If the target of a rule does not exist or the timestamp of the target is older
than any of the prerequisites, the target is out of date. My question is that
if some files of prerequisites does not exist, how can we compare the timestamp.
For example:
foo : boo
@echo foo
boo :
@echo boo
Then if foo already exists, but the rule for "boo" does not create the file
boo, so how compare the timestamp of foo and boo.
When I run the makefile, the console prints :
boo
foo
I want to know why the make parses the makefile like this.
A more general problem is that how the make treat files which doesn't exist and
can't be created by explicit rules and implicit rules.
Thank you for your answer!
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