Check this out!
Suppose I have this Makefile
:
a: b touch $@ b: touch $@ # A common chain of build steps %-GENERATED.c: %-generate touch $@ %.o: %.c touch $@ %.so: %-GENERATED.o touch $@ xxx-GENERATED.o: CFLAGS += adsf # Imitates .d files created with "gcc -MMD". Does not exist on the initial build ifneq ($(wildcard xxx.so),) xxx-GENERATED.o: xxx-GENERATED.c endif
This is all very simple build-system stuff. Let's see how it works:
$ rm -rf a b xxx-GENERATED.c xxx-GENERATED.o xxx.so [start from a clean slate] $ touch xxx-generate xxx.h [Files that would be available in a project exist; xxx-generate is some tool] [that would generate xxx-GENERATED.c ] $ touch a ["a" exists but the file "b" it depends on does not] $ make a xxx.so touch b touch a touch xxx-GENERATED.c touch xxx-GENERATED.o touch xxx.so rm xxx-GENERATED.c [It built everything, but then deleted xxx-GENERATED.c] $ make a xxx.so remake: 'a' is up to date. touch xxx-GENERATED.c touch xxx-GENERATED.o touch xxx.so [It knew to not rebuild "a", but the missing xxx-GENERATED.c caused it to] [re-build stuff ]
Well that's not good. What if we add .SECONDARY:
to the end of the Makefile
to mark everything as a secondary file?
$ rm -rf a b xxx-GENERATED.c xxx-GENERATED.o xxx.so $ touch xxx-generate xxx.h $ touch a $ make a xxx.so remake: 'a' is up to date. touch xxx-GENERATED.c touch xxx-GENERATED.o touch xxx.so [It didn't bother rebuilding "a" even though its prerequisites "b" doesn't] [exist. But it didn't delete the xxx-GENERATED.c at least ] $ make a xxx.so remake: 'a' is up to date. remake: 'xxx.so' is up to date. [It knew to not rebuild anything. Great.]
So it doesn't work right with or without .SECONDARY:
, but it's much closer
with it. The solution is to mark everything as not an intermediate file.
mrbuild cannot do this without a bleeding-edge version of GNU Make
, but users
of mrbuild can do this by explicitly mentioning specific files in rules. This
would suffice:
___dummy___: file1 file2
Detailed notes are in a commit in mrbuild (mrbuild 1.13) and in a post to LKML by Masahiro Yamada.