How do you force a makefile to rebuild a target?

Question

The -B switch to make, whose long form is --always-make, tells make to disregard timestamps and make the specified targets. This may defeat the purpose of using make, but it may be what you need.

You could declare one or more of your targets to be phony.

A phony target is one that is not really the name of a file; rather it is just a name for a recipe to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.

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A phony target should not be a prerequisite of a real target file; if it is, its recipe will be run every time make goes to update that file. As long as a phony target is never a prerequisite of a real target, the phony target recipe will be executed only when the phony target is a specified goal

One trick that used to be documented in a Sun manual for make is to use a (non-existent) target '.FORCE'. You could do this by creating a file, force.mk, that contains:

.FORCE:
$(FORCE_DEPS): .FORCE

Then, assuming your existing makefile is called makefile, you could run:

make FORCE_DEPS=release -f force.mk -f makefile release

Since .FORCE does not exist, anything that depends on it will be out of date and rebuilt.

All this will work with any version of make; on Linux, you have GNU Make and can therefore use the .PHONY target as discussed.

It is also worth considering why make considers release to be up to date. This could be because you have a touch release command in amongst the commands executed; it could be because there is a file or directory called 'release' that exists and has no dependencies and so is up to date. Then there's the actual reason...

Someone else suggested .PHONY which is definitely correct. .PHONY should be used for any rule for which a date comparison between the input and the output is invalid. Since you don't have any targets of the form output: input you should use .PHONY for ALL of them!

All that said, you probably should define some variables at the top of your makefile for the various filenames, and define real make rules that have both input and output sections so you can use the benefits of make, namely that you'll only actually compile things that are necessary to copmile!

Edit: added example. Untested, but this is how you do .PHONY

.PHONY: clean    
clean:
    $(clean)