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Chapter 8: Functions for Transforming Text 93

8.8 The value Function

The value function provides a way for you to use the value of a variable without having it

expanded. Please note that this does not undo expansions which have already occurred; for

example if you create a simply expanded variable its value is expanded during the deﬁnition;

in that case the value function will return the same result as using the variable directly.

The syntax of the value function is:

$(value variable)

Note that variable is the name of a variable, not a reference to that variable. Therefore

you would not normally use a ‘$’ or parentheses when writing it. (You can, however, use a

variable reference in the name if you want the name not to be a constant.)

The result of this function is a string containing the value of variable, without any

expansion occurring. For example, in this makeﬁle:

FOO = $PATH

all:

@echo $(FOO)

@echo $(value FOO)

The ﬁrst output line would be ATH, since the “$P” would be expanded as a make variable,

while the second output line would be the current value of your $PATH environment variable,

since the value function avoided the expansion.

The value function is most often used in conjunction with the eval function (see

Section 8.9 [Eval Function], page 93).

8.9 The eval Function

The eval function is very special: it allows you to deﬁne new makeﬁle constructs that

are not constant; which are the result of evaluating other variables and functions. The

argument to the eval function is expanded, then the results of that expansion are parsed

as makeﬁle syntax. The expanded results can deﬁne new make variables, targets, implicit

or explicit rules, etc.

The result of the eval function is always the empty string; thus, it can be placed virtually

anywhere in a makeﬁle without causing syntax errors.

It’s important to realize that the eval argument is expanded twice; ﬁrst by the eval

function, then the results of that expansion are expanded again when they are parsed

as makeﬁle syntax. This means you may need to provide extra levels of escaping for “$”

characters when using eval. The value function (see Section 8.8 [Value Function], page 93)

can sometimes be useful in these situations, to circumvent unwanted expansions.

Here is an example of how eval can be used; this example combines a number of concepts

and other functions. Although it might seem overly complex to use eval in this example,

rather than just writing out the rules, consider two things: ﬁrst, the template deﬁnition (in

PROGRAM_template) could need to be much more complex than it is here; and second, you

might put the complex, “generic” part of this example into another makeﬁle, then include

it in all the individual makeﬁles. Now your individual makeﬁles are quite straightforward.

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Roland Ver. 4.5 Informações Técnicas Página 103