4.3 Including other files

It is cool to have main.d and array.d. But they are only informational up to this point. We can, indeed, include them in the Makefile:

CC = gcc # g++ for c++ programs
CXXFLAGS = -O -Wall -g
LDFLAGS = -g
SRC = main.c array.c
EXE = main
  
$(EXE):  $(SRC:.c=.o)

#array.o:  array.c array.h
#	gcc $(CXXFLAGS) -c array.c

#main.o:  main.c array.h
#	gcc $(CXXFLAGS) -c main.c
  
%.d: %.c
	 set -e; gcc $(CXXFLAGS) $^ -MM \
	| sed 's/\($*\)\.o[ :]*/\1.o $@ : /g' > $@; \
	[ -s $@ ] || rm -f $@ 
  
clean:
	rm -f $(EXE) $(CSRC:.c=.o) $(CSRC:.c=.d) 

include $(SRC:.c=.d)

Here, the statement ``include $(SRC=.c=.d) is saying for each source file listed in SRC, include its .d (dependency) file. Because these files already specify that array.o depends on array.c and array.h, there is no need to explicitly state that rule anymore. The # symbol states that whatever follows on the same line should be ignored.

Now, go ahead and update array.h, then run make again.

We only have one problem now: cc is used as our compiler, not gcc. This is because the rules included in main.d and array.d only state the dependency, but not the command to generate the .o files. This leaves everything to the default settings, which uses cc to compile C programs. On most systems, cc is a ``shortcut'' (symlink) to gcc.

Note that the variable CC specifies the compiler to use. By default, the program cc is used. You can change the definition of CC to gcc for C programs, or g++ for C++ programs. You can even specify a particular version of compiler, such as gcc-3.1.

The variable LDFLAGS is similar to to CXXFLAGS, but instead of specifying compile-time flags, it specifies link time flags. Here we only specify -g to include debug information in the executable.

The variables CC, CXXFLAGS and LDFLAGS are special because they are used by the default Makefile rules. You need to use these specific names if implicit rules (like those contained in .d files) are used.

The special target clean is used if you want to delete all the created files.