make: Syntax of Functions
8.1 Function Call Syntax
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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.
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.