GNU Make: details regarding intermediate files

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.