Marking a makefile dependency as optional or other

问题:

I have a Makefile with a pair of rules like the following:

file.c: filesvn

filesvn: .svn/entries
     action1
     action2

Within the svn repository, this of course works fine. The file is dependent upon living in a subversion repository. When exported from the repository, this does not work (No rule to make target...), and I would like to fix that. I have tried to simply bring a previously-generated version of filesvn out into the exported directory, but Make still insists on verifying filesvn's dependency.

Simply deleting the dependency of .svn/entries does work, of course, but then the spirit of the rule is broken since watching for a revision update is the goal.

Is there a way to get Make to not care that the .svn/entries file is not there?

The merits of such a technique are not really part of the question. I can't fundamentally change this, but if there is a change that maintains the spirit that might work. An answer of "You can't" is perfectly valid of course. :)

回答1:

You can add a target that generates copy of actual makefile with changes that drop the svn dependency, run make with it and then deletes it.

回答2:

You can use the wildcard function to ignore the file unless it exists:

filesvn: $(wildcard .svn/entries)

Aside: Subversion 1.7 changes the format of the local working copy, so this rule would stop working even in working copies.

回答3:

Pattern % rules

For pattern rules, you need .SECONDEXPANSION:

.SECONDEXPANSION:
%.a: %.b $$(wildcard $$*.c)
    cc $< -o $@

See also: How can I make a pattern rule dependency optional in a Makefile?

Tested in Make 4.1.