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C Markdown parser. Fast. SAX-like interface. Compliant to CommonMark specification.
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MD4C Readme

MD4C stands for "Markdown for C" and, unsurprisingly, it is a C Markdown parser implementation.

What is Markdown

In short, Markdown is the markup language this file is written in.

The following resources can explain more if you are unfamiliar with it:

What is MD4C

MD4C is C Markdown parser with the following features:

  • Compliance: Generally MD4C aims to be compliant to the latest version of CommonMark specification. Right now we are fully compliant to CommonMark 0.29.

  • Extensions: MD4C supports some commonly requested and accepted extensions. See below.

  • Compactness: MD4C is implemented in one source file and one header file.

  • Embedding: MD4C is easy to reuse in other projects, its API is very straightforward: There is actually just one function, md_parse().

  • Push model: MD4C parses the complete document and calls callback functions provided by the application for each start/end of block, start/end of a span, and with any textual contents.

  • Portability: MD4C builds and works on Windows and Linux, and it should be fairly simple to make it run also on most other systems.

  • Encoding: MD4C can be compiled to recognize ASCII-only control characters, UTF-8 and, on Windows, also UTF-16, i.e. what is on Windows commonly called just "Unicode". See more details below.

  • Permissive license: MD4C is available under the MIT license.

  • Performance: MD4C is very fast.

Using MD4C

Application has to include the header md4c.h and link against MD4C library; or alternatively it may include md4c.h and md4c.c directly into its source base as the parser is only implemented in the single C source file.

The main provided function is md_parse(). It takes a text in Markdown syntax as an input and a pointer to a structure which holds pointers to several callback functions.

As md_parse() processes the input, and it calls the appropriate callbacks (when entering or leaving any Markdown block or span; and when outputting any textual content of the document), allowing application to convert it into another format or render it onto the screen.

More comprehensive guide can be found in the header md4c.h and also on MD4C wiki.

Example implementation of simple renderer is available in the md2html directory which implements a conversion utility from Markdown to HTML.

Markdown Extensions

The default behavior is to recognize only elements defined by the CommonMark specification.

However with appropriate flags, the behavior can be tuned to enable some extensions:

  • With the flag MD_FLAG_COLLAPSEWHITESPACE, non-trivial whitespace is collapsed into a single space.

  • With the flag MD_FLAG_TABLES, GitHub-style tables are supported.

  • With the flag MD_FLAG_TASKLISTS, GitHub-style task lists are supported.

  • With the flag MD_FLAG_STRIKETHROUGH, strike-through spans are enabled (text enclosed in tilde marks, e.g. ~foo bar~).

  • With the flag MD_FLAG_PERMISSIVEURLAUTOLINKS permissive URL autolinks (not enclosed in < and >) are supported.

  • With the flag MD_FLAG_PERMISSIVEAUTOLINKS, ditto for e-mail autolinks.

  • With the flag MD_FLAG_PERMISSIVEWWWAUTOLINKS permissive WWW autolinks (without any scheme specified; http: is assumed) are supported.

The syntax of the extensions is described on MD4C wiki.

Few features (those some people see as mis-features) of CommonMark specification may be disabled:

  • With the flag MD_FLAG_NOHTMLSPANS or MD_FLAG_NOHTMLBLOCKS, raw inline HTML or raw HTML blocks respectively are disabled.

  • With the flag MD_FLAG_NOINDENTEDCODEBLOCKS, indented code blocks are disabled.

Input/Output Encoding

The CommonMark specification generally assumes UTF-8 input, but under closer inspection, Unicode plays any role in few very specific situations when parsing Markdown documents:

  • For detection of word boundary when processing emphasis and strong emphasis, some classification of Unicode character (whitespace, punctuation) is used.

  • For (case-insensitive) matching of a link reference with corresponding link reference definition, Unicode case folding is used.

  • For translating HTML entities (e.g. &amp;) and numeric character references (e.g. &#35; or &#xcab;) into their Unicode equivalents. However MD4C leaves this translation on the renderer/application; as the renderer is supposed to really know output encoding and whether it really needs to perform this kind of translation. (Consider that a renderer converting Markdown to HTML may leave the entities untranslated and defer the work to a web browser.)

MD4C relies on this property of the CommonMark and the implementation is, to a large degree, encoding-agnostic. Most of MD4C code only assumes that the encoding of your choice is compatible with ASCII, i.e. that the codepoints below 128 have the same numeric values as ASCII.

Any input MD4C does not understand is simply seen as part of the document text and sent to the renderer's callback functions unchanged.

The two situations where MD4C has to understand Unicode are handled accordingly to the following preprocessor macros:

  • If preprocessor macro MD4C_USE_UTF8 is defined, MD4C assumes UTF-8 for word boundary detection and case-folding.

  • On Windows, if preprocessor macro MD4C_USE_UTF16 is defined, MD4C uses WCHAR instead of char and assumes UTF-16 encoding in those situations. (UTF-16 is what Windows developers usually call just "Unicode" and what Win32API works with.)

  • By default (when none of the macros is defined), ASCII-only mode is used even in the specific situations. That effectively means that non-ASCII whitespace or punctuation characters won't be recognized as such and that case-folding is performed only on ASCII letters (i.e. [a-zA-Z]).

(Adding support for yet another encodings should be relatively simple due the isolation of the respective code.)


MD4C is covered with MIT license, see the file

Reporting Bugs

If you encounter any bug, please be so kind and report it. Unheard bugs cannot get fixed. You can submit bug reports here:

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