How to setup the caption and listings package to insert code?

Question

Possible Duplicate:
Caption parbox 6.0 pt too wide

I used caption and listings package in my .tex file as below:

\documentclass{article}
\usepackage{listings}
\usepackage{xcolor}
\lstset{ %Settings for listings package.
%language=[ANSI]{C},
extendedchars=false,% 重要！解决中文跨页出错的问题
backgroundcolor=\color{lightgray},
basicstyle={\ttfamily},
breakatwhitespace=false,
breaklines=true,
captionpos=t,
commentstyle=\color{olive},
%directivestyle=\color{blue},
extendedchars=false,
frame=tb,%single,%shadowbox
framerule=0pt,
keywordstyle=\color{blue}\bfseries,
morekeywords={*,define,*,include...},
numbers=left,
numberstyle=\tiny,
numbersep=5pt,
rulesepcolor=\color{red!20!green!20!blue!20},
showspaces=false,
showstringspaces=false,
showtabs=false,
%stepnumber=2,
stringstyle=\color{purple},
tabsize=4,
escapeinside=``,
xleftmargin={2em},
xrightmargin={2em},
%title=\lstname
}
\usepackage{caption}
\DeclareCaptionFont{white}{\color{white}}
\DeclareCaptionFormat{listing}{\colorbox{gray}{\parbox{\textwidth}{#1#2#3}}}
\captionsetup[lstlisting]{format=listing,labelfont=white,textfont=white}
\begin{document}
\begin{lstlisting}[language={[ANSI]C},label=example,caption=example]
int main(int argc, char ** argv)
{
    printf("Hello!\n");
    return 0;
}
\end{lstlisting}
\end{document}

But the caption has been eclipsed by the listings.

My problems are:

1. How can I do to avoid the eclipse?

2. How can I to set caption's left and right margin as in the listings?