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Chapter 9: How to Run make 103

3. Use the command ‘make -t ’ to mark all the object ﬁles as up to date. The next time

you run make, the changes in the header ﬁles will not cause any recompilation.

If you have already changed the header ﬁle at a time when some ﬁles do need recom-

pilation, it is too late to do this. Instead, you can use the ‘-o file’ ﬂag, which marks a

speciﬁed ﬁle as “old” (see Section 9.7 [Summary of Options], page 104). This means that

the ﬁle itself will not be remade, and nothing else will be remade on its account. Follow

this procedure:

1. Recompile the source ﬁles that need compilation for reasons independent of the partic-

ular header ﬁle, with ‘make -o headerfile’. If several header ﬁles are involved, use a

separate ‘-o’ option for each header ﬁle.

2. Touch all the object ﬁles with ‘make -t’.

9.5 Overriding Variables

An argument that contains ‘=’ speciﬁes the value of a variable: ‘v=x’ sets the value of the

variable v to x. If you specify a value in this way, all ordinary assignments of the same

variable in the makeﬁle are ignored; we say they have been overridden by the command line

argument.

The most common way to use this facility is to pass extra ﬂags to compilers. For example,

in a properly written makeﬁle, the variable CFLAGS is included in each recipe that runs the

C compiler, so a ﬁle foo.c would be compiled something like this:

cc -c $(CFLAGS) foo.c

Thus, whatever value you set for CFLAGS aﬀects each compilation that occurs. The

makeﬁle probably speciﬁes the usual value for CFLAGS, like this:

CFLAGS=-g

Each time you run make, you can override this value if you wish. For example, if you

say ‘make CFLAGS=’-g -O’’, each C compilation will be done with ‘cc -c -g -O’. (This also

illustrates how you can use quoting in the shell to enclose spaces and other special characters

in the value of a variable when you override it.)

The variable CFLAGS is only one of many standard variables that exist just so that you

can change them this way. See Section 10.3 [Variables Used by Implicit Rules], page 115,

for a complete list.

You can also program the makeﬁle to look at additional variables of your own, giving

the user the ability to control other aspects of how the makeﬁle works by changing the

variables.

When you override a variable with a command line argument, you can deﬁne either a

recursively-expanded variable or a simply-expanded variable. The examples shown above

make a recursively-expanded variable; to make a simply-expanded variable, write ‘:=’ or

‘::=’ instead of ‘=’. But, unless you want to include a variable reference or function call in

the value that you specify, it makes no diﬀerence which kind of variable you create.

There is one way that the makeﬁle can change a variable that you have overridden. This

is to use the override directive, which is a line that looks like this: ‘override variable =

value’ (see Section 6.7 [The override Directive], page 68).

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Roland Ver. 4.5 Technical Information Seite 113