# [Tex/LaTex] Unclean .aux file causes “file ended while scanning use of \@newl@bel” error. Why is it not purged

auxiliary-filescompilingerrors

I am working on TeX Live 2013 with TeXShop (MacTex2013 distribution)

Expected behavior is, that in the case an error occurs during compilation the .aux file is purged. Subsequent compilation must therefore be done twice, to get the references right.

In the below "minimal" example, this does not happen. The .aux file remains half written causing a "file ended while scanning use of \@newl@bel" error.

Even minimal changes lead to the .aux file being correctly purged. Such as:

• Change of text in headlines or captions. Even a change of Ü to \"U.
• Omitting a \label{…} anywhere.
• Omitting a \cite{…} anywhere (providing a .bib resource has no effect).
• Not using biblatex or hyperref.
• Changing the page breaks.

Here is the self-contained file.

\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage{csquotes}
%\usepackage[T1]{fontenc}

\usepackage{biblatex}

\usepackage{hyperref}

% ================================================

\begin{document}

\clearpage

\section{Geomaxrie der ganzzahligen linearen Optimierung}

\label{Sec.Einfuhrung}

. [figure]

\begin{figure}
\caption{Caption}
\label{Fig.Beispiel-ILP}
\end{figure}

\section{Gitterbasisreduktion und Voronoi Algorithmus}
\label{Sec.Voronoi}

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cite\cite{Voronoi}

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label\label{Thm.Voronoi}

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label\label{Alg.VoronoiFilter}

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label\label{Alg.VoronoiFilter.Bedingung}

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label\label{Alg.CVPP}

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label\label{Alg.VoronoiRelevant}

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label\label{Alg.Voronoi}

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\clearpage
\section{Ellipsoid-Überdeckungen und M-Ellipsoide}
\label{Sec.Ellipsoid-Methode.M-Ellipsoide}

. [figure]

\begin{figure}
.
\caption{Löwner-John-Ellipsoid mit Anwendung in der Ellipsoid Methode}
\label{Fig.MinEllipsoid}
\end{figure}

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label\label{Thm.InertEllipsoid}

label\label{Thm.MEllipsoid}

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. [figure]

\begin{figure}
\caption{Label}
\label{Fig.M-Ellipsoidoo}
\end{figure}

label\label{Alg.ConvexBodyEnumerationInformal}

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\clearpage
\section{Berechnungsmodell}
\label{Sec.Berechnungsmodell}

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cite\cite{M-Ellipsoid}

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label\label{Def.SEP}

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\section{M-Ellipsoid Algorithmen}

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label\label{Theorem}

Cause error here: $A simple error at this point results in the .aux file not being cleaned. Even marginal changes in the document above make the effect disappear \end{document}  To reproduce the bug, do the following: 1. Observe the single$ at the end of the document causing an error
2. Compile (.aux remains in an unclean state)
3. Correct the error
4. Compile again (half written .aux causes error)
5. Delete .aux file to recover

I know what happens and how to recover. My question is:

or better

## Is there a way to prevent it?

In my current document, for every error I make while writing in the last section, I have to manually delete the .aux files to recover. Since I compile fairly often while writing, this is an annoyance and I'd like to prevent it.

From what I understand the output to an .aux file is buffered. In case of an error, compilation stops and the current buffer remains unwritten. At this point .aux file contains the buffer chunks already written, which may break anywhere, leaving incomplete definitions.

Normally I then don't work with the TeX engine on the console, but simply correct my error and restart.

I do not understand, what triggers the deletion of the .aux file, which, as the example shows does not work reliably. Possibly due to the biblatex or hyperref package. If you could shed some light on that?

Also of interest:

• Is there something to control the deletion of the .aux file after an error?
• Are there TeX/LaTeX commands I can use to manually flush the output buffer for the .aux file? I could use this command right before the section I am working on. That way, I probably won't have to deal with a corrupt .aux file that often anymore.
• Is there an Option for the pdflatex program so that the output buffers are flushed after an error. In most cases the error does not affect the .aux file, so there is no reason in not finish writing it. In the cases where it does, I would have to delete it then.

There are several related questions here on StackExchange:

With the help of When is the aux file read and written? and Barbara Beetons kind assistance I was able to determine what goes wrong and how to prevent it.

This is the problem:

Normally I then don't work with the TeX engine on the console, but simply correct my error and restart.

This is what I should have done on the console:

• Type ?+Return to get an idea of what can be done.
• Simply hit Return repeatedly until Tex is finished.
• Type X+Return to finish the run immediately.
• Type E+Return to have the cursor jump to the point where the error occurred.

What happens is, that TeX buffers the \write commands to the .aux file in two stages. First stage, it waits until a page is shipped out to get the page numbers right. Second stage, it waits until it sees fit to issue a write() system call, to reduce the number of (potentially slow) system calls. This second buffer seemingly does not care, if the chunks it writes to file are self-contained legal TeX since it will complete the file before the TeX run is finished anyway.

In the environment of TeXShop however it is tempting to let TeX hang unfinished after an error. Thus the second output puffer is not flushed and the .aux file possibly left unfinished and in an illegal state.

My personal conclusion from this: Always let TeX finish.

The assumption that TeX will have a chance to finish it's run after it stopped for an error was very reasonable before windowed environments, where it became easy and normal to leave processes hanging in the background.

Things that could be done on software side to facilitate this user behavior:

• Change Tex's second buffer behavior to issue a write() system call in case of an error or to break only into TeX-legal chunks.
• Adjust the environment (TexShop) to either clean the .aux file after a process abortion or provide default input to always correctly finish the process.

Things that the user can do, other than always letting TeX finish:

• Use \include{...}. That way the .aux files of the sections before are closed an finished by the time the error occurs. Chances are the .aux file of your current section isn't large enough yet and no TeX-illegal parts were written.
• Hack something involving \immediate\closeout\@mainaux followed by an \openout\@mainaux and a copy of the contents so they don't get overwritten. This will effectively flush the output buffer. Does not seem worth the effort though.