CODEWITHSUNDEEP
linuxmakefilegnu-make
HighCommander4
HighCommander4

Reputation: 52739

Make and last modification times of directories on Linux

Consider the following makefile:

foo :
    mkdir foo

foo/a.out : foo a.in
    cp a.in foo/a.out

foo/b.out : foo b.in
    cp b.in foo/b.out

and the following interaction with it, starting from a directory that contains files a.in and b.in and nothing else:

$ make foo/a.out
mkdir foo
cp a.in foo/a.out

So far, so good.

$ make foo/b.out
cp b.in foo/b.out

Still good, but now:

$ make foo/a.out   # a.out should be up to date now!
cp a.in foo/a.out

It rebuilds the a.out target, even though none of its prerequisites have changed.

It seems like what's happening is that when foo/b.out is created, the last modification time of the directory foo is updated, so it picks that up as having "changed".

Is there a way to avoid this? For example, is there a way to declare that foo/a.out depends on foo only in that foo has to exist, and the creating of files inside foo doesn't cause foo to be considered out of date?

Upvotes: 1

Views: 702

Answers (3)

tripleee
tripleee

Reputation: 189387

I like @Beta's answer, but it is not portable. For a simple portable workaround, create a sentinel file when you create the directory, and depend on the sentinel file instead.

foo/.dir:
        mkdir -p foo
        touch $@
foo/a.out: a.in foo/.dir
        cp $< $@
foo/b.out: b.in foo/.dir
        cp $< $@

This can be further simplified with a pattern rule:

foo/%.out: %.in foo/.dir
        cp $< $@

Upvotes: 2

B. Szonye
B. Szonye

Reputation: 280

Instead of making the targets dependent on the directory, you can simply create the directory unconditionally in their build rules:

foo/a.out: a.in
        mkdir -p foo
        cp a.in foo/a.out

foo/b.out: b.in
        mkdir -p foo
        cp b.in foo/b.out

This avoids problems with using a directory as a prerequisite.

Upvotes: 1

Beta
Beta

Reputation: 99094

As an authority on Make recently pointed out:

"The one thing you must NEVER do is use a directory as a simple prerequisite. The rules the filesystem uses to update the modified time on directories do not work well with make."

But I think this will do what you want:

foo :
    mkdir foo

foo/a.out : a.in | foo
    cp a.in foo/a.out

foo/b.out : b.in | foo
    cp b.in foo/b.out

Upvotes: 4

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