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Chapter 8: Functions for Transforming Text 83

8 Functions for Transforming Text

Functions allow you to do text processing in the makeﬁle to compute the ﬁles to operate

on or the commands to use in recipes. You use a function in a function call, where you

give the name of the function and some text (the arguments) for the function to operate

on. The result of the function’s processing is substituted into the makeﬁle at the point of

the call, just as a variable might be substituted.

8.1 Function Call Syntax

A function call resembles a variable reference. It can appear anywhere a variable reference

can appear, and it is expanded using the same rules as variable references. A function call

looks like this:

$(function arguments)

or like this:

${function arguments}

Here function is a function name; one of a short list of names that are part of make. You

can also essentially create your own functions by using the call built-in function.

The arguments are the arguments of the function. They are separated from the function

name by one or more spaces or tabs, and if there is more than one argument, then they are

separated by commas. Such whitespace and commas are not part of an argument’s value.

The delimiters which you use to surround the function call, whether parentheses or braces,

can appear in an argument only in matching pairs; the other kind of delimiters may appear

singly. If the arguments themselves contain other function calls or variable references, it is

wisest to use the same kind of delimiters for all the references; write ‘$(subst a,b,$(x))’,

not ‘$(subst a,b,${x})’. This is because it is clearer, and because only one type of delim-

iter is matched to ﬁnd the end of the reference.

The text written for each argument is processed by substitution of variables and function

calls to produce the argument value, which is the text on which the function acts. The

substitution is done in the order in which the arguments appear.

Commas and unmatched parentheses or braces cannot appear in the text of an argument

as written; leading spaces cannot appear in the text of the ﬁrst argument as written. These

characters can be put into the argument value by variable substitution. First deﬁne variables

comma and space whose values are isolated comma and space characters, then substitute

these variables where such characters are wanted, like this:

comma:= ,

empty:=

space:= $(empty) $(empty)

foo:= a b c

bar:= $(subst $(space),$(comma),$(foo))

# bar is now ‘a,b,c’.

Here the subst function replaces each space with a comma, through the value of foo, and

substitutes the result.

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