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Chapter 15: Makeﬁle Conventions 151

15.3 Variables for Specifying Commands

Makeﬁles should provide variables for overriding certain commands, options, and so on.

In particular, you should run most utility programs via variables. Thus, if you use Bison,

have a variable named BISON whose default value is set with ‘BISON = bison’, and refer to

it with $(BISON) whenever you need to use Bison.

File management utilities such as ln, rm, mv, and so on, need not be referred to through

variables in this way, since users don’t need to replace them with other programs.

Each program-name variable should come with an options variable that is used to supply

options to the program. Append ‘FLAGS’ to the program-name variable name to get the

options variable name—for example, BISONFLAGS. (The names CFLAGS for the C compiler,

YFLAGS for yacc, and LFLAGS for lex, are exceptions to this rule, but we keep them because

they are standard.) Use CPPFLAGS in any compilation command that runs the preprocessor,

and use LDFLAGS in any compilation command that does linking as well as in any direct use

of ld.

If there are C compiler options that must be used for proper compilation of certain ﬁles,

do not include them in CFLAGS. Users expect to be able to specify CFLAGS freely themselves.

Instead, arrange to pass the necessary options to the C compiler independently of CFLAGS,

by writing them explicitly in the compilation commands or by deﬁning an implicit rule, like

this:

CFLAGS = -g

ALL_CFLAGS = -I. $(CFLAGS)

.c.o:

$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) $<

Do include the ‘-g’ option in CFLAGS, because that is not required for proper compilation.

You can consider it a default that is only recommended. If the package is set up so that it

is compiled with GCC by default, then you might as well include ‘-O’ in the default value

of CFLAGS as well.

Put CFLAGS last in the compilation command, after other variables containing compiler

options, so the user can use CFLAGS to override the others.

CFLAGS should be used in every invocation of the C compiler, both those which do

compilation and those which do linking.

Every Makeﬁle should deﬁne the variable INSTALL, which is the basic command for

installing a ﬁle into the system.

Every Makeﬁle should also deﬁne the variables INSTALL_PROGRAM and INSTALL_DATA.

(The default for INSTALL_PROGRAM should be $(INSTALL); the default for INSTALL_DATA

should be ${INSTALL} -m 644.) Then it should use those variables as the commands for

actual installation, for executables and non-executables respectively. Minimal use of these

variables is as follows:

$(INSTALL_PROGRAM) foo $(bindir)/foo

$(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a

However, it is preferable to support a DESTDIR preﬁx on the target ﬁles, as explained in

the next section.

It is acceptable, but not required, to install multiple ﬁles in one command, with the ﬁnal

argument being a directory, as in:

$(INSTALL_PROGRAM) foo bar baz $(bindir)

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