%% 
%%  An UIT Edition example
%% 
%%  Example 06-15-2 on page 117.
%% 
%%  Copyright (C) 2010 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{exaartplain}
\pagestyle{empty}
\setlength\textwidth{375.57637pt}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\usepackage{amsmath}
\makeatletter
\newcommand\xLongLeftRightArrow[2][]{\ext@arrow 0055{\LongLeftRightArrowfill@}{#1}{#2}}
\def\LongLeftRightArrowfill@{\arrowfill@\Leftarrow\Relbar\Rightarrow}
\newcommand\xlongleftrightarrow[2][]{\ext@arrow 0055{\longleftrightarrowfill@}{#1}{#2}}
\def\longleftrightarrowfill@{\arrowfill@\leftarrow\relbar\rightarrow}
\makeatother
\StopShownPreambleCommands
\begin{document}
$\Leftarrow\Relbar\Rightarrow$
$\Leftarrow\mkern-3mu\Relbar\mkern-3mu\Rightarrow$
$\xLongLeftRightArrow[\text{UUU}]{\text{OOOOOOO}}$
$\xlongleftrightarrow[y=f(x,v,w)]{x=f(w)}$
\end{document}