Make

make is an utility that determines automatically which pieces of a program need to be recompiled, and issues the commands to rebuild them. make uses a file called Makefile, which describes the relationships among files in the program.

References

• GNU 'Make'
  • Manual

Quick Reference

Automatic Variables

Make Automatic Variables (most frequent)

$@	Target of the rule (if multi target, the one that cause the rule's command to run)
$%	Target member name, when target is an archive member
$<	Name of 1st prerequisite
$?	All newer prerequisites (more recent than the target)
$^	All prerequisites
$+	All prerequisites, incl. duplicates
$*	The stem with which the rule matched (e.g. if target is dir/a.foo.b, a pattern like a.%.b gives a stem dir/foo)

Automatic Variables - F and D variants:

$(@D)	$(@F)	Resp. the directory (w/o trailing /) and filename part of the target
$(*D)	$(*F)	Resp. the directory (w/o trailing /) and filename part of the stem
$(<D)	$(<F)	Resp. the directory (w/o trailing /) and filename part of the 1st prerequisite
$(^D)	$(^F)	Resp. the directory (w/o trailing /) and filename part of the all prerequisite
$(+D)	$(+F)	Resp. the directory (w/o trailing /) and filename part of the all prerequisites, incl. duplicates
$(?D)	$(?F)	Resp. the directory (w/o trailing /) and filename part of the newer prerequisites

How to Use Variables

Recursive vs. simple expansion

# Recursive expansion
CFLAGS = $(INCLUDES) -O
INCLUDES = -Ifoo -Ibar

all:
	echo $(CFLAGS)           # will echo '-Ifoo -Ibar -O'

# Simple expansion
x := foo
y := $(x) bar                    # expansion occurs here, once for all (y = foo bar)
x := later                       # y still = foo bar

Substitution References

foo = a.o b.o c.o
bar = $(foo:.o=.c)               # bar = a.c b.c c.c

Using % (equiv. to $(patsubst a,b,$(var)):

foo = a.o b.o c.o
bar = $(foo:%.o=%.c)             # bar = a.c b.c c.c

Tips

Automatic Prerequisites

A standard solution (inspired from Makefile manual):

#----- BEGIN Automatic-Prerequisites -----
# - see http://www.gnu.org/software/make/manual/make.html#Automatic-Prerequisites
# - Note that .d files are automatically regenerated because of the include command below.
# - Only 1 .d file generated, indep. of defines. So if include files depend on defines, the .d files will *NOT* be updated.

-include $(SOURCES:.cpp=.d)

%.d: %.cpp
	$(CPP) $(CFLAGS) $(INCPATHS) -MM -MT $*.o -MT $*.d -MF $@ $<
#----- END Automatic-Prerequisites -----

A more advanced solution, where object files are stored in directory $(VARDIR), and have a suffic $(VARIANT):

#----- BEGIN Automatic-Prerequisites -----
# - see http://www.gnu.org/software/make/manual/make.html#Automatic-Prerequisites
# - Note that .d files are automatically regenerated because of the include command below.
# - Only 1 .d file generated, indep. of defines. So if include files depend on defines, the .d files will *NOT* be updated.
# - [CHANGES] .o and .d files are stored in directory $(VARDIR)/, and .o have suffix $(VARIANT)

-include $(addprefix $(VARDIR)/, $(notdir $(SOURCES:.cpp=.d) ) )

$(VARDIR)/%.d: $(SRCDIR)/%.cpp
	$(CPP) $(CFLAGS) $(INCPATHS) -MM -MT $(VARDIR)/$*$$"(VARIANT)".o -MT $*.d -MF $@ $<
#----- END Automatic-Prerequisites -----

Limitations:

• Only one .d file generated, indep. of $(DEFINES), so if $(DEFINES) changed, and include files depends on these defines, the .d files will not be updated.

A solution would be to name the .d files according to relevant defines, as we do for the .o files

Frequent Mistakes

Use TABS to prefix commands in rules, *NOT* SPACES

Each rule in a makefile gives the set of commands that must be executed to build a given target from a given set of dependencies. These commands MUST be indented with a TAB character, not with SPACES. So make sure that the editor does not automatically change these tabs into spaces.