# Fixing errors in generated files
## Introduction
Sometimes Overleaf will report errors which arise from files with names such as `output.toc`, `output.bbl`, etc., but those files *don’t even exist in your project*. So where do such files *come from* and, more importantly, how do you *locate* and *fix* the errors?
“But I don’t even have this output.toc file in my project!”
These “mysterious files” are artifacts: ancillary files generated during the LaTeX compilation process, but they can also trigger errors which usually originate from coding issues within your project’s source files. This help article uses a [real error-ridden example](#playing-detective-locating-errors-in-generated-files) to show how to track down such errors to their point-of-origin within your project source file(s).
## Files generated during the LaTeX compilation process
Apart from the output `.pdf` file, the LaTeX compilation process generates several file types which contain information about labels, cross-references, page numbers etc. Some ancillary file types are not directly created by the LaTeX compiler, but by other processing tools used during typesetting, for example, formatted reference lists and indices. These generated files must be read by the compiler on subsequent runs, so that the final `.pdf` file is populated with the correct lists, citations and cross-references.
pdflatex\tobibtex\topdflatex\topdflatex
On a local LaTeX installation you may need to execute each intermediate step yourself, or you might use a build tool such as [make](https://www.gnu.org/software/make/manual/), [latexmk](https://ctan.org/pkg/latexmk) or [arara](https://ctan.org/pkg/arara), to automate this chain of processes. [Overleaf uses latexmk](https://docs.overleaf.com/managing-projects-and-files/the-latexmkrc-file), and the following video shows what happens every time you click the “Recompile” button in your Overleaf project:
{% embed url="" fullWidth="false" %}
## Compile errors reported from generated files
Coding errors in the main project `.tex` file may be carried through to the generated files. For example, an unescaped `&` in `\section{Introduction & Background}` would make its way into the generated `.toc` file. When the generated `.toc` file is subsequently read-in by the next `pdflatex` compile run, an error message is then raised as “coming from the .toc file”. However, current LaTeX logs and error-reporting do not make any connection from lines in the `.toc` file (and other generated files) to their originating location in the source `.tex` file: it may not be immediately obvious where the original error should be corrected.
Here’s an example of how errors are presented in another LaTeX editor: whilst you can click on the error message to open the related line in the generated `.toc` or `.aux` file, there isn’t a path to go from the error to the originating location in the `.tex` file. And even if you correct the error by replacing `&` with `\&` in the `.toc` file, that doesn’t really solve the problem: the next time you compile the `.tex` file, the `.toc` file is regenerated and the same error will be rewritten to the `.toc` file.
When an error is reported as coming from generated files, the correct way to solve them is to locate the actual originating error in the source files, and correct those. The error messages usually include contextual information, some text or code surrounding the error, so doing a text search in the source files will help to locate them. The errors themselves are often common: you may look up some common errors on [this help page](https://www.overleaf.com/learn/latex/Errors), or in the [TeX FAQ list](https://texfaq.org/#errors).
## First, try clearing the generated files
Sometimes, errors in the generated files are leftovers from a previous compile. Even if you had corrected the errors in your `.tex` file and recompiled, the generated files, still containing the errors, might still be read by the compiler, producing the same errors as before. You can try deleting the generated files before you recompile. Instructions on how to do this on Overleaf can be found [here](https://docs.overleaf.com/troubleshooting-and-support/clearing-the-project-cache).
If you’re still seeing the same errors even after deleting all generated files and recompiling, it’s likely that there are genuine errors in your source files.
## Playing detective: locating errors in generated files
The following sections offer some tips on where to look for such errors. We’ll use the following *purposely error-ridden* example project for our sleuthing practice. Ready? Let’s go!
[ Open an *error-ridden* Overleaf project!](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=Demo+of+LaTeX+compile+errors+reported+from+generated+files\&main_document=main.tex\&snip_uri=https%3A%2F%2Fassets.ctfassets.net%2Fnrgyaltdicpt%2F5qdEqgUidaYDHjOlBoKM7K%2F55395921a114edb36ed3d51d7b1c6edb%2Fdemoerrors.zip) The project should look something like this 😲:
Image showing LaTeX project errors
Errors from .aux files
The `.aux` file contains information about use of `\label{...}` declarations and `\cite{...}` in the `.tex` file, so these are useful things to check.
Let’s look at this first error from our example error-ridden project.
Example error in an .aux file due to bad \label declaration
The error `Missing \endcsname inserted.` at an `\alpha` may seem quite cryptic at first glance, and appears not to be that helpful in our debugging. But we see from the text (context) of the error message that it’s occurring somewhere near to some code that has `sec:\alpha`, which is associated with some label or counter numbered 1.1. Therefore, we may want to look carefully at any `\label{...}` in our `.tex` files that contain the snippet `sec:\alpha`.
You should soon find your way to line 37 of our example project:
```
\label{sec:\alpha}
```
The argument passed to `\label{...}` should not contain any LaTeX commands or control sequences, such as `\alpha`. Therefore, correcting the label to something like `\label{sec:alpha}`, as well as any corresponding `\ref{...}`, will clear this error.
Here’s another error reported from the `.aux` file, in our example project:
Example error in .aux file due to space in \cite{...} key
This time the compiler complains of a white space in a `\citation`; where the key starts with “That”. So we would want to search for `\cite{That ...}` in our `.tex` file... and we find it on line 23:
```
... justo \cite{That mandatory talk} eget magna ...
```
Citation keys should not contain space characters, so the spaces in this `\cite{...}`, as well as the corresponding BIBTeX key in the `.bib` file, should be removed.
Errors from .toc files
The `.toc` file contains entries for `\tableofcontents`. These are populated from sectional headings in a LaTeX document, e.g., `\chapter{...}`, `\section{...}`, etc. Therefore, if you see errors reported from `.toc` files, it would be useful to check your sectional headings that contain text reported in the error message.
Example error in an .toc file due to unescaped & in \section{...}
This looks like a sectional heading that would be numbered “1”, and contains the text `Introduction & Welcome`. A quick search in the `.tex` file will bring us to line 12:
```
\section{Introduction & Welcome}
```
The `&` is unescaped here, so we need to change it to `\&` in the `.tex` file.
Another similar example:
Example error in a .toc file due to math command not in math mode in \section{...} key
Similarly our hunch would be to look for a sectional heading that would be numbered “1.1”, and likely containing `Setting \alpha`. Our hunch turns out to be correct, on line 36 of the `.tex` file:
```
\subsection{Setting \alpha}
```
`\alpha` is a math command but we forgot to put it in math mode here, so we need to correct this to:
```
\subsection{Setting $\alpha$}
```
or
```
\subsection{Setting \(\alpha\)}
```
Sometimes the errors arise in the `.toc` file due to errors in the code used to style the entries in the table of contents:
Example error in an .toc file due to improper code when styling the ToC entry
Here, searching with `\itshae` will lead us to a typo on line 5 of our example `.tex`: it should be `\itshape` instead. In this case, the error can be located in the preamble of our `.tex` file, so we can go ahead and correct it. But if you are using a `.cls` or `.sty` file provided by your institution and you suspect the errors are due to misconfiguration in the provided files, you should contact the author or maintainer of the template or package so they can update the template.
Errors from .lof and .lot files
The `.lof` file contains entries for `\listoffigures`, and are populated by `\caption{...}` of `figure` environments.
Similarly, the `.lot` file uses `\caption{...}` from `table` environments, to generate entries for `\listoftables`.
Therefore, if there are LaTeX errors from `.lof` or `.lot` files, you should check the `\caption{...}` which contains the relevant text snippets.
Based on the error message here, we will search for `Effects of \alpha and var_coeff` in our `\caption{...}`s, especially for `figure`s:
Example error in a .lof file due to math command not in math mode in figure \caption{...} key
and come to line 19 in our `.tex` file:
```
\caption{Effects of \alpha and var_coeff}
```
The `\alpha` should be placed in math mode; and if `var_coeff` is supposed to be variable name, the underscore needs to be escaped. Don’t use `\verb|var_coeff|` because this can be quite disastrous in `\caption{...}`, too!
```
\caption{Effects of $\alpha$ and \texttt{var\_coeff}}
```
Similarly, looking at this error, we would search for `some sample data with n^i` in `table` `\caption{...}`, with a hunch that we forgot to put the `n^i` in math mode, too.
Example error in a .lot file due to superscript not in math mode in table \caption{...} key
And so we correct line 31:
```
\caption{Some sample data with n^i}
```
to:
```
\caption{Some sample data with $n^i$}
```
Errors from .bbl files
The `.bbl` file contains entries for the reference list (bibliography), and is generated by BIBTeX after processing the `.bib` file, using styling information from the `.bst` file. Errors reported from `.bbl` usually originate from the `.bib` file.
Here’s a common error which we often see in Overleaf support:
Example error in a .bbl file due to unescaped & in .bib file
Searching for `This & that` in the `.bib` file of the example project will land us on line 13, where we realise that the `&` should be escaped as `\&` or replaced with the word `and`.
```
title = {This & That},
```
Authors often encounter errors related to Unicode characters in `.bbl` (and therefore `.bib`) files—especially if the information sources had been copied-and-pasted from elsewhere:
Example error in a .bbl file due to unsupported Unicode character in .bib file
Copy-pasting the ♣ character from the error message and searching for it within the `.bib` file will bring you to line 3:
```
title = {Research ♣ Development},
```
Some Unicode characters are not supported by the `latex` and `pdflatex` compilers. To avoid the error, you may try [changing your project’s compiler](https://docs.overleaf.com/getting-started/recompiling-your-project/selecting-a-tex-live-version-and-latex-compiler) to `XeLaTeX` or `LuaLaTeX`. But even then you need to ensure your project is compatible with those compilers, and that you have used a font which contains a glyph to represent ♣ so that it is rendered correctly in the output PDF. Therefore, it would be better to use the relevant LaTeX commands:
```
title = {Research $\clubsuit$ Development},
```
Sometimes, though, the character might be much harder to spot:
Example error in a .bbl file due to unsupported, unprintable Unicode character in .bib file
Here, U+3000 is a [Unicode Character `IDEOGRAPHIC SPACE`](https://www.fileformat.info/info/unicode/char/3000/index.htm), and so indeed is a white space. It may be hard to copy this exact character from the error message so that you can search it in the `.bib` file. So we’ll have to try another way to locate it, by using a `\DeclareUnicodeCharacter` to replace `U+3000` with something more visible! Add the following lines in your project’s preamble, before `\begin{document}`:
```
\usepackage{xcolor}
\DeclareUnicodeCharacter{3000}{\textcolor{red}{BAD!!}}
```
Compiling your project will then make the error disappear; but you should look carefully in the output PDF for the BAD!!:
So now we know that the bad character is between `of` and `Wondrous`, for the bibliography item by “An Author”. It looks quite innocuous on line 5 of the `.bib` file since it’s invisible: but if you retype that `...of Wondrous...` manually with a normal space-bar, the error will go away after a recompile.
```
journal = {Journal of Wondrous Musings},
```
**A note about `biblatex`**
If you are using the `biblatex` package, then similar errors in the `.bib` files would likely all be reported as coming from the `\printbibliography` line in your `.tex` file. The debugging process is similar: use the short text snippets in the error messages to search in your `.bib` file to locate the offending lines, and correct them.
Errors from .bst files
`.bst` files are bibliography styles that BIBTeX uses to process and format the reference list. If you see errors from `.bst` files, this might be due to bugs in the `.bst` itself, especially if it’s a very, very old style file that might no longer be compatible with recent LaTeX packages. But it may be actually due to errors in the `.bib` file, too. The most common error in this category looks like this:
Bst-error-example
The entry with BIBTeX key `Someone:etal:2008` has the `author` field on line 12 of the `.bib` file:
```
author = {Strange Someone, Night Stranger, and Random Person},
```
In a `.bib` file, [a list of multiple names](https://learn.overleaf.com/learn/Bibliography_management_with_bibtex#Multiple_authors) should be separated by `and`, not commas nor `&`. Therefore this `author` field should be corrected to:
```
author = {Strange Someone and Night Stranger and Random Person}
```
Errors from .ttt and .fff files
These file types are only generated if you load the [`endfloat` package](https://www.ctan.org/pkg/endfloat), which is used to force all table and figure environments to be moved to the end of the document, per the requirements of some academic journals during article submission.
To see example errors from these generated files, uncomment line 3 `\usepackage{endfloat}` of the `.tex` file in our example project.
Ttt-error-example
The `.ttt` file contain contents from all `table` environments in the document, so we would search for `dummy & data` in the `.tex` file, paying attention to tables. We would come to line 29 in the `.tex` file, where we see the problem: we have three columns, one more than the two declared. So one extra column may need to be deleted; or we should insert `\\` after `dummy` to start a new row; or perhaps we had intended to declare three columns `c | c | c` after all!
fff error example
Similarly, the `.fff` file contains content from all `figure` environments, and searching for that typo-ed `\includegraphcs` would bring us to line 18 of the `.tex` file, where we can correct it to `\includegraphics`.
