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68 GNU make

The ﬁrst line deﬁnes the CFLAGS variable with a reference to another variable, includes.

(CFLAGS is used by the rules for C compilation; see Section 10.2 [Catalogue of Built-In

Rules], page 112.) Using ‘=’ for the deﬁnition makes CFLAGS a recursively-expanded variable,

meaning ‘$(includes) -O’ is not expanded when make processes the deﬁnition of CFLAGS.

Thus, includes need not be deﬁned yet for its value to take eﬀect. It only has to be deﬁned

before any reference to CFLAGS. If we tried to append to the value of CFLAGS without using

‘+=’, we might do it like this:

CFLAGS := $(CFLAGS) -pg # enable profiling

This is pretty close, but not quite what we want. Using ‘:=’ redeﬁnes CFLAGS as a simply-

expanded variable; this means make expands the text ‘$(CFLAGS) -pg’ before setting the

variable. If includes is not yet deﬁned, we get ‘ -O -pg’, and a later deﬁnition of includes

will have no eﬀect. Conversely, by using ‘+=’ we set CFLAGS to the unexpanded value

‘$(includes) -O -pg’. Thus we preserve the reference to includes, so if that variable gets

deﬁned at any later point, a reference like ‘$(CFLAGS)’ still uses its value.

6.7 The override Directive

If a variable has been set with a command argument (see Section 9.5 [Overriding Variables],

page 103), then ordinary assignments in the makeﬁle are ignored. If you want to set the

variable in the makeﬁle even though it was set with a command argument, you can use an

override directive, which is a line that looks like this:

override variable = value

override variable := value

To append more text to a variable deﬁned on the command line, use:

override variable += more text

See Section 6.6 [Appending More Text to Variables], page 66.

Variable assignments marked with the override ﬂag have a higher priority than all other

assignments, except another override. Subsequent assignments or appends to this variable

which are not marked override will be ignored.

The override directive was not invented for escalation in the war between makeﬁles

and command arguments. It was invented so you can alter and add to values that the user

speciﬁes with command arguments.

For example, suppose you always want the ‘-g’ switch when you run the C compiler, but

you would like to allow the user to specify the other switches with a command argument

just as usual. You could use this override directive:

override CFLAGS += -g

You can also use override directives with define directives. This is done as you might

expect:

override define foo =

bar

endef

See the next section for information about define.

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Roland Ver. 4.5 Technical Information Seite 78