timestamp

Make uses the timestamp of a target to determine if it needs to be rebuilt or not. Let’s consider this scenario :

a.o: a.c a.h
    gcc -c $@ -o $<

libproject.a : a.o b.o c.o
    ar rcs $@ $^

if you add a comment to a.h, a.o gets rebuild, but because you only added a comment, a.o is unchanged after the rebuild. Its timestamp changes though, and this will trigger the useless link of libproject.a, and all other artefacts down the build path.

If you have a code generator, this is even worse, as many files get involved.

Yamake solves this issue by considering the digest of the files, and not their timestamp. In our example, as a.o has same digest after the build, the rebuild is not propagated.