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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.