Re: make: existence-only prerequisites

["J. Lewis Muir" <jlmuir%imca-cat.org@localhost>]

On 02/14, Edgar Fuß wrote:
> TRC> and the traditional approach is to use a dummy file in the directory:
> TRC> 
> TRC> foo/.stamp:
> TRC> 	touch foo/.stamp
> Which a) creates a file you may have to ignore afterwards and b) updates the 
> directory's time stamp causing gratuitous command execution later.

Hello, Edgar!

True about point a.  I'm not sure about point b, though.  What is the
scenario you're thinking of that causes gratuitous command execution
later?

The use case I've come across is that there are targets to be created
inside a directory, and the directory needs to exist before creating the
targets in the directory, so the targets need to have the directory as a
prerequisite.

For example, here the target build/buffer.o is created inside the build
directory:

  build/.dirstamp
  	mkdir -p build
  	touch build/.dirstamp

  build/buffer.o: build/.dirstamp buffer.c
  	cc -c buffer.c -o build/buffer.o

The build directory's timestamp is only changed once: the build
directory is obviously first created with a timestamp, and then the
.dirstamp file is created inside it thus changing the build directory's
timestamp.  Since the build/buffer.o target has build/.dirstamp as
a prerequisite, the build directory gets created before executing
the build/buffer.o target commands, but that directory creation and
.dirstamp file creation only happen once.  Subsequent invocations of
make will not cause the timestamp of the build directory to change
(unless, of course, a target is created or updated in it).

Regards,

Lewis