Using make's Predefined Macros

The next example performs exactly the same function, but demonstrates the use of make's predefined macros for the indicated compilation commands. Using predefined macros eliminates the need to edit makefiles when the underlying compilation environment changes. Macros also provide access to the CFLAGS macro (and other FLAGS macros) for supplying compiler options from the command line. Predefined macros are also used extensively within make's implicit rules. The predefined macros in the following makefile are listed below. [Predefined macros are used more extensively than in earlier versions of make. Not all of the predefined macros shown here are available with earlier versions. ] They are generally useful for compiling C programs.

COMPILE.C

The cc command line; composed of the values of CC, CFLAGS, and CPPFLAGS, as follows, along with the -c option.

COMPILE.c=$(CC) $(CFLAGS) $(CPPFLAGS) -c

The root of the macro name, COMPILE, is a convention used to indicate that the macro stands for a compilation command line (to generate an object, or .o file). The .c suffix is a mnemonic device to indicate that the command line applies to .c (C source) files.

Macro names that end in the string FLAGS pass options to a related compiler-command macro. It is good practice to use these macros for consistency and portability. It is also good practice to note the desired default values for them in the makefile. The complete list of all predefined macros is shown in Table 4-9.

LINK.c

The basic cc command line to link object files, such as COMPILE.c, but without the -c option and with a reference to the LDFLAGS macro:

LINK.c=$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS)

CC

The value cc. (You can redefine the value to be the path name of an alternate C compiler.)

CFLAGS

Options for the cc command; none by default.

CPPFLAGS

Options for cpp; none by default.

LDFLAGS

Options for the link editor, ld; none by default.

# Makefile for compiling two C sources 
CFLAGS= -O 
.KEEP_STATE:

functions: main.o data.o 
          $(LINK.c) -o functions main.o data.o 
main.o: main.c 
          $(COMPILE.c) main.c 
data.o: data.c 
          $(COMPILE.c) data.c 
clean: 
         	rm functions main.o data.o