How to make a simple line with two sided arrow?

Question

\documentclass[aps,amsmath,article,amsfonts,11pt]{revtex4}
\usepackage{graphicx}
\usepackage{epstopdf}
\usepackage{epsfig,graphicx}
 \usepackage[english]{babel}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{latexsym}
\usepackage{graphics,bm}
\usepackage{dcolumn}
 \usepackage{bm}
\usepackage{natbib}
\usepackage{rotating}
\usetikzlibrary{arrows}
\pgfarrowsdeclarecombine{twotriang}{twotriang}%
{triangle 90}{triangle 90}{triangle 90}{triangle 90}
%\setlength\topmargin{0.0005 in}


\begin{document}
  C1  \xleftrightarrow{\text{\hspace*{1cm}   }} C2 

I am using this code but nothing happening. I want to make a two sided arrow between two function like this:- C1 <----------------> C2 I am using here dash line. but i need solid line here. If anyone can figure it out, most welcome.

Answer 1

You don't really need TiKZ for that. \xleftrightarrow is not defined by default. Here is how to do it. It has to be used in math mode.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath, old-arrows}

\makeatletter
\newcommand\xleftrightarrow[2][]{%
 \ext@arrow 9999{\longleftrightarrowfill@}{#1}{#2}}
\newcommand\longleftrightarrowfill@{%
 \arrowfill@ \leftarrow \relbar \rightarrow }
\makeatother


\begin{document}

C1 $\xleftrightarrow{\hspace*{1cm}}$ C2

\end{document}