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Cute decorative scissors, cutting through your code.

Yxml - A small, fast and correct* XML parser

*But see the Bugs and Limitations and Conformance Issues below.

Yxml is a small (6 KiB) non-validating yet mostly conforming XML parser written in C. Its primary goals are small binary size, simplicity and correctness. It also happens to be pretty fast.

The code can be obtained from the git repo and is available under a permissive MIT license. The only two files you need are yxml.c and yxml.h, which can easily be included and compiled as part of your project. Complete API documentation is available in the manual.

The API follows a simple and mostly buffer-less design, and only consists of three functions:

void yxml_init(yxml_t *x, void *buf, size_t bufsize);
yxml_ret_t yxml_parse(yxml_t *x, int ch);
yxml_ret_t yxml_eof(yxml_t *x);

Be aware that simple is not necessarily easy or convenient. The API is relatively low-level and designed to integrate into pretty much any application and for any use case. This includes incrementally parsing data from a socket in an event-driven fashion and parsing large XML files on memory-restricted devices. It is possible to implement a more convenient and high-level API on top of yxml, but I’m not very fond of libraries that do more than what I strictly need.

There are no tarball releases available at the moment. The API is relatively stable, but I won’t currently promise any ABI stability. Dynamic linking against yxml is therefore not a very good idea.


But let’s not be too optimistic, because there are also…

Bugs and Limitations

These issues may be addressed in future versions.

Conformance Issues

These conformance issues are the result of the byte-oriented and minimal design of yxml and I do not intent to fix these directly within the library. The intention is to make sure that all of the above mentioned issues can be fixed on top of yxml (by the application, or by a wrapper) if strict conformance is required, but the required functionality to support custom entity references and DTD handling has not been implemented yet.


And now follows a list of things that are not part of the core XML specification and are not directly supported. As with the conformance issues, these features can be implemented on top of yxml.


Yxml is used in a few products. Let me know if I missed one.


The following benchmark compares expat, libxml2 and Mini-XML with yxml. A strlen(3) implementation is also included as an indication of the “theoretical” minimum.

                                  SIZE                  PERFORMANCE
LIB      VER    LICENSE              OBJ    STATIC       WIKI  DISCOGS
strlen                                      25 816       0.16     0.09
expat    2.1.0  MIT              162 139   194 432       1.47     1.09
libxml2  2.9.1  MIT              464 328   518 816       2.53     1.75
mxml     2.7    LGPL2+static      32 733    75 832      12.38     7.80
yxml     git    MIT                5 971    31 416       1.15     0.74

The code for these benchmarks is available in the bench/ directory on git. Some explanatory notes:

And just for fun, here’s the same comparison when compiled with -Os, i.e. optimized for small size. Interestingly enough, Mini-XML actually runs faster with -Os than with -O2.

                                  SIZE                  PERFORMANCE
LIB       VER    LICENSE             OBJ    STATIC       WIKI  DISCOGS
strlen                                      25 816       0.16     0.09
expat     2.1.0  MIT             113 314   145 632       1.58     1.20
libxml2   2.9.1  MIT             356 948   412 256       3.01     2.08
mxml      2.7    LGPL2+static     27 725    71 704      11.70     7.44
yxml      git    MIT               4 955    30 392       1.67     1.02

Validating vs. non-validating

TL;DR: yxml does not accept garbage XML documents, it will correctly handle and report issues if the input does not strictly follow the XML grammar.

The terms validating and non-validating have specific meanings within the context of XML. A validating parser is one that reads the doctype declaration (DTD) associated with a document, and validates that the contents of the document follow the rules described in the DTD. A DTD may also include instructions on how to parse the document, including the definition of custom entity references (&whatever;) and instructions on how attribute values or element contents should be normalized before passing its data to the application.

A non-validating parser is one that ignores the DTD and happily parses documents that do not follow the rules described in that DTD. They (usually) don’t support entity references and will not normalize attribute values or element contents. A non-validating parser still has to verify that the XML document follows the XML syntax rules.

It should be noted that a lot of XML documents found in the wild are not described with a DTD, but instead use an alternative technology such as XML schema. Wikipedia has more information on this. Using a validating parser for such documents would only add bloat and may introduce potential security vulnerabilities.