Calling a function means causing the function to run and do its job. A function call is an expression and its value is the value returned by the function.
A function call consists of the function name followed by the arguments
in parentheses. awk expressions are what you write in the
call for the arguments. Each time the call is executed, these
expressions are evaluated, and the values become the actual arguments. For
example, here is a call to
foo() with three arguments (the first
being a string concatenation):
foo(x y, "lose", 4 * z)
CAUTION: Whitespace characters (spaces and TABs) are not allowed between the function name and the open-parenthesis of the argument list. If you write whitespace by mistake, awk might think that you mean to concatenate a variable with an expression in parentheses. However, it notices that you used a function name and not a variable name, and reports an error.