Escaping from the escaping hell

Question

Given the following MWE, where the argument for the code function contains an underscore:

\documentclass [11pt,oneside,onecolumn]{article}
\usepackage{listings}
\usepackage{filecontents}

\begin{filecontents*}{hello_world.c}
#include <stdio.h>

int main(void) {
  printf("Hello World!\n");
  // return 0;
}
\end{filecontents*}


\makeatletter
\def\code{\@ifnextchar[{\@with}{\@without}}
\def\@with[#1]#2{
}
\def\@without#1{
  \section{#1}
  \lstinputlisting[]{#1}
}
\makeatother

\begin{document}
\code{hello_world.c}
\end{document}

The same argument #1 is used in two different functions: section, which requires the underscore to be escaped, and lstinputlisting, which requires the underscore not to be escaped.

If the user escapes the filename, compilation fails because of section. If the user doesn't escape the filename, listings cannot find the filename.

How to solve the issue?

Answer 1

Please try to make your MWE self contained (as in this answer) it really helps when answering.

You can use the eTeX primitive \detokenize to make _ safe and then T1 font encoding so a _ character gets printed as an underscore:

\documentclass [11pt,oneside,onecolumn]{article}
\usepackage[T1]{fontenc}
\makeatletter
\usepackage{listings}

\def\code{\@ifnextchar[{\@with}{\@without}}%
\def\@with[#1]#2{%
}
\def\@without#1{%
  \section{\protect\detokenize{#1}}%
  \lstinputlisting[]{#1}%
}

\makeatother

\begin{document}

\tableofcontents

\code{hello_world.c}
\end{document}

Answer 2

There are several packages that takes care of typesetting file names, e.g. url, path. The following example uses package url. I have assumed that the optional argument of \code has the purpose to pass options to the lstinputlisting command. In this case the hacking with \@ifnextchar is not necessary.

\documentclass [11pt,oneside,onecolumn]{article}
\usepackage[T1]{fontenc}
\usepackage{listings}
\usepackage{url}
\usepackage{makerobust}

% Define \file that typeset a file name using the url package
% that also takes care of hyphenation issues, ...
\newcommand*{\file}{}% to get an error if \file is already defined
\DeclareUrlCommand{\file}{\urlstyle{sf}}
% Different styles are available, e.g.:
% sf: sans serif font (\sffamily)
% rm: roman font (\rmfamily)
% tt: typewriter font (\ttfamily)
\MakeRobustCommand{\file}
% as robust command \file do not need \protect

\newcommand*{\code}[2][]{%
  \section{\file{#2}}%
  \lstinputlisting[{#1}]{#2}%
}

\makeatother

\begin{document}

\tableofcontents

\code{hello_world.c}

\end{document}