Why \hyperlink{\rightmark}{} does not work in fancyhead (and inside some other macros)?

Question

\fancyhead[LO,RE]{\hyperlink{\rightmark}{\bfseries\rightmark}}

(\leftmark is all the same) cause error:

TeX capacity exceeded, sorry [input stack size...

Why? (here, \rightmark has double function: the name and text argument of hyperlink (actually it is the full name of a section (e.g. 1.2 mysection) by which a hypertarget was created before inside \section{...} macro.)

Here is a siple working code. My idea is to have chapter and section names automatically hyperlinked in header. This code works but if you uncomment \usepackage[]{babel} it produces the problematic error.

---

\documentclass[pdftex,a4paper,12pt,oneside]{book}%

%\usepackage[magyar,english]{babel}
\usepackage{fancyhdr}
\usepackage{xifthen}
\usepackage{xstring}
\usepackage{hyperref}
\hypersetup{colorlinks,
                  linktoc=all,
                hypertexnames=false,
                unicode=true,
                bookmarksnumbered=false,
            pdfmenubar=true,
              pdftoolbar=true}

%------- Chapter link ------------
\newcommand{\chapterlink}[1]{\addtocounter{chapter}{1}\hypertarget{\thechapter\ #1}{}\addtocounter{chapter}{-1}}
\newcommand{\Chapter}[2][]{\chapterlink{#2}\ifthenelse{\equal{#1}{}}{\chapter{#2}}{\chapter{#1}{#2}}}

%------- Section link ------------
\newcommand{\sectionlink}[1]{\addtocounter{section}{1}\hypertarget{\thesection\ #1}{}\addtocounter{section}{-1}}
\newcommand{\Section}[2][]{\sectionlink{#2}\ifthenelse{\equal{#1}{}}{\section{#2}}{\section{#1}{#2}}}

            \newcommand{\mainheader}
            {
                \pagestyle{fancy}
                \fancyhf{} 
                \renewcommand{\chaptermark}[1]{\markboth{\thechapter\ ##1}{}}
                \renewcommand{\sectionmark}[1]{\markright{\thesection\ ##1}} 

                \fancyhead[LO,RE]{\hyperlink{\leftmark}{\bfseries\StrBehind{\leftmark}{\ }}\ :\ \hyperlink{\rightmark}{\bfseries\StrBehind{\rightmark}{\ }}}  

                \fancyhead[LE,RO]{\bfseries\thepage}
                \renewcommand{\headrulewidth}{0.5pt}
                \renewcommand{\footrulewidth}{0pt}
                \addtolength{\headheight}{2.5pt} 
                \fancypagestyle{plain}{\fancyhead{}\renewcommand{\headrulewidth}{0pt}} 
            }

%-----------------------------------

\begin{document}

\mainheader

\Chapter{Chapter one}

chapter text chapter text chapter text chapter text chapter text chapter text 

\newpage

\Section{Section one}

section text section text section text section text section text section text 

\newpage

section text section text section text section text section text section text 

\end{document}

---

Investigating the meaning of the macro \rightmark I have found the follwing:

macro:->\expandafter \@rightmark \firstmark \@empty \@empty

So I had a look into \firstmark

without \usepackage[magyar]{babel}: {1:Chapter-One}{3:Section-One}

%1 and 3 are my private counter values to be unique id-s in hyperlink, text is chapter name and section name

with \usepackage[magyar]{babel}: {\protect \foreignlanguage {magyar}{\protect \bbl@restore@actives 1:Chapter-One}}{\protect \foreignlanguage {magyar}{\protect \bbl@restore@actives 3:Section-One}}

Surely not fully expandable :)

But this investigation gives me the clue to solve my problem. \meaning\firstmark can be parsed and split into substrings to get that part of the text that I wanted to use to be \hyperlink name and text.

(If anybody like to see the whole MWE I readyly show it.)

Answer 1

As stated in my answer to your similar question Nested macros and hyperlink problem, the first argument of \hyperlink and the matching \hypertarget must be a fully expandable string. This string is only used as an internal name, similar to the name for \label and the matching \ref.

You should therefore try to use something else and not the section text as label. Something which is always expandable, but still different for every section. I would suggest using:

\fancyhead[LO,RE]{\hyperlink{section-\thesection}{\bfseries\rightmark}}

where \thesection expands to the section number. (If there is any issue with that use \arabic{section} instead, which is always expandable.)