How do I get GNU make to remove intermediate directories created by implicit rules?

Question

GNU make automatically removes intermediate files created by implicit rules, by calling rm filename at the end. This obviously doesn't work if one of the targets was actually a directory. Take the following example:

.PHONY: all 

all: test.target

%.target: tempdir.% 
  touch $@

tempdir.%:
  mkdir -p $@

make -n reveals the action plan:

mkdir -p tempdir.test
touch test.target
rm tempdir.test

Is it possible to get GNU make to correctly dispose of intermediate directories? Perhaps by changing rm to rm -rf?

Answer 1

There is no way to make this happen. Although GNU make prints the command "rm", really internally it's running the unlink(2) system call directly and not invoking a shell command. There is no way to configure or modify the command that GNU make runs (except by changing the source code of course).

However, I feel I should point out that it's just not going to work to use a directory as a normal prerequisite of a target. GNU make uses time-last-modified comparison to tell when targets are up to date or not, and the time-last-modified of a directory does not follow the standard rules. The TLM of a directory is updated every time a file (or subdirectory) in that directory is created, deleted, or renamed. This means you will created the directory, then have a bunch of files that depend on it: the first one is built and has timestamp N. The last one is built and has timestamp N+x. That also sets the directory's timestamp to N+x. Then the next time you run make, it will notice that the first one has an older timestamp (N) than one of its prerequisites (the directory, at N+x), and rebuild.

And this will happen forever, until it can build the remaining "out of date" prerequisites fast enough that their timestamp is not newer than the directory.

And, if you were to drop a temporary file or editor backup file or something in that directory, it would start all over again.

Just don't do it.

Some people use an explicit shell command to create directories. Some people create them as a side-effect of the target creation. Some people use order-only prerequisites to ensure they're created on time.