$ make DEBUG=1
g++ -c -DDEBUG=1 -o foo.o foo.c
$ make DEBUG=1
make: Nothing to be done for `all'.
$ make
g++ -c -DDEBUG= -o foo.o foo.c
$ make CPPFLAGS+=-DFOO=foo
g++ -DFOO=foo -c -DDEBUG= -o foo.o foo.c
g++ -DFOO=foo -c -o bar.o bar.c
Of course, changing a variable that isn't referenced doesn't cause anything to be updated. Here's an example starting from a clean build and then redefining SOMEVAR.
$ make
g++ -c -DDEBUG= -o foo.o foo.c
g++ -c -o bar.o bar.c
$ make SOMEVAR=42
make: Nothing to be done for `all'.
To understand how this works the first place to look is inside a .sig file. The .sig files are automatically generated by signature for each rule that uses the $(call do,...) (the details of how are later on).
Here, for example, is the contents of the foo.sig file after the first clean build was run:
$(eval @ := foo.o)
$(eval % := )
$(eval < := foo.c)
$(eval ? := foo.force)
$(eval ^ := foo.c foo.force)
$(eval + := foo.c foo.force)
$(eval * := foo)
foo.o: foo.force
$(if $(call sne,$(COMPILE.C) -DDEBUG=$(DEBUG) -o $@ $<,g++ -c -DDEBUG= -o foo.o foo.c),$(shell touch foo.force))
Next comes the line foo.o: foo.force. This says that foo.o must be rebuilt is foo.force is newer. It's this line that causes foo.o to get rebuilt when the commands change, and it's the next line that touches foo.force if the commands have changed.
The long $(if ...) statement uses the GMSL (see http://gmsl.sf.net/) sne (string not equal) to compare the current commands for foo.o (by expanding them) against their value the last time they were expanded. If the commands have changed then $(shell touch foo.force) is called.
Since the .sig files are processed when the Makefile is being parsed (they are just Makefile's themselves read using include), all the .force files will have been updated before any rules run. And so this small .sig file does all the work of forcing an object file to rebuild when the commands change.
The .sig files themselves are created by signature:
include gmsl
last_target :=
dump_var = \$$(eval $1 := $($1))
define new_rule
@echo "$(call map,dump_var,@ % < ? ^ + *)" > $S
@$(if $(wildcard $F),,touch $F)
@echo $@: $F >> $S
endef
define do
$(eval S := $*.sig)$(eval F := $*.force)$(eval C := $1)
$(if $(call sne,$@,$(last_target)),$(call new_rule),$(eval last_target := $@))
@echo "$(subst $$,\$$,$$(if $$(call sne,$1,$C),$$(shell touch $F)))" >> $S
$C
endef
About the author
John Graham-Cumming is Co-Founder at Electric Cloud, Inc . Prior to joining Electric Cloud, John was a Venture Consultant with Accel Partners, VP of Internet Technology at Interwoven, Inc. (IWOV), VP of Engineering at Scriptics Corporation (acquired by Interwoven), and Chief Architect at Optimal Networks, Inc. John holds BA and MA degrees in Mathematics and Computation and a Doctorate in Computer Security from Oxford University. John is the creator of the highly acclaimed open source POPFile project. He also holds two patents in network analysis and has others pending.
