like the first one. Can anybody see what I'm doing wrong in my 2nd makefile?

Thanks.

Answer 1

There's another way to do the same thing, perhaps more elegantly:

$(JSON_FILES): out/%.php: %.json
    action

This will create a rule for every file listed in JSON_FILES, so if it were defined JSON_FILES= foo.json bar.json, then this would be equivalent to:

out/foo.php: foo.json
    action

out/bar.php: bar.json
    action

You could then use $(wildcard *.json) to set that value if you really wanted all the .json files, or set it manually if the rule is unique to just a few.