I prefer to include listings in fixed width font as that is
how the compiler reads them. To do that the following works well:
\documentclass[11pt,twoside]{report}
\usepackage{verbatim}
\usepackage{lscape}
\usepackage{multicol}
%when ready for section containing listing do this:
\begin{landscape}
\begin{multicols}{2}
\section{Code Listings}
\subsection{{\tt file1.c}}
{\tiny
\verbatiminput{src/file1.c}}