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70 GNU make

not. However, if you are using the flavor (see Section 8.11 [Flavor Function], page 95) and

origin (see Section 8.10 [Origin Function], page 94) functions, there is a diﬀerence between

a variable that was never set and a variable with an empty value. In such situations you

may want to use the undefine directive to make a variable appear as if it was never set.

For example:

foo := foo

bar = bar

undefine foo

undefine bar

$(info $(origin foo))

$(info $(flavor bar))

This example will print “undeﬁned” for both variables.

If you want to undeﬁne a command-line variable deﬁnition, you can use the override

directive together with undefine, similar to how this is done for variable deﬁnitions:

override undefine CFLAGS

6.10 Variables from the Environment

Variables in make can come from the environment in which make is run. Every environment

variable that make sees when it starts up is transformed into a make variable with the same

name and value. However, an explicit assignment in the makeﬁle, or with a command

argument, overrides the environment. (If the ‘-e’ ﬂag is speciﬁed, then values from the

environment override assignments in the makeﬁle. See Section 9.7 [Summary of Options],

page 104. But this is not recommended practice.)

Thus, by setting the variable CFLAGS in your environment, you can cause all C compi-

lations in most makeﬁles to use the compiler switches you prefer. This is safe for variables

with standard or conventional meanings because you know that no makeﬁle will use them

for other things. (Note this is not totally reliable; some makeﬁles set CFLAGS explicitly and

therefore are not aﬀected by the value in the environment.)

When make runs a recipe, variables deﬁned in the makeﬁle are placed into the environ-

ment of each shell. This allows you to pass values to sub-make invocations (see Section 5.7

[Recursive Use of make], page 50). By default, only variables that came from the environ-

ment or the command line are passed to recursive invocations. You can use the export

directive to pass other variables. See Section 5.7.2 [Communicating Variables to a Sub-

make], page 52, for full details.

Other use of variables from the environment is not recommended. It is not wise for

makeﬁles to depend for their functioning on environment variables set up outside their

control, since this would cause diﬀerent users to get diﬀerent results from the same makeﬁle.

This is against the whole purpose of most makeﬁles.

Such problems would be especially likely with the variable SHELL, which is normally

present in the environment to specify the user’s choice of interactive shell. It would be very

undesirable for this choice to aﬀect make; so, make handles the SHELL environment variable

in a special way; see Section 5.3.2 [Choosing the Shell], page 45.

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