lowdown — simple markdown translator

lowdown is a Markdown translator producing HTML5, roff documents in the ms and man formats, LaTeX, gemini, OpenDocument, and terminal output. The open source C source code has no dependencies.

The tools are documented in lowdown(1) and lowdown-diff(1), the language in lowdown(5), and the library interface in lowdown(3).

To get and use lowdown, check if it’s available from your system’s package manager. If not, download, verify, and unpack the source. Then build:

% ./configure
% make
% make regress
# make install install_libs

lowdown is a BSD.lv project. Its portability to OpenBSD, NetBSD, FreeBSD, Mac OS X, Linux (glibc and musl), Solaris, and IllumOS is enabled by oconfigure.

One major difference between lowdown and other Markdown formatters it that it internally converts to an AST instead of directly formatting output. This enables some semantic analysis of the content such as with the difference engine, which shows the difference between two markdown trees in markdown.

Output

lowdown produces HTML5 output in XML mode with -thtml, LaTeX documents with -tlatex, “flat” OpenDocument XML documentx (OpenDocument version 1.3) with -tfodt, Gemini with -tgemini, roff documents with -tms and -tman¹ outputs (via groff or mandoc, or directly on ANSI terminals with -tterm.

The -tlatex and -tms are commonly used for PDF documents, -tman for manpages, -thtml or -tgemini for web, and -tterm for the command line.

By way of example: this page, index.md, renders as index.latex.pdf with LaTeX (via -tlatex), index.mandoc.pdf with mandoc (via -tman), or index.nroff.pdf with groff (via -tms).

-tman -tterm -tms

Only -thtml and -tlatex allow images and equations, though -tms has limited image support with encapsulated postscript.

Input

Beyond traditional Markdown syntax support, lowdown supports the following Markdown features and extensions:

• autolinking
• fenced code
• tables
• superscripts (traditional and GFM)
• footnotes
• disabled inline HTML
• “smart typography”
• metadata
• commonmark (in progress)
• definition lists
• extended attributes
• task lists
• admonitions

Examples

Want to quickly review your Markdown in a terminal window?

lowdown -tterm README.md | less -R

I usually use lowdown when writing sblg articles when I’m too lazy to write in proper HTML5. (sblg is a simple tool for knitting together blog articles into a blog feed.) This basically means wrapping the output of lowdown in the elements indicating a blog article. I do this in my Makefiles:

.md.xml:
     ( echo "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>" ; \
       echo "<article data-sblg-article=\"1\">" ; \
       echo "<header>" ; \
       echo "<h1>" ; \
       lowdown -X title $< ; \
       echo "</h1>" ; \
       echo "<aside>" ; \
       lowdown -X htmlaside $< ; \
       echo "</aside>" ; \
       echo "</header>" ; \
       lowdown $< ; \
       echo "</article>" ; ) >$@

Note: you’ll want to make sure that the title and aside are properly HTML formatted, as -X will not escape for the output format.

If you just want a straight-up HTML5 file, use standalone mode:

lowdown -s -o README.html README.md

This can use the document’s meta-data to populate the title, CSS file, and so on.

The troff output modes work well to make PS or PDF files, although they will omit equations and only use local PS/EPS images in -tms mode. The extra groff arguments in the following invocation are for UTF-8 processing (-k), tables (-t), and clickable links and a table of contents (-mspdf).

If outputting PDF, use the pdfroff script instead of -Tpdf output. This allows image generation to work properly. If not, a blank square will be output in places of your images.

lowdown -stms README.md | groff -itk -mspdf > README.ps
lowdown -stms README.md | pdfroff -itk -mspdf > README.pdf

The same can be effected with systems using mandoc:

lowdown -stman README.md | mandoc -Tps > README.ps
lowdown -stman README.md | mandoc -Tpdf > README.pdf

More support for PDF (and other print formats) is available with the -tlatex output.

lowdown -stlatex README.md | pdflatex

For terminal output, troff or mandoc may be used in their respective -Tutf8 or -Tascii modes. Alternatively, lowdown can render directly to ANSI terminals with UTF-8 support:

lowdown -tterm README.md | less -R

Read lowdown(1) for details on running the system.

Library

lowdown is also available as a library, lowdown(3). This is what’s used internally by lowdown(1) and lowdown-diff(1).

Testing

The canonical Markdown tests are available as part of a regression framework within the system. Just use make regress to run these and many other tests.

If you have valgrind installed, make valgrind will run all regression tests with all output modes and store any leaks or bad behaviour. These are output to the screen at the conclusion of all tests.

The CI runner in lowdown’s GitHub repository runs both valgrind and regular regression tests, the latter with the compiler’s -fsanitize options enabled, on each push.

I’ve extensively run AFL against the compiled sources with no failures—definitely a credit to the hoedown authors (and those from whom they forked their own sources). I’ll also regularly run the system through valgrind, also without issue. The afl/in directory contains a series of small input files that may be used in longer AFL runs.

Code layout

The code is neatly layed out and heavily documented internally.

First, start in library.c. (The main.c file is just a caller to the library interface.) Both the renderer (which renders the parsed document contents in the output format) and the document (which generates the parse AST) are initialised.

The parse is started in document.c. It is preceded by meta-data parsing, if applicable, which occurs before document parsing but after the BOM. The document is parsed into an AST (abstract syntax tree) that describes the document as a tree of nodes, each node corresponding an input token. Once the entire tree has been generated, the AST is passed into the front-end renderers, which construct output depth-first.

There are a variety of renderers supported: html.c for HTML5 output, nroff.c for -ms and -man output, latex.c for LaTeX, gemini.c for Gemini, odt.c for OpenDocument, term.c for terminal output, and a debugging renderer tree.c.

Installing

You’ll need a C compiler with essential build tools (make, cc, etc.). First, configure the system:

./configure

You can pass variables like PREFIX and such here. To install the binaries, run:

make install

For libraries, you can additionally run:

make install_libs

This may be split into install_shared and install_static for shared and static libraries, respectively.

Example

For example, consider the following:

## Hello **world**

First, the outer block (the subsection) would begin parsing. The parser would then step into the subcomponent: the header contents. It would then render the subcomponents in order: first the regular text “Hello”, then a bold section. The bold section would be its own subcomponent with its own regular text child, “world”.

When run through the -Ttree output, it would generate:

LOWDOWN_ROOT
  LOWDOWN_DOC_HEADER
  LOWDOWN_HEADER
    LOWDOWN_NORMAL_TEXT
      data: 6 Bytes: Hello 
    LOWDOWN_DOUBLE_EMPHASIS
      LOWDOWN_NORMAL_TEXT
        data: 5 Bytes: world

This tree would then be passed into a front-end, such as the HTML5 front-end with -thtml. The nodes would be appended into a buffer, which would then be passed back into the subsection parser. It would paste the buffer into <h2> blocks (in HTML5) or a .SH block (troff outputs).

Finally, the subsection block would be fitted into whatever context it was invoked within.

Compatibility

lowdown is fully compatible with the original Markdown syntax as checked by the Markdown test suite, last version 1.0.3. This suite is available as part of the make regress functionality.

There are many other extensions to Markdown implemented in lowdown: see the manpages for specifics.

How Can You Help?

Want to hack on lowdown? Of course you do.

• Using a perfect hash (such as gperf) for entities.

• RTL languages are totally unrepresented. It’s difficult but not impossible to refactor term.c to internally create a sequence of lines, instead of a stream of contiguous output, allowing for a post-processing RTL pass.

• There are bits and bobs remaining to be fixed or implemented. You can always just search for TODO, XXX, or FIXME in the source code. This is your best bet.

• If you want a larger project, a -tpdf seems most interesting (and quite difficult given that UTF-8 need be present).

• Implement a parser for mathematics such that eqn or similar may be output.

Coverage

The regression suite executes the lowdown code in many ways. It’s easy to track this by examining run-time coverage using gcov (or the clang alternative). To examine how the regression suite covers the sources, run make coverage. The current coverage is as follows: