example34c.tex

\documentclass[oneside]{book}
\usepackage{kantlipsum}
\usepackage{boxedminipage}

\usepackage[twoside]{fancyhdr}
\pagestyle{fancy}
\usepackage[fit]{truncate}
\fancyhf{}
\fancyhead[LE,RO]{\nouppercase{%
         \truncate{0.5\headwidth}{\rightmark}}} % Section title
\fancyhead[LO,RE]{\nouppercase{%
         \truncate{0.5\headwidth}{\leftmark}}} % Chapter title
\fancyfoot[LE,RO]{\thepage}

\begin{document}

\thispagestyle{plain}
\tableofcontents

\bigskip

\noindent
\begin{boxedminipage}{\textwidth}
This is example 34c in the \texttt{fancyhdr} documentation.

This document has some very long chapter and section titles. We use the package \texttt{trunc} so that these titles are automatically truncated in the header.
Check that the table of contents contains the long titles and the header contains the truncated titles where necessary.

\begin{verbatim}
\usepackage[fit]{truncate}
\fancyhead[LE,RO]{\nouppercase{%
         \truncate{0.5\headwidth}{\rightmark}}} % Section title
\fancyhead[LO,RE]{\nouppercase{%
         \truncate{0.5\headwidth}{\leftmark}}} % Chapter title
\fancyfoot[C]{\thepage}

\chapter{This is a very long chapter title to see if we can
         give fancyhdr a shorter one that fits in the header}
 . . .
\section{This is a very long section title that will not fit
         in the header}
\end{verbatim}
\end{boxedminipage}

\newpage
\pagestyle{fancy}
\chapter{This is a very long chapter title to see if we can
         give fancyhdr a shorter one that fits in the header}
\thispagestyle{fancy}

\kant[2-4]

\section{Normal section title}

\kant[5-6]

\section{This is a very long section title that will not fit
         in the header}

\kant[7]

\end{document}