How to cut a section title in the header

Question

I redefine my headers as in the following MWE:

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{fancyhdr}
\fancypagestyle{monstyle}{%
\fancyhf{} % remove everything
\renewcommand{\headrule}{\rule[1.5ex]{\headwidth}{1pt}}
  \lhead{%
\leftmark%
  }
}
\begin{document}
\pagestyle{monstyle}
\section{le tres tres long titre qui prend trop de place sur la ligne}
\end{document}

The section title could be very long, like in my MWE, and i want to cut it at a fixed length (say 15 letters). My attempt is to use the command \StrLeft from the package xstring : I replace the line

\leftmark%

by

    \StrLeft{\leftmark}{15}...%

But i get compilation errors which i do not understand:

Argument of \@iiparbox has an extra }. 

EDIT I try to use Mico's solution, but i encounter now a problem with the TOC :

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{fancyhdr}
\usepackage{blindtext}

%% Redefine the short title
\let\origsection\section % 
\renewcommand{\section}[1]{% 
\def\ShortSecName{\StrLeft{#1}{10}...}
\origsection[\protect\ShortSecName]{#1}%
}

\begin{document}
\tableofcontents
\section{mon titre tres long tres tres long}
\blindtext\blindtext\blindtext
\section{mon titre tres long tres tres long}
\blindtext\blindtext\blindtext
\end{document}

I get the error :

! LaTeX Error: Something's wrong--perhaps a missing \item.

If i remove \tableofcontents then it compiles.

Answer 1

If you want to keep all the functionality of \section and get the original title in the table of contents, here is how

\makeatletter
\let\cutsec@ori@section\section
\def\section{\@ifstar\cutsec@ssection\cutsec@section}
\def\cutsec@ssection{\cutsec@ori@section*}
\def\cutsec@section{\@dblarg\cutsec@@section}
\def\cutsec@@section[#1]#2{%
  \cutsec@ori@section[#1]{#2}%
  \StrLeft{#2}{10}[\temp]%
  \expandafter\markboth{\expandafter\MakeUppercase\expandafter{\temp...}}{}}
\makeatother

However, such a way of doing is risky when the section title has accented characters, unless you use an 8 bit encoding (latin-1, for instance) and refrain from explicit accents such as \'el\`eve, that is, you write élève. It's definitely bound to break with UTF-8 and pdflatex (not with UTF-8 engines such as XeLaTeX and LuaLaTeX).

Answer 2

An alternative approach would be using titleps instead of fancyhdr (caveat: I'm the author of titleps):

\documentclass[12pt]{article}
\usepackage{xstring}
\usepackage{titleps}

\newpagestyle{monstyle}{
   \headrule
   \sethead{\thepage}
           {}
           {\StrLeft{\sectiontitle}{15}...}}

\begin{document}
\pagestyle{monstyle}
\section{le tres tres long titre qui prend trop de place sur la ligne}
\end{document}

\sectiontitle is a macro containing just the title, not a mark with part of the header (formatted), and therefore it can be handled easily.

Answer 3

You need to \protect the \StrLeft command. You don't explain what exactly you do with redefining the \leftmark command, so I suggest you use the \protect\StrLeft approach in the setup of the \section[]{} command:

\section[\protect\StrLeft{Le tres tres long titre qui prend 
  trop de place sur la ligne}{15}]{Le tres tres long titre 
  qui prend trop de place sur la ligne}

Addendum One can automate this process by redefining the \section command suitably, say along the lines of the following MWE (minimum working example):

\documentclass[12pt]{article}
\usepackage{xstring,fancyhdr}
\fancypagestyle{monstyle}{%
  \fancyhf{} % remove everything
  \renewcommand{\headrule}{\rule[1.5ex]{\headwidth}{1.2pt}}
  \lhead{\leftmark}
  }
\pagestyle{monstyle}  

\let\origsection\section       %  Save the original \section command
\renewcommand{\section}[2][15]{%  Default value for the first, optional arg: "15"
    \origsection[\protect\StrLeft{#2}{#1}]{#2}}
\begin{document}

\section[23]{Le tr{\`e}s tr{\`e}s long titre qui prend trop de place sur la ligne}

Il y avait une fois que \ldots
\end{document}

Note that the redefined \section command takes two arguments. The first argument, the number of characters to be retained for the short title or running head, is optional and defaults to 15. The second argument is the "regular" (possibly very long) section header. If you don't want any shortening of the section header, just supply a number -- is 1,000 large enough? -- that exceeds the number of characters in the section header.

Notes: (i) Should you need to create a section header without automatically setting the page's running head as well, you could use the command \origsection (which contains, unsurprisingly, the original version of the \section command). (ii) A side effect of this setup is that the \tableofcontents and \section* commands won't work correctly; let me know if this is a concern to you, and I'll figure out a solution.

Answer 4

You could use the truncate package:

\documentclass[12pt]{article}
\usepackage{fancyhdr}
\usepackage[breakall]{truncate}
\fancypagestyle{monstyle}{%
\fancyhf{} % remove everything

\renewcommand{\headrule}{\rule[1.5ex]{\headwidth}{1pt}}
\lhead{\truncate{15em}{\leftmark}}}

\begin{document}
\pagestyle{monstyle}
\section{le tres tres long titre qui prend trop de place sur la ligne}
\end{document}