The commands in built-in implicit rules make liberal use of certain
predefined variables. You can alter the values of these variables in
the makefile, with arguments to
make, or in the environment to
alter how the implicit rules work without redefining the rules
themselves. You can cancel all variables used by implicit rules with
the ‘-R’ or ‘--no-builtin-variables’ option.
For example, the command used to compile a C source file actually says ‘$(CC) -c $(CFLAGS) $(CPPFLAGS)’. The default values of the variables used are ‘cc’ and nothing, resulting in the command ‘cc -c’. By redefining ‘CC’ to ‘ncc’, you could cause ‘ncc’ to be used for all C compilations performed by the implicit rule. By redefining ‘CFLAGS’ to be ‘-g’, you could pass the ‘-g’ option to each compilation. All implicit rules that do C compilation use ‘$(CC)’ to get the program name for the compiler and all include ‘$(CFLAGS)’ among the arguments given to the compiler.
The variables used in implicit rules fall into two classes: those that are
names of programs (like
CC) and those that contain arguments for the
CFLAGS). (The “name of a program” may also contain
some command arguments, but it must start with an actual executable program
name.) If a variable value contains more than one argument, separate them
The following tables describe of some of the more commonly-used predefined
variables. This list is not exhaustive, and the default values shown here may
not be what are selected by
make for your environment. To see the
complete list of predefined variables for your instance of GNU
can run ‘make -p’ in a directory with no makefiles.
Here is a table of some of the more common variables used as names of programs in built-in rules: makefiles.
Here is a table of variables whose values are additional arguments for the programs above. The default values for all of these is the empty string, unless otherwise noted.