16.3 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)