User-Defined functions are defined with the pattern keyword:

                                   function

    To define a function to find the maximum of two arguments:

    function max(a1,a2) {
        return a1 > a2 ? a1 : a2;
    }

    The function would return either numeric if both arguments are numeric.
    If either  argument is  a string,  then the  function would  return the
    string highest in the ASCII collating sequence.

    A function may be defined with  no arguments.  To define a  function to
    find the maximum value in the current input record:

    function max_input() {
        local i;
        local max = $1;

        for ( i = 2 ; i <= NF ; i++ ) if ( max < $i ) max = $i;
        return max;
    }

    This example is notable in the use of 'local' variables in the function
    definition.   The 'local'  variable 'i'  and 'max'  disappear upon exit
    from the function.