How do I access the width of text from the preamble?

Question

I've got some macros that I want to set up in my preamble. The problem is that they need to know what the width of a typical line of text will be in the document.

If I do something like,

%% Everything set up correctly
\documentclass{article}
\usepackage[margin=1in,showframe]{geometry}
\setlength{\parindent}{0pt}

\newlength{\testlength}
\setlength{\testlength}{\textwidth}
\newcommand{\myrule}{\rule{\testlength}{4pt}}

\usepackage{lipsum}
\begin{document}

\myrule

\lipsum[1-10]

\end{document}

Then \myrule has the correct width.

But if I do something like

%% FAIL
\documentclass{article}
\usepackage[margin=1in,showframe,twocolumn]{geometry}
\setlength{\parindent}{0pt}

\newlength{\testlength}
\setlength{\testlength}{\textwidth}
\newcommand{\myrule}{\rule{\testlength}{4pt}}

\usepackage{lipsum}
\begin{document}

\myrule

\lipsum[1-10]

\end{document}

I don't get the desired result.

I might be inclined to set the width of my macro as

\setlength{\testlength}{\linewidth}

or

\setlength{\testlength}{\columnwidth}

But both of these fail to get the correct length (apart from the fact that \linewidth, as I understand it, shouldn't really make too much sense in the preamble).

How can my macro know from within the preamble what the proper textwidth should be?

I should note that I'm not particularly concerned about what happens if, let's say, the macro is called from within a multicols environment from the multicol package. In other words, the macro should have an absolute length, not a length relative to a particular environment.

Answer 1

You could write the length you're interested in to the aux file and retrieve it the next time round:

\documentclass{article}
\usepackage[margin=1in,showframe,twocolumn]{geometry}
\setlength{\parindent}{0pt}

\newlength{\testlength}

\makeatletter
\def\getlength{\immediate\write\@mainaux{\noexpand\global\noexpand\testlength=\the\linewidth}}
\makeatother

\newcommand{\myrule}{\rule{\testlength}{4pt}}

\usepackage{lipsum}
\begin{document}

\getlength
\myrule

\lipsum[1-10]

\end{document}

yields

after two runs.