Roland Ver. 4.5 Informações Técnicas descarregar pdf (Página 49)

Chapter 4: Writing Rules 39

main.o : main.c defs.h

Thus you no longer have to write all those rules yourself. The compiler will do it for you.

Note that such a rule constitutes mentioning main.o in a makeﬁle, so it can never be

considered an intermediate ﬁle by implicit rule search. This means that make won’t ever

remove the ﬁle after using it; see Section 10.4 [Chains of Implicit Rules], page 117.

With old make programs, it was traditional practice to use this compiler feature to

generate prerequisites on demand with a command like ‘make depend’. That command

would create a ﬁle depend containing all the automatically-generated prerequisites; then

the makeﬁle could use include to read them in (see Section 3.3 [Include], page 13).

In GNU make, the feature of remaking makeﬁles makes this practice obsolete—you need

never tell make explicitly to regenerate the prerequisites, because it always regenerates any

makeﬁle that is out of date. See Section 3.5 [Remaking Makeﬁles], page 14.

The practice we recommend for automatic prerequisite generation is to have one makeﬁle

corresponding to each source ﬁle. For each source ﬁle name.c there is a makeﬁle name.d

which lists what ﬁles the object ﬁle name.o depends on. That way only the source ﬁles that

have changed need to be rescanned to produce the new prerequisites.

Here is the pattern rule to generate a ﬁle of prerequisites (i.e., a makeﬁle) called name.d

from a C source ﬁle called name.c:

%.d: %.c

@set -e; rm -f $@; \

$(CC) -M $(CPPFLAGS) $< > $@.$$$$; \

sed ’s,$$*$\.o[ :]*,\1.o $@ : ,g’ < $@.$$$$ > $@; \

rm -f $@.$$$$

See Section 10.5 [Pattern Rules], page 118, for information on deﬁning pattern rules. The

‘-e’ ﬂag to the shell causes it to exit immediately if the $(CC) command (or any other

command) fails (exits with a nonzero status).

With the GNU C compiler, you may wish to use the ‘-MM’ ﬂag instead of ‘-M’. This omits

prerequisites on system header ﬁles. See Section “Options Controlling the Preprocessor” in

Using GNU CC, for details.

The purpose of the sed command is to translate (for example):

main.o : main.c defs.h

into:

main.o main.d : main.c defs.h

This makes each ‘.d’ ﬁle depend on all the source and header ﬁles that the corresponding

‘.o’ ﬁle depends on. make then knows it must regenerate the prerequisites whenever any of

the source or header ﬁles changes.

Once you’ve deﬁned the rule to remake the ‘.d’ ﬁles, you then use the include directive

to read them all in. See Section 3.3 [Include], page 13. For example:

sources = foo.c bar.c

include $(sources:.c=.d)

(This example uses a substitution variable reference to translate the list of source ﬁles ‘foo.c

bar.c’ into a list of prerequisite makeﬁles, ‘foo.d bar.d’. See Section 6.3.1 [Substitution

Refs], page 62, for full information on substitution references.) Since the ‘.d’ ﬁles are

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