Makefile target dependency to generated files

问题:

I have a question regarding the behavior of Make when running targets that are dependent on generated files.

Given the source tree and Makefile below, when I run this it takes two runs to complete the "build" even though everything was generated on the first run.

$ ls -R
.:
bar  foo  Makefile

Makefile

all: foobar

work:
    mkdir -p work

work/foo: work foo
    cp foo work/foo

work/bar: work bar
    cp bar work/bar

foobar: work/foo work/bar

make

$ make
mkdir -p work
cp foo work/foo
cp bar work/bar
$ ls -R
.:
work/  bar  CMakeLists.txt  foo  Makefile

./work:
bar  foo
$ make
cp foo work/foo
$ make
make: Nothing to be done for 'all'.

Why does this happen?

回答1:

You need to make the directory an order-only prerequisite, otherwise the targets will be remade each time the directory changes; during the second invocation work is newer than work/foo because work/bar was created after work/foo, so work's timestamp is newer than work/foo.

work/foo: foo | work
    cp foo work/foo

work/bar: bar | work
    cp bar work/bar

Or more concisely

work/foo work/bar: work/%: % | work
    cp $< $@