CODEWITHSUNDEEP
makefilegnu-make
Xiphias
Xiphias

Reputation: 4716

Difference between $(VARIABLE) and % pattern rule

I am slowly fighting my way through the gnu make documentation jungle (although it's excellent — just a lot).

I encountered pattern rules:

%.png: file1 file2
    run command here

There are also wildcards usable as targets:

FILES = $(wildcard *.png)

Now I could write:

$(FILES): file1 file2
    run command here

Is there any difference between the first and the second use case?

Upvotes: 2

Views: 52

Answers (1)

BRPocock
BRPocock

Reputation: 13914

In the case of %.png: If any other rule, in turn, asks for some file, which does not itself have an explicit rule, but ends in .png, then this rule will be invoked.

So, %.png: %.jpeg (for example) says, “unless there is a more specific rule, here is how to convert from a JPEG to a PNG image.”

$(FILES) is enumerating a list of dependencies that you might want to create. It's not really the sort of thing for the target of a rule, but rather, for the dependencies of one.

For example, a phony target like images.zip: $(FILES) could be used to say that all of the $(FILES) must be up-to-date before making images.zip.

Using wildcard sources is, of course, rather “dangerous” as missing files won't be detected by make, so it's possible to have a “falsely positive” result of a build.

Upvotes: 2

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