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 delimiter is matched to find the end of the reference.

Each argument is expanded before the function is invoked, unless otherwise noted below. The substitution is done in the order in which the arguments appear.

Special Characters

When using characters that are special to make as function arguments, you may need to hide them. GNU make doesn’t support escaping characters with backslashes or other escape sequences; however, because arguments are split before they are expanded you can hide them by putting them into variables.

Characters you may need to hide include:

• Commas
• Initial whitespace in the first argument
• Unmatched open parenthesis or brace
• An open parenthesis or brace if you don’t want it to start a matched pair

For example, you can 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.