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REQUEST EXIT

The XSLT C library for GNOME

libxslt

Libxslt is the XSLT C library developed for the GNOME project. XSLT itself is a an XML language to define transformation for XML. Libxslt is based on libxml2 the XML C library developed for the GNOME project. It also implements most of the EXSLT set of processor-portable extensions functions and some of Saxon's evaluate and expressions extensions.

People can either embed the library in their application or use xsltproc the command line processing tool. This library is free software and can be reused in commercial applications (see the intro)

External documents:

Logo designed by Marc Liyanage.

Introduction

This document describes libxslt, the XSLT C library developed for the GNOME project.

Here are some key points about libxslt:

Libxslt is a C implementation
Libxslt is based on libxml for XML parsing, tree manipulation and XPath support
It is written in plain C, making as few assumptions as possible, and sticking closely to ANSI C/POSIX for easy embedding. Should works on Linux/Unix/Windows.
This library is released under the MIT Licence
Though not designed primarily with performances in mind, libxslt seems to be a relatively fast processor.

Documentation

There are some on-line resources about using libxslt:

Check the API documentation automatically extracted from code comments (using the program apibuild.py, developed for libxml, together with the xsl script 'newapi.xsl' and the libxslt xsltproc program).
Look at the mailing-list archive.
Of course since libxslt is based on libxml, it's a good idea to at least read libxml description

Reporting bugs and getting help

If you need help with the XSLT language itself, here are a number of useful resources:

I strongly suggest to subscribe to XSL-list, check the XSL-list archives
The XSL FAQ.
The tutorial written by Paul Grosso and Norman Walsh is a very good on-line introdution to the language.
The only Zvon XSLT tutorial details a lot of constructs with examples.
Jeni Tennison's XSLT pages provide links to a lot of answers
the Gallery of XSLT Tricks provides non-standard use case of XSLT
And I suggest to buy Michael Kay "XSLT Programmer's Reference" book published by Wrox if you plan to work seriously with XSLT in the future.

Well, bugs or missing features are always possible, and I will make a point of fixing them in a timely fashion. The best way to report a bug is to use the GNOME bug tracking database (make sure to use the "libxslt" module name). Before filing a bug, check the list of existing libxslt bugs to make sure it hasn't already been filed. I look at reports there regularly and it's good to have a reminder when a bug is still open. Be sure to specify that the bug is for the package libxslt.

For small problems you can try to get help on IRC, the #xml channel on irc.gnome.org (port 6667) usually have a few person subscribed which may help (but there is no guarantee and if a real issue is raised it should go on the mailing-list for archival).

There is also a mailing-list xslt@gnome.org for libxslt, with an on-line archive. To subscribe to this list, please visit the associated Web page and follow the instructions.

Alternatively, you can just send the bug to the xslt@gnome.org list, if it's really libxslt related I will approve it.. Please do not send me mail directly especially for portability problem, it makes things really harder to track and in some cases I'm not the best person to answer a given question, ask the list instead. Do not send code, I won't debug it (but patches are really appreciated!).

Please note that with the current amount of virus and SPAM, sending mail to the list without being subscribed won't work. There is *far too many bounces* (in the order of a thousand a day !) I cannot approve them manually anymore. If your mail to the list bounced waiting for administrator approval, it is LOST ! Repost it and fix the problem triggering the error. Also please note that emails with a legal warning asking to not copy or redistribute freely the information they contain are NOT acceptable for the mailing-list, such mail will as much as possible be discarded automatically, and are less likely to be answered if they made it to the list, DO NOT post to the list from an email address where such legal requirements are automatically added, get private paying support if you can't share information.

Check the following too before posting:

use the search engine to get information related to your problem.
make sure you are using a recent version, and that the problem still shows up in those
check the list archives to see if the problem was reported already, in this case there is probably a fix available, similarly check the registered open bugs
make sure you can reproduce the bug with xsltproc, a very useful thing to do is run the transformation with -v argument and redirect the standard error to a file, then search in this file for the transformation logs just preceding the possible problem
Please send the command showing the error as well as the input and stylesheet (as an attachment)

Then send the bug with associated information to reproduce it to the xslt@gnome.org list; if it's really libxslt related I will approve it. Please do not send mail to me directly, it makes things really hard to track and in some cases I am not the best person to answer a given question, ask on the list.

To be really clear about support:

Support or help request MUST be sent to the list or on bugzilla in case of problems, so that the Question and Answers can be shared publicly. Failing to do so carries the implicit message "I want free support but I don't want to share the benefits with others" and is not welcome. I will automatically Carbon-Copy the xslt@gnome.org mailing list for any technical reply made about libxml2 or libxslt.
There is no guarantee for support, if your question remains unanswered after a week, repost it, making sure you gave all the detail needed and the information requested.
Failing to provide information as requested or double checking first for prior feedback also carries the implicit message "the time of the library maintainers is less valuable than my time" and might not be welcome.

Of course, bugs reports with a suggested patch for fixing them will probably be processed faster.

If you're looking for help, a quick look at the list archive may actually provide the answer, I usually send source samples when answering libxslt usage questions. The auto-generated documentation is not as polished as I would like (I need to learn more about Docbook), but it's a good starting point.

How to help

You can help the project in various ways, the best thing to do first is to subscribe to the mailing-list as explained before, check the archives and the GNOME bug database::

provide patches when you find problems
provide the diffs when you port libxslt to a new platform. They may not be integrated in all cases but help pinpointing portability problems and
provide documentation fixes (either as patches to the code comments or as HTML diffs).
provide new documentations pieces (translations, examples, etc ...)
Check the TODO file and try to close one of the items
take one of the points raised in the archive or the bug database and provide a fix. Get in touch with me before to avoid synchronization problems and check that the suggested fix will fit in nicely :-)

Downloads

The latest versions of libxslt can be found on the xmlsoft.org server. (NOTE that you need the libxml2, libxml2-devel, libxslt and libxslt-devel packages installed to compile applications using libxslt.) Igor Zlatkovic is now the maintainer of the Windows port, he provides binaries. CSW provides Solaris binaries, and Steve Ball provides Mac Os X binaries.

Snapshot:

Code from the GNOME GIT base libxslt module, updated hourly libxslt-git-snapshot.tar.gz.

Contribs:

I do accept external contributions, especially if compiling on another platform, get in touch with me to upload the package. I will keep them in the contrib directory

Libxslt is also available from GIT:

See libxslt Git web. To checkout a local tree use:
```
git clone git://git.gnome.org/libxslt
```
The libxml2 module is also present there

FAQ

Troubles compiling or linking programs using libxslt
Usually the problem comes from the fact that the compiler doesn't get the right compilation or linking flags. There is a small shell script xslt-config which is installed as part of libxslt usual install process which provides those flags. Use

xslt-config --cflags

to get the compilation flags and

xslt-config --libs

to get the linker flags. Usually this is done directly from the Makefile as:

CFLAGS=`xslt-config --cflags`

LIBS=`xslt-config --libs`

Note also that if you use the EXSLT extensions from the program then you should prepend -lexslt to the LIBS options
passing parameters on the xsltproc command line doesn't work
xsltproc --param test alpha foo.xsl foo.xml

the param does not get passed and ends up as ""

In a nutshell do a double escaping at the shell prompt:

xsltproc --param test "'alpha'" foo.xsl foo.xml

i.e. the string value is surrounded by " and ' then terminated by ' and ". Libxslt interpret the parameter values as XPath expressions, so the string ->alpha<- is intepreted as the node set matching this string. You really want ->'alpha'<- to be passed to the processor. And to allow this you need to escape the quotes at the shell level using ->"'alpha'"<- .

or use

xsltproc --stringparam test alpha foo.xsl foo.xml
Is there C++ bindings ?
Yes for example xmlwrapp , see the related pages about bindings

News

See the git page to get a description of the recent commits.

Those are the public releases made:

1.1.27: Sep 12 2012

Portability:
xincludestyle wasn't protected with LIBXML_XINCLUDE_ENABLED (Michael Bonfils),
Portability fix for testThreads.c (IlyaS),
FreeBSD portability fixes (Pedro F. Giffuni),
check for gmtime - on mingw* hosts will enable date-time function (Roumen Petrov),
use only native crypto-API for mingw* hosts (Roumen Petrov),
autogen: Only check for libtoolize (Colin Walters),
minimal mingw support (Roumen Petrov),
configure: acconfig.h is deprecated since autoconf-2.50 (Stefan Kost),
Fix a small out of tree compilation issue (Hao Hu),
Fix python generator to not use deprecated xmllib (Daniel Veillard),
link python module with python library (Frederic Crozat)
Documentation:
Tiny doc improvement (Daniel Veillard),
Various documentation fixes for docs on internals (C. M. Sperberg-McQueen)
Bug fixes:
Report errors on variable use in key (Daniel Veillard),
The XSLT namespace string is a constant one (Daniel Veillard),
Fix handling of names in xsl:attribute (Nick Wellnhofer),
Reserved namespaces in xsl:element and xsl:attribute (Nick Wellnhofer),
Null-terminate result string of cry:rc4_decrypt (Nick Wellnhofer),
EXSLT date normalization fix (James Muscat),
Exit after compilation of invalid func:result (Nick Wellnhofer),
Fix for EXSLT func:function (Nick Wellnhofer),
Rewrite EXSLT string:replace to be conformant (Nick Wellnhofer),
Avoid a heap use after free error (Chris Evans),
Fix a dictionary string usage (Chris Evans),
Output should not include extraneous newlines when indent is off (Laurence Rowe),
document('') fails to return stylesheets parsed from memory (Jason Viers),
xsltproc should return an error code if xinclude fails (Malcolm Purvis),
Forwards-compatible processing of unknown top level elements (Nick Wellnhofer),
Fix system-property with unknown namespace (Nick Wellnhofer),
Hardening of code checking node types in EXSLT (Daniel Veillard),
Hardening of code checking node types in various entry point (Daniel Veillard),
Cleanup of the pattern compilation code (Daniel Veillard),
Fix default template processing on namespace nodes (Daniel Veillard),
Fix a bug in selecting XSLT elements (Daniel Veillard),
Fixed bug #616839 (Daniel Mustieles),
Fix some case of pattern parsing errors (Abhishek Arya),
preproc: fix the build (Stefan Kost),
Fix a memory leak with xsl:number (Daniel Veillard),
Fix a problem with ESXLT date:add() with January (money_seshu Dronamraju),
Fix a memory leak if compiled with Windows locale support (Daniel Veillard),
Fix generate-id() to not expose object addresses (Daniel Veillard),
Fix curlies support in literals for non-compiled AVTs (Nick Wellnhofer),
Allow whitespace in xsl:variable with select (Nick Wellnhofer),
Small fixes to locale code (Nick Wellnhofer),
Fix bug 602515 (Nick Wellnhofer),
Fix popping of vars in xsltCompilerNodePop (Nick Wellnhofer),
Fix direct pattern matching bug (Nick Wellnhofer)
Improvements:
Add the saxon:systemId extension (Mike Hommey),
Add an append mode to document output (Daniel Veillard),
Add new tests to EXTRA_DIST (Nick Wellnhofer),
Test for bug #680920 (Nick Wellnhofer),
fix regresson in Various "make distcheck" and other fixes (Roumen Petrov),
Various "make distcheck" and other fixes (Daniel Richard G),
Fix portability to upcoming libxml2-2.9.0 (Daniel Veillard),
Adding --system flag support to autogen.sh (Daniel Veillard),
Allow per-context override of xsltMaxDepth, introduce xsltMaxVars (J�r�me Carretero),
autogen.sh: Honor NOCONFIGURE environment variable (Colin Walters),
configure: support silent automake rules if possible (Stefan Kost),
Precompile patterns in xsl:number (Nick Wellnhofer),
Fix some warnings in the refactored code (Nick Wellnhofer),
Adding new generated files (Daniel Veillard),
profiling: add callgraph report (Stefan Kost)
Cleanups:
Big space and tabs cleanup (Daniel Veillard),
Fix authors list (Daniel Veillard),
Cleanups some of the test makefiles (Daniel Richard),
Remove .cvsignore files which are not needed anymore (Daniel Veillard),
Cleanup some misplaced spaces and tabs (Daniel Veillard),
Augment list of ignored files (Daniel Veillard),
configure: remove checks for isinf and isnan as those are not used anyway (Stefan Kost),
Point to GIT for source code and a bit of cleanup (Daniel Veillard),
Get rid of specific build setup and STATIC_BINARIES (Daniel Veillard)

1.1.26: Sep 24 2009

Improvement: Add xsltProcessOneNode to exported symbols for lxml (Daniel Veillard)
Bug fixes: Fix an idness generation problem (Daniel Veillard), 595612 Try to fix some locking problems (Daniel Veillard), Fix a crash on misformed imported stylesheets (Daniel Veillard)

1.1.25: Sep 17 2009

Features: Add API versioning and various cleanups (Daniel Veillard), xsl:sort lang support using the locale (Nick Wellnhofer and Roumen Petrov)
Documentation: Fix the download links for Solaris (Daniel Veillard), Fix makefile and spec file to include doc in rpm (Daniel Veillard)
Portability: Make sure testThreads is linked with pthreads (Daniel Veillard), Fix potential crash on debug of extensions Solaris (Ben Walton), applied patch from Roumen Petrov for mingw cross compilation problems (Roumen Petrov), patch from Richard Jones to build shared libs with MinGW cross-compiler (Richard Jones), fix include path when compiling with MinGW (Roumen Petrov), portability fixes ( Nick Wellnhofer and Roumen Petrov)
Bug fixes: Big fixes of pattern compilations (Nick Wellnhofer), Fix uses of xmlAddChild for error handling (Daniel Veillard), Detect deep recusion on function calls (Daniel Veillard), Avoid an error in namespace generation (Martin), Fix importing of encoding from included stylesheets (Nick Wellnhofer), Fix problems with embedded stylesheets and namespaces (Martin), QName parsing fix for patterns (Martin), Crash compiling stylesheet with DTD (Martin), Fix xsl:strip-space with namespace and wildcard (Nick Wellnhofer), Fix a mutex deadlock on unregistered extensions (Nix), 567192 xsltproc --output option ignore --xinclude (Joachim Breitner), Fix redundant headers in list (Daniel Veillard), 134754 Configure's --with-html-dir related fixes (Julio M. Merino Vidal), 305913 a serious problem in extensions reentrancy (Daniel Veillard), Fix an idness issue when building the tree (Daniel Veillard), Fixed indexing error reported by Ron Burk on the mailing list. (William M. Brack), prevent some unchecked pointer accesses (Jake Goulding), fix for CVE-2008-2935 libexslt RC4 encryption/decryption functions Daniel (Daniel Veillard), avoid a quadratic behaviour when hitting duplicates (Daniel Veillard), 544829 fixed option --with-debugger (Arun Ragnavan), 541965 fixed incorrect argument popping in exsltMathAtan2Function (William M. Brack), fix problem with string check for element-available (Ron Burk), 539741 added code to handle literal within an AVT (William M. Brack)
Improvements: Allow use of EXSLT outside XSLT (Martin), Support Esperanto locale (Nick Wellnhofer), Change how attributes are copied for id and speed (Daniel Veillard), Add API versioning and various cleanups (Daniel Veillard), Adding a test program to check thread reentrancy (Daniel Veillard), big patch from finishing xsl:sort lang support (Roumen Petrov), add xsl:sort lang support using the locale (Nick Wellnhofer)
Cleanups: Label xsltProcessOneNode as static (Daniel Veillard), git setup (Daniel Veillard), fixed typo detected by new libxml2 code (William M. Brack), xsltExtFunctionLookup was defined but never implemented (Ralf Junker)

1.1.24: May 13 2008

Documentation: man page fix (Vincent Lefevre).
Bug fixes: pattern bug fix, key initialization problems, exclusion of unknown namespaced element on top of stylesheets, python generator syntactic cleanup (Martin)

1.1.23: Apr 8 2008

Documentation: fix links for Cygwin DocBook setup (Philippe Bourcier), xsltParseStylesheetDoc doc fix (Jason Viers), fix manpage default maxdepth value
Bug fixes: python segfault (Daniel Gryniewicz), week-in-year bug fix (Maurice van der Pot), fix python iterator problem (William Brack), avoid garbage collection problems on str:tokenize and str:split and function results (William Brack and Peter Pawlowski) superfluous re-generation of keys (William Brack), remove superfluous code in xsltExtInitTest (Tony Graham), func:result segfault fix (William Brack), timezone offset problem (Peter Pawlowski),
Portability fixes: old gcrypt support fix (Brent Cowgill), Python portability patch (Stephane Bidoul), VS 2008 fix (Rob Richard)

1.1.22: Aug 23 2007

Bug fixes: RVT cleanup problems (William Brack), exclude-result-prefix bug (William Brack), stylesheet compilation error handling (Rob Richards).
Portability fixes: improve build with VS2005 (Rob Richards), fixing build on AIX (Bjorn Wiberg), fix the security file checks on Windows (Roland Schwarz and Rob Richards).
Improvement: add an --encoding option to xsltproc (Drazen Kacar).

1.1.21: Jun 12 2007

Bug fixes: out of memory allocation errors (William Brack), namespace problem on compound predicates (William Brack), python space/tab inconsistencies (Andreas Hanke), hook xsl:message to per transformation error callbacks (Shaun McCance), cached RVT problem (William Brack), XPath context maintainance on choose (William Brack), memory leaks in the math module (William Brack), exclude-result-prefix induced namespace problem (William Brack)
Build: configure setup for TRIO_REPLACE_STDIO (William Brack)
Documentation: updated after change from CVs to SVN (William Brack)

1.1.20: Jan 17 2007

Portability fixes: strict aliasing fix (Marcus Meissner), BSD portability patches (Roland Illig)
Bug fixes: Result Value Tree handling fix (William Brack), function parameters fix (William), uninitialized variable (Kjartan Maraas), empty text node handling (William), plugin support and test fixes (William), fragment support fixes (William)
Improvements: python stylesheet compare and transform context access (Nic Ferrier), EXSLT string replace support (Joel Reed), xsltproc better low level error handling (Mike Hommey and William)

1.1.19: Nov 29 2006

Bug fixes: entities within attributes (William Brack), Python detection problem (Joseph Sacco), in-scope namespace bug (Mike Hommey), Result value tree caching bug (William Brack)

1.1.18: Oct 26 2006

portability and build fixes: DESTDIR problem, build paths in python shared lib, compile when libxml2 doesn't support XInclude (Gary Coady).
bug fixes: a number of namespace related bugs (Kasimier Buchcik), parameters bugs (Kasimier Buchcik), proximity position in predicates of match patterns (Kasimier), exslt-node-set troubles with strings (Kasimier), CDATA serialization, Python floats and booleans XPath conversions, XInclude support fixes, RVT cleanup problem (William Brack and Kasimier), attribute checking in stylesheets (Kasimier), xsltForEach context problem (Kasimier), security check should pass full URLs (Shane Corgatelli), security cleanup patch (Mikhail Zabaluev), some python accessor for stylesheet were broken, memory errors when compiling stylesheets (Mike Hommey), EXSLT current date end-of-month problem (William Brack).
improvements: refactoring of namespace handling, value-of impleemntation and template internal processing (Kasimier Buchcik), new xsltproc flag to apply Xinclude to stylesheets.
documentation: xsltproc man pages (Daniel Leidert), tests updates (William Brack), various typo fixes (Daniel Leidert), comments on versions macros (Peter Breitenlohner).

1.1.17: Jun 6 2006

portability fixes: python detection
bug fixes: some regression tests, attribute/namespaces output (Kasimier Buchcik), problem in mixed xsl:value-of and xsl:text uses (Kasimier)
improvements: internal refactoring (Kasimier Buchcik), use of the XPath object cache in libxml2-2.6.25 (Kasimier)

1.1.16: May 01 2006

portability fixes: EXSLT date/time on Solaris and IRIX (Albert Chin), HP-UX build (Albert Chin),
build fixes: Python detection(Joseph Sacco), plugin configurei (Joel Reed)
bug fixes: pattern compilation fix(William Brack), EXSLT date/time fix (Thomas Broyer), EXSLT function bug, potential loop on variable eval, startup race (Christopher Palmer), debug statement left in python (Nic Ferrier), various cleanup based on Coverity reports), error on Out of memory condition (Charles Hardin), various namespace prefixes fixes (Kasimier Buchcik),
improvement: speed up sortingi, start of internals refactoring (Kasimier Buchcik)
documentation: man page fixes and updates (Daniel Leidert)

1.1.15: Sep 04 2005

build fixes: Windows build cleanups and updates (Igor Zlatkovic), remove jhbuild warnings
bug fixes: negative number formatting (William Brack), number formatting per mille definition (William Brack), XInclude default values (William), text copy bugs (William), bug related to xmlXPathContext size, reuse libxml2 memory management for text nodes, dictionary text bug, forbid variables in match (needs libxml2-2.6.21)
improvements: EXSLT dyn:map (Mark Vakoc),
documentation: EXSLT date and time functions namespace in man (Jonathan Wakely)

1.1.14: Apr 02 2005

bug fixes: text node on stylesheet document without a dictionary (William Brack), more checking of XSLT syntax, calling xsltInit() multiple times, mode values interning raised by Mark Vakoc, bug in pattern matching with ancestors, bug in patterna matching with cascading select, xinclude and document() problem, build outside of source tree (Mike Castle)
improvement: added a --nodict mode to xsltproc to check problems for docuemtns without dictionaries

1.1.13: Mar 13 2005

build fixes: 64bits cleanup (William Brack), python 2.4 test (William), LIBXSLT_VERSION_EXTRA on Windows (William), Windows makefiles fixes (Joel Reed), libgcrypt-devel requires for RPM spec.
bug fixes: exslt day-of-week-in-month (Sal Paradise), xsl:call-template should not change the current template rule (William Brack), evaluation of global variables (William Brack), RVT's in XPath predicates (William), namespace URI on template names (Mark Vakoc), stat() for Windows patch (Aleksey Gurtovoy), pattern expression fixes (William Brack), out of memory detection misses (William), parserOptions propagation (William), exclude-result-prefixes fix (William), // patten fix (William).
extensions: module support (Joel Reed), dictionary based speedups trying to get rid of xmlStrEqual as much as possible.
documentation: added Wiki (Joel Reed)

1.1.12: Oct 29 2004

build fixes: warnings removal (William).
bug fixes: attribute document pointer fix (Mark Vakoc), exslt date negative periods (William Brack), generated tree structure fixes, namespace lookup fix, use reentrant gmtime_r (William Brack), exslt:funtion namespace fix (William), potential NULL pointer reference (Dennis Dams, William), force string interning on generated documents.
documentation: update of the second tutorial (Panagiotis Louridas), add exslt doc in rpm packages, fix the xsltproc man page.

1.1.11: Sep 29 2004

bug fixes: xsl:include problems (William Brack), UTF8 number pattern (William), date-time validation (William), namespace fix (William), various Exslt date fixes (William), error callback fixes, leak with namespaced global variable, attempt to fix a weird problem #153137
improvements: exslt:date-sum tests (Derek Poon)
documentation: second tutorial by Panagiotis Lourida

1.1.10: Aug 31 2004

build fix: NUL in c file blocking compilation on Solaris, Windows build (Igor Zlatkovic)
fix: key initialization problem (William Brack)
documentation: fixed missing man page description for --path

1.1.9: Aug 22 2004

build fixes: missing tests (William Brack), Python dependancies, Python on 64bits boxes, --with-crypto flag (Rob Richards),
fixes: RVT key handling (William), Python binding (William and Sitsofe Wheeler), key and XPath troubles (William), template priority on imports (William), str:tokenize with empty strings (William), #default namespace alias behaviour (William), doc ordering missing for main document (William), 64bit bug (Andreas Schwab)
improvements: EXSLT date:sum added (Joel Reed), hook for document loading for David Hyatt, xsltproc --nodtdattr to avoid defaulting DTD attributes, extend xsltproc --version with CVS stamp (William).
Documentation: web page problem reported by Oliver Stoeneberg

1.1.8: July 5 2004

build fixes: Windows runtime options (Oliver Stoeneberg), Windows binary package layout (Igor Zlatkovic), libgcrypt version test and link (William)
documentation: fix libxslt namespace name in doc (William)
bug fixes: undefined namespace message (William Brack), search engine (William), multiple namespace fixups (William), namespace fix for key evaluation (William), Python memory debug bindings,
improvements: crypto extensions for exslt (Joel Reed, William)

1.1.7: May 17 2004

build fix: warning about localtime_r on Solaris
bug fix: UTF8 string tokenize (William Brack), subtle memory corruption, linefeed after comment at document level (William), disable-output-escaping problem (William), pattern compilation in deep imported stylesheets (William), namespace extension prefix bug, libxslt.m4 bug (Edward Rudd), namespace lookup for attribute, namespaced DOCTYPE name

1.1.6: Apr 18 2004

2 bug fixes about keys fixed one by Mark Vakoc

1.1.5: Mar 23 2004

performance: use dictionary lookup for variables
remove use of _private from source documents
cleanup of "make tests" output
bugfixes: AVT in local variables, use localtime_r to avoid thread troubles (William), dictionary handling bug (William), limited number of stubstitutions in AVT (William), tokenize fix for UTF-8 (William), superfluous namespace (William), xsltproc error code on <xsl:message> halt, OpenVMS fix, dictionary reference counting change.

1.1.4: Feb 23 2004

bugfixes: attributes without doc (Mariano Su�rez-Alvarez), problem with Yelp, extension problem
display extension modules (Steve Little)
Windows compilation patch (Mark Vadoc), Mingw (Mikhail Grushinskiy)

1.1.3: Feb 16 2004

Rewrote the Attribute Value Template code, new XPath compilation interfaces, dictionary reuses for XSLT with potential for serious performance improvements.
bug fixes: portability (William Brack), key() in node-set() results (William), comment before doctype (William), math and node-set() problems (William), cdata element and default namespace (William), behaviour on unknown XSLT elements (Stefan Kost), priority of "//foo" patterns (William), xsl:element and xsl:attribute QName check (William), comments with -- (William), attribute namespace (William), check for ?> in PI (William)
Documentations: cleanup (John Fleck and William)
Python: patch for OS-X (Gianni Ceccarelli), enums export (Stephane bidoul)

1.1.2: Dec 24 2003

Documentation fixes (John Fleck, William Brack), EXSLT documentation (William Brack)
Windows compilation fixes for MSVC and Mingw (Igor Zlatkovic)
Bug fixes: exslt:date returning NULL strings (William Brack), namespaces output (William Brack), key and namespace definition problem, passing options down to the document() parser, xsl:number fixes (William Brack)

1.1.1: Dec 10 2003

code cleanup (William Brack)
Windows: Makefile improvements (Igor Zlatkovic)
documentation improvements: William Brack, libexslt man page (Jonathan Wakely)
param in EXSLT functions (Shaun McCance)
XSLT debugging improvements (Mark Vakoc)
bug fixes: number formatting (Bjorn Reese), exslt:tokenize (William Brack), key selector parsing with | reported by Oleg Paraschenko, xsl:element with computed namespaces (William Brack), xslt:import/include recursion detection (William Brack), exslt:function used in keys (William Brack), bug when CDATA_SECTION are foun in the tree (William Brack), entities handling when using XInclude.

1.1.0: Nov 4 2003

Removed DocBook SGML broken support
fix xsl:key to work with PIs
Makefile and build improvement (Graham Wilson), build cleanup (William Brack), macro fix (Justin Fletcher), build outside of source tree (Roumen Petrov)
xsltproc option display fix (Alexey Efimov), --load-trace (Crutcher Dunnavant)
Python: never use stdout for error
extension memory error fix (Karl Eichwalder)
header path fixes (Steve Ball)
added saxon:line-number() to libexslt (Brett Kail)
Fix some tortuous template problems when using predicates (William Brack)
Debugger status patch (Kasimier Buchcik)
Use new libxml2-2.6.x APIs for faster processing
Make sure xsl:sort is empty
Fixed a bug in default processing of attributes
Removes the deprecated breakpoint library
detect invalid names on templates (William Brack)
fix exslt:document (and similar) base handling problem

1.0.33: Sep 12 2003

This is a bugfix only release

error message missing argument (William Brack)
mode not cascaded in template fallbacks (William Brack)
catch redefinition of parameter/variables (William Brack)
multiple keys with same namespace name (William Brack)
patch for compilation using MingW on Windows (Mikhail Grushinskiy)
header export macros for Windows (Igor Zlatkovic)
cdata-section-elements handling of namespaced names
compilation without libxml2 XPointer support (Mark Vadoc)
apply-templates crash (William Brack)
bug with imported templates (William Brack)
imported attribute-sets merging bug (DocBook) (William Brack)

1.0.32: Aug 9 2003

bugfixes: xsltSaveResultToFile() python binding (Chris Jaeger), EXSLT function (William Brack), RVT for globals (William Brack), EXSLT date (William Brack),
speed of large text output, xsl:copy with attributes, strip-space and namespaces prefix, fix for --path xsltproc option, EXST:tokenize (Shaun McCance), EXSLT:seconds (William Brack), sort with multiple keys (William Brack), checking of { and } for attribute value templates (William Brack)
Python bindings for extension elements (Sean Treadway)
EXSLT:split added (Shaun McCance)
portability fixes for HP-UX/Solaris/IRIX (William Brack)
doc cleanup

1.0.31: Jul 6 2003

bugfixes: xsl:copy on namespace nodes, AVT for xsl:sort order, fix for the debugger (Keith Isdale), output filename limitation, trio.h and triodef.h added (Albert Chin), EXSLT node-set (Peter Breitenlohner), xsltChoose and whitespace (Igor Zlatkovic),
stylesheet compilation (Igor Zlatkovic), NaN and sort (William Brack), RVT bug introduced in 1.0.30
avoid generating " (fix in libxml2-2.5.8)
fix 64bit cleaness problem and compilation troubles introduced in 1.0.30
Windows makefile generation (Igor Zlatkovic)
HP-UX portability fix

1.0.30: May 4 2003

Fixes and new APIs to handle Result Value Trees and avoid leaks
Fixes for: EXSLT math pow() function (Charles Bozeman), global parameter and global variables mismatch, a segfault on pattern compilation errors, namespace copy in xsl:copy-of, python generator problem, OpenVMS trio update, premature call to xsltFreeStackElem (Igor), current node when templates applies to attributes

1.0.29: Apr 1 2003

performance improvements especially for large flat documents
bug fixes: Result Value Tree handling, XML IDs, keys(), extra namespace declarations with xsl:elements.
portability: python and trio fixes (Albert Chin), python on Solaris (Ben Phillips)

1.0.28: Mar 24 2003

fixed node() in patterns semantic.
fixed a memory access problem in format-number()
fixed stack overflow in recursive global variable or params
cleaned up Result Value Tree handling, and fixed a couple of old bugs in the process

1.0.27: Feb 24 2003

bug fixes: spurious xmlns:nsX="" generation, serialization bug (in libxml2), a namespace copy problem, errors in the RPM spec prereqs
Windows path canonicalization and document cache fix (Igor)

1.0.26: Feb 10 2003

Fixed 3 serious bugs in document() and stylesheet compilation which could lead to a crash

1.0.25: Feb 5 2003

Bug fix: double-free for standalone stylesheets introduced in 1.0.24, C syntax pbm, 3 bugs reported by Eric van der Vlist
Some XPath and XInclude related problems were actually fixed in libxml2-2.5.2
Documentation: emphasize taht --docbook is not for XML docs.

1.0.24: Jan 14 2003

bug fixes: imported global varables, python bindings (St�phane Bidoul), EXSLT memory leak (Charles Bozeman), namespace generation on xsl:attribute, space handling with imports (Daniel Stodden), extension-element-prefixes (Josh Parsons), comments within xsl:text (Matt Sergeant), superfluous xmlns generation, XInclude related bug for numbering, EXSLT strings (Alexey Efimov), attribute-sets computation on imports, extension module init and shutdown callbacks not called
HP-UX portability (Alexey Efimov), Windows makefiles (Igor and Stephane Bidoul), VMS makefile updates (Craig A. Berry)
adds xsltGetProfileInformation() (Michael Rothwell)
fix the API generation scripts
API to provide the sorting routines (Richard Jinks)
added XML description of the EXSLT API
added ESXLT URI (un)escaping (J�rg Walter)
Some memory leaks have been found and fixed
document() now support fragment identifiers in URIs

1.0.23: Nov 17 2002

Windows build cleanup (Igor)
Unix build and RPM packaging cleanup
Improvement of the python bindings: extension functions and activating EXSLT
various bug fixes: number formatting, portability for bounded string functions, CData nodes, key(), @*[...] patterns
Documentation improvements (John Fleck)
added libxslt.m4 (Thomas Schraitle)

1.0.22: Oct 18 2002

Updates on the Windows Makefiles
Added a security module, and a related set of new options to xsltproc
Allowed per transformation error handler.
Fixed a few bugs: node() semantic, URI escaping, media-type, attribute lists

1.0.21: Sep 26 2002

Bug fixes: match="node()", date:difference() (Igor and Charlie Bozeman), disable-output-escaping
Python bindings: style.saveResultToString() from Ralf Mattes
Logos from Marc Liyanage
Mem leak fix from Nathan Myers
Makefile: DESTDIR fix from Christophe Merlet, AMD x86_64 (Mandrake), Windows (Igor), Python detection
Documentation improvements: John Fleck

1.0.20: Aug 23 2002

Windows makefile updates (Igor) and x86-64 (Frederic Crozat)
fixed HTML meta tag saving for Mac/IE users
possible leak patches from Nathan Myers
try to handle document('') as best as possible depending in the cases
Fixed the DocBook stylesheets handling problem
Fixed a few XSLT reported errors

1.0.19: July 6 2002

EXSLT: dynamic functions and date support bug fixes (Mark Vakoc)
xsl:number fix: Richard Jinks
xsl:format-numbers fix: Ken Neighbors
document('') fix: bug pointed by Eric van der Vlist
xsl:message with terminate="yes" fixes: William Brack
xsl:sort order support added: Ken Neighbors
a few other bug fixes, some of them requiring the latest version of libxml2

1.0.18: May 27 2002

a number of bug fixes: attributes, extra namespace declarations (DocBook), xsl:include crash (Igor), documentation (Christian Cornelssen, Charles Bozeman and Geert Kloosterman), element-available (Richard Jinks)
xsltproc can now list teh registered extensions thanks to Mark Vakoc
there is a new API to save directly to a string xsltSaveResultToString() by Morus Walter
specific error registration function for the python API

1.0.17: April 29 2002

cleanup in code, XSLT debugger support and Makefiles for Windows by Igor
a C++ portability fix by Mark Vakoc
EXSLT date improvement and regression tests by Charles Bozeman
attempt to fix a bug in xsltProcessUserParamInternal

1.0.16: April 15 2002

Bug fixes: strip-space, URL in HTML output, error when xsltproc can't save
portability fixes: OSF/1, IEEE on alphas, Windows, Python bindings

1.0.15: Mar 25 2002

Bugfixes: XPath, python Makefile, recursive attribute sets, @foo[..] templates
Debug of memory alocation with valgind
serious profiling leading to significant improvement for DocBook processing
revamp of the Windows build

1.0.14: Mar 18 2002

Improvement in the XPath engine (libxml2-2.4.18)
Nasty bug fix related to exslt:node-set
Fixed the python Makefiles, cleanup of doc comments, Windows portability fixes

1.0.13: Mar 8 2002

a number of bug fixes including "namespace node have no parents"
Improvement of the Python bindings
Charles Bozeman provided fixes and regression tests for exslt date functions.

1.0.12: Feb 11 2002

Fixed the makefiles especially the python module ones
half a dozen bugs fixes including 2 old ones

1.0.11: Feb 8 2002

Change of Licence to the MIT Licence
Added a beta version of the Python bindings, including support to extend the engine with functions written in Python
A number of bug fixes
Charlie Bozeman provided more EXSLT functions
Portability fixes

1.0.10: Jan 14 2002

Windows fixes for Win32 from Igor
Fixed the Solaris compilation trouble (Albert)
Documentation changes and updates: John Fleck
Added a stringparam option to avoid escaping hell at the shell level
A few bug fixes

1.0.9: Dec 7 2001

Makefile patches from Peter Williams
attempt to fix the compilation problem associated to prelinking
obsoleted libxsltbreakpoint now deprecated and frozen to 1.0.8 API
xsltproc return codes are now significant, John Fleck updated the documentation
patch to allow as much as 40 steps in patterns (Marc Tardif), should be made dynamic really
fixed a bug raised by Nik Clayton when using doctypes with HTML output
patches from Keith Isdale to interface with xsltdebugger

1.0.8: Nov 26 2001

fixed an annoying header problem, removed a few bugs and some code cleanup
patches for Windows and update of Windows Makefiles by Igor
OpenVMS port instructions from John A Fotheringham
fixed some Makefiles annoyance and libraries prelinking information

1.0.7: Nov 10 2001

remove a compilation problem with LIBXSLT_PUBLIC
Finishing the integration steps for Keith Isdale debugger
fixes the handling of indent="no" on HTML output
fixes on the configure script and RPM spec file

1.0.6: Oct 30 2001

bug fixes on number formatting (Thomas), date/time functions (Bruce Miller)
update of the Windows Makefiles (Igor)
fixed DOCTYPE generation rules for HTML output (me)

1.0.5: Oct 10 2001

some portability fixes, including Windows makefile updates from Igor
fixed a dozen bugs on XSLT and EXSLT (me and Thomas Broyer)
support for Saxon's evaluate and expressions extensions added (initial contribution from Darren Graves)
better handling of XPath evaluation errors

1.0.4: Sep 12 2001

Documentation updates from John fleck
bug fixes (DocBook FO generation should be fixed) and portability improvements
Thomas Broyer improved the existing EXSLT support and added String, Time and Date core functions support

1.0.3: Aug 23 2001

XML Catalog support see the doc
New NaN/Infinity floating point code
A few bug fixes

1.0.2: Aug 15 2001

lot of bug fixes, increased the testsuite
a large chunk of EXSLT is implemented
improvements on the extension framework
documentation improvements
Windows MSC projects files should be up-to-date
handle attributes inherited from the DTD by default

1.0.1: July 24 2001

initial EXSLT framework
better error reporting
fixed the profiler on Windows
bug fixes

1.0.0: July 10 2001

a lot of cleanup, a lot of regression tests added or fixed
added a documentation for writing extensions
fixed some variable evaluation problems (with William)
added profiling of stylesheet execution accessible as the xsltproc --profile option
fixed element-available() and the implementation of the various chunking methods present, Norm Walsh provided a lot of feedback
exclude-result-prefixes and namespaces output should now work as expected
added support of embedded stylesheet as described in section 2.7 of the spec

0.14.0: July 5 2001

lot of bug fixes, and code cleanup
completion of the little XSLT-1.0 features left unimplemented
Added and implemented the extension API suggested by Thomas Broyer
the Windows MSC environment should be complete
tested and optimized with a really large document (DocBook Definitive Guide) libxml/libxslt should really be faster on serious workloads

0.13.0: June 26 2001

lots of cleanups
fixed a C++ compilation problem
couple of fixes to xsltSaveTo()
try to fix Docbook-xslt-1.4 and chunking, updated the regression test with them
fixed pattern compilation and priorities problems
Patches for Windows and MSC project mostly contributed by Yon Derek
update to the Tutorial by John Fleck
William fixed bugs in templates and for-each functions
added a new interface xsltRunStylesheet() for a more flexible output (incomplete), added -o option to xsltproc

0.12.0: June 18 2001

fixed a dozen of bugs reported
HTML generation should be quite better (requires libxml-2.3.11 upgrade too)
William fixed some problems with document()
Fix namespace nodes selection and copy (requires libxml-2.3.11 upgrade too)
John Fleck added a tutorial
Fixes for namespace handling when evaluating variables
XInclude global flag added to process XInclude on document() if requested
made xsltproc --version more detailed

0.11.0: June 1 2001

Mostly a bug fix release.

integration of catalogs from xsltproc
added --version to xsltproc for bug reporting
fixed errors when handling ID in external parsed entities
document() should hopefully work correctly but ...
fixed bug with PI and comments processing
William fixed the XPath string functions when using unicode

0.10.0: May 19 2001

cleanups to make stylesheet read-only (not 100% complete)
fixed URI resolution in document()
force all XPath expression to be compiled at stylesheet parsing time, even if unused ...
Fixed HTML default output detection
Fixed double attribute generation #54446
Fixed {{ handling in attributes #54451
More tests and speedups for DocBook document transformations
Fixed a really bad race like bug in xsltCopyTreeList()
added a documentation on the libxslt internals
William Brack and Bjorn Reese improved format-number()
Fixed multiple sort, it should really work now
added a --docbook option for SGML DocBook input (hackish)
a number of other bug fixes and regression test added as people were submitting them

0.9.0: May 3 2001

lot of various bugfixes, extended the regression suite
xsltproc should work with multiple params
added an option to use xsltproc with HTML input
improved the stylesheet compilation, processing of complex stylesheets should be faster
using the same stylesheet for concurrent processing on multithreaded programs should work now
fixed another batch of namespace handling problems
Implemented multiple level of sorting

0.8.0: Apr 22 2001

fixed ansidecl.h problem
fixed unparsed-entity-uri() and generate-id()
sort semantic fixes and priority prob from William M. Brack
fixed namespace handling problems in XPath expression computations (requires libxml-2.3.7)
fixes to current() and key()
other, smaller fixes, lots of testing with N Walsh DocBook HTML stylesheets

0.7.0: Apr 10 2001

cleanup using stricter compiler flags
command line parameter passing
fix to xsltApplyTemplates from William M. Brack
added the XSLTMark in the regression tests as well as document()

0.6.0: Mar 22 2001

another beta
requires 2.3.5, which provide XPath expression compilation support
document() extension should function properly
fixed a number or reported bugs

0.5.0: Mar 10 2001

fifth beta
some optimization work, for the moment 2 XSLT transform cannot use the same stylesheet at the same time (to be fixed)
fixed problems with handling of tree results
fixed a reported strip-spaces problem
added more reported/fixed bugs to the test suite
incorporated William M. Brack fix for imports and global variables as well as patch for with-param support in apply-templates
a bug fix on for-each

0.4.0: Mar 1 2001

fourth beta test, released at the same time of libxml2-2.3.3
bug fixes
some optimization
started implement extension support, not finished
implemented but not tested multiple file output

0.3.0: Feb 24 2001

third beta test, released at the same time of libxml2-2.3.2
lot of bug fixes
some optimization
added DocBook XSL based testsuite

0.2.0: Feb 15 2001

second beta version, released at the same time as libxml2-2.3.1
getting close to feature completion, lot of bug fixes, some in the HTML and XPath support of libxml
start becoming usable for real work. This version can now regenerate the XML 2e HTML from the original XML sources and the associated stylesheets (in section I of the XML REC)
Still misses extension element/function/prefixes support. Support of key() and document() is not complete

0.1.0: Feb 8 2001

first beta version, released at the same time as libxml2-2.3.0
lots of bug fixes, first "testing" version, but incomplete

0.0.1: Jan 25 2001

first alpha version released at the same time as libxml2-2.2.12
Framework in place, should work on simple examples, but far from being feature complete

The xsltproc tool

This program is the simplest way to use libxslt: from the command line. It is also used for doing the regression tests of the library.

It takes as first argument the path or URL to an XSLT stylesheet, the next arguments are filenames or URIs of the inputs to be processed. The output of the processing is redirected on the standard output. There is actually a few more options available:

orchis:~ -> xsltproc
Usage: xsltproc [options] stylesheet file [file ...]
   Options:
      --version or -V: show the version of libxml and libxslt used
      --verbose or -v: show logs of what's happening
      --output file or -o file: save to a given file
      --timing: display the time used
      --repeat: run the transformation 20 times
      --debug: dump the tree of the result instead
      --novalid: skip the DTD loading phase
      --noout: do not dump the result
      --maxdepth val : increase the maximum depth
      --html: the input document is(are) an HTML file(s)
      --docbook: the input document is SGML docbook
      --param name value : pass a (parameter,value) pair
      --nonet refuse to fetch DTDs or entities over network
      --warnnet warn against fetching over the network
      --catalogs : use the catalogs from $SGML_CATALOG_FILES
      --xinclude : do XInclude processing on document input
      --profile or --norman : dump profiling information 
orchis:~ ->

DocBook

The duck picture

DocBook is an XML/SGML vocabulary particularly well suited to books and papers about computer hardware and software.

xsltproc and libxslt are not specifically dependant on DocBook, but since a lot of people use xsltproc and libxml2 for DocBook formatting, here are a few pointers and information which may be helpful:

The DocBook homepage at Oasis you should find pointers there on all the lastest versions of the DTDs and XSLT stylesheets
DocBook: The Definitive Guide is the official reference documentation for DocBook.
DocBook Open Repository contains a lot of information about DocBook
Bob Stayton provides a lot of resources and consulting services around DocBook.
Here is a shell script to generate XML Catalogs for DocBook 4.1.2 . If it can write to the /etc/xml/ directory, it will set-up /etc/xml/catalog and /etc/xml/docbook based on the resources found on the system. Otherwise it will just create ~/xmlcatalog and ~/dbkxmlcatalog and doing:
export XMLCATALOG=$HOME/xmlcatalog

should allow to process DocBook documentations without requiring network accesses for the DTd or stylesheets
I have uploaded a small tarball containing XML Catalogs for DocBook 4.1.2 which seems to work fine for me too
Informations on installing a Windows DocBook processing setup based on Cygwin (using the binaries from the official Windows port should be possible too)
Alexander Kirillov's page on Using DocBook XML 4.1.2 (RPM packages)
Tim Waugh's xmlto front-end conversion script
Linux Documentation Project DocBook-Install-mini-HOWTO
ScrollKeeper the open documentation cataloging project has a DocBook section
Dan York presentation on Publishing using DocBook XML

Do not use the --docbook option of xsltproc to process XML DocBook documents, this option is only intended to provide some (limited) support of the SGML version of DocBook.

Points which are not DocBook specific but still worth mentionning again:

if you think DocBook processing time is too slow, make sure you have XML Catalogs pointing to a local installation of the DTD of DocBook. Check the XML Catalog page to understand more on this subject.
before processing a new document, use the command
xmllint --valid --noout path_to_document

to make sure that your input is valid DocBook. And fixes the errors before processing further. Note that XSLT processing may work correctly with some forms of validity errors left, but in general it can give troubles on output.

The programming API

Okay this section is clearly incomplete. But integrating libxslt into your application should be relatively easy. First check the few steps described below, then for more detailed information, look at the generated pages for the API and the source of libxslt/xsltproc.c and the tutorial.

Basically doing an XSLT transformation can be done in a few steps:

configure the parser for XSLT:
xmlSubstituteEntitiesDefault(1);

xmlLoadExtDtdDefaultValue = 1;
parse the stylesheet with xsltParseStylesheetFile()
parse the document with xmlParseFile()
apply the stylesheet using xsltApplyStylesheet()
save the result using xsltSaveResultToFile() if needed set xmlIndentTreeOutput to 1

Steps 2,3, and 5 will probably need to be changed depending on you processing needs and environment for example if reading/saving from/to memory, or if you want to apply XInclude processing to the stylesheet or input documents.

Python and bindings

There is a number of language bindings and wrappers available for libxml2, the list below is not exhaustive. Please contact the xml-bindings@gnome.org (archives) in order to get updates to this list or to discuss the specific topic of libxml2 or libxslt wrappers or bindings:

Matt Sergeant developped XML::LibXML and XML::LibXSLT, Perl wrappers for libxml2/libxslt as part of the AxKit XML application server
Dave Kuhlman provides and earlier version of the libxml/libxslt wrappers for Python
Petr Kozelka provides Pascal units to glue libxml2 with Kylix, Delphi and other Pascal compilers
Wai-Sun "Squidster" Chia provides bindings for Ruby and libxml2 bindings are also available in Ruby through the libgdome-ruby module maintained by Tobias Peters.
Steve Ball and contributors maintains libxml2 and libxslt bindings for Tcl
Peter Jones maintains C++ bindings for libxslt within xmlwrapp
Mike Phillips provides a module using libxslt for PHP.
LibxmlJ is an effort to create a 100% JAXP-compatible Java wrapper for libxml2 and libxslt as part of GNU ClasspathX project.
Patrick McPhee provides Rexx bindings fof libxml2 and libxslt, look for RexxXML.
Satimage provides XMLLib osax. This is an osax for Mac OS X with a set of commands to implement in AppleScript the XML DOM, XPATH and XSLT.

The libxslt Python module depends on the libxml2 Python module.

The distribution includes a set of Python bindings, which are guaranteed to be maintained as part of the library in the future, though the Python interface have not yet reached the completeness of the C API.

St�phane Bidoul maintains a Windows port of the Python bindings.

Note to people interested in building bindings, the API is formalized as an XML API description file which allows to automate a large part of the Python bindings, this includes function descriptions, enums, structures, typedefs, etc... The Python script used to build the bindings is python/generator.py in the source distribution.

To install the Python bindings there are 2 options:

If you use an RPM based distribution, simply install the libxml2-python RPM and the libxslt-python RPM.
Otherwise use the libxml2-python module distribution corresponding to your installed version of libxml2 and libxslt. Note that to install it you will need both libxml2 and libxslt installed and run "python setup.py build install" in the module tree.

The distribution includes a set of examples and regression tests for the python bindings in the python/tests directory. Here are some excepts from those tests:

basic.py:

This is a basic test of XSLT interfaces: loading a stylesheet and a document, transforming the document and saving the result.

import libxml2
import libxslt

styledoc = libxml2.parseFile("test.xsl")
style = libxslt.parseStylesheetDoc(styledoc)
doc = libxml2.parseFile("test.xml")
result = style.applyStylesheet(doc, None)
style.saveResultToFilename("foo", result, 0)
style.freeStylesheet()
doc.freeDoc()
result.freeDoc()

The Python module is called libxslt, you will also need the libxml2 module for the operations on XML trees. Let's have a look at the objects manipulated in that example and how is the processing done:

styledoc : is a libxml2 document tree. It is obtained by parsing the XML file "test.xsl" containing the stylesheet.
style : this is a precompiled stylesheet ready to be used by the following transformations (note the plural form, multiple transformations can resuse the same stylesheet).
doc : this is the document to apply the transformation to. In this case it is simply generated by parsing it from a file but any other processing is possible as long as one get a libxml2 Doc. Note that HTML tree are suitable for XSLT processing in libxslt. This is actually how this page is generated !
result : this is a document generated by applying the stylesheet to the document. Note that some of the stylesheet information may be related to the serialization of that document and as in this example a specific saveResultToFilename() method of the stylesheet should be used to save it to a file (in that case to "foo").

Also note the need to explicitely deallocate documents with freeDoc() except for the stylesheet document which is freed when its compiled form is garbage collected.

extfunc.py:

This one is a far more complex test. It shows how to modify the behaviour of an XSLT transformation by passing parameters and how to extend the XSLT engine with functions defined in python:

import libxml2
import libxslt
import string

nodeName = None
def f(ctx, str):
    global nodeName

    #
    # Small check to verify the context is correcly accessed
    #
    try:
        pctxt = libxslt.xpathParserContext(_obj=ctx)
        ctxt = pctxt.context()
        tctxt = ctxt.transformContext()
        nodeName = tctxt.insertNode().name
    except:
        pass

    return string.upper(str)

libxslt.registerExtModuleFunction("foo", "http://example.com/foo", f)

This code defines and register an extension function. Note that the function can be bound to any name (foo) and how the binding is also associated to a namespace name "http://example.com/foo". From an XSLT point of view the function just returns an upper case version of the string passed as a parameter. But the first part of the function also read some contextual information from the current XSLT processing environement, in that case it looks for the current insertion node in the resulting output (either the resulting document or the Result Value Tree being generated), and saves it to a global variable for checking that the access actually worked.

For more information on the xpathParserContext and transformContext objects check the libray internals description. The pctxt is actually an object from a class derived from the libxml2.xpathParserContext() with just a couple more properties including the possibility to look up the XSLT transformation context from the XPath context.

styledoc = libxml2.parseDoc("""
<xsl:stylesheet version='1.0'
  xmlns:xsl='http://www.w3.org/1999/XSL/Transform'
  xmlns:foo='http://example.com/foo'
  xsl:exclude-result-prefixes='foo'>

  <xsl:param name='bar'>failure</xsl:param>
  <xsl:template match='/'>
    <article><xsl:value-of select='foo:foo($bar)'/></article>
  </xsl:template>
</xsl:stylesheet>
""")

Here is a simple example of how to read an XML document from a python string with libxml2. Note how this stylesheet:

Uses a global parameter bar
Reference the extension function f
how the Namespace name "http://example.com/foo" has to be bound to a prefix
how that prefix is excluded from the output
how the function is called from the select

style = libxslt.parseStylesheetDoc(styledoc)
doc = libxml2.parseDoc("<doc/>")
result = style.applyStylesheet(doc, { "bar": "'success'" })
style.freeStylesheet()
doc.freeDoc()

that part is identical, to the basic example except that the transformation is passed a dictionary of parameters. Note that the string passed "success" had to be quoted, otherwise it is interpreted as an XPath query for the childs of root named "success".

root = result.children
if root.name != "article":
    print "Unexpected root node name"
    sys.exit(1)
if root.content != "SUCCESS":
    print "Unexpected root node content, extension function failed"
    sys.exit(1)
if nodeName != 'article':
    print "The function callback failed to access its context"
    sys.exit(1)

result.freeDoc()

That part just verifies that the transformation worked, that the parameter got properly passed to the engine, that the function f() got called and that it properly accessed the context to find the name of the insertion node.

pyxsltproc.py:

this module is a bit too long to be described there but it is basically a rewrite of the xsltproc command line interface of libxslt in Python. It provides nearly all the functionalities of xsltproc and can be used as a base module to write Python customized XSLT processors. One of the thing to notice are:

libxml2.lineNumbersDefault(1)
libxml2.substituteEntitiesDefault(1)

those two calls in the main() function are needed to force the libxml2 processor to generate DOM trees compliant with the XPath data model.

Library internals

Introduction

This document describes the processing of libxslt, the XSLT C library developed for the GNOME project.

Note: this documentation is by definition incomplete and I am not good at spelling, grammar, so patches and suggestions are really welcome.

Basics

XSLT is a transformation language. It takes an input document and a stylesheet document and generates an output document:

the XSLT processing model

Libxslt is written in C. It relies on libxml, the XML C library for GNOME, for the following operations:

parsing files
building the in-memory DOM structure associated with the documents handled
the XPath implementation
serializing back the result document to XML and HTML. (Text is handled directly.)

Keep it simple stupid

Libxslt is not very specialized. It is built under the assumption that all nodes from the source and output document can fit in the virtual memory of the system. There is a big trade-off there. It is fine for reasonably sized documents but may not be suitable for large sets of data. The gain is that it can be used in a relatively versatile way. The input or output may never be serialized, but the size of documents it can handle are limited by the size of the memory available.

More specialized memory handling approaches are possible, like building the input tree from a serialization progressively as it is consumed, factoring repetitive patterns, or even on-the-fly generation of the output as the input is parsed but it is possible only for a limited subset of the stylesheets. In general the implementation of libxslt follows the following pattern:

KISS (keep it simple stupid)
when there is a clear bottleneck optimize on top of this simple framework and refine only as much as is needed to reach the expected result

The result is not that bad, clearly one can do a better job but more specialized too. Most optimization like building the tree on-demand would need serious changes to the libxml XPath framework. An easy step would be to serialize the output directly (or call a set of SAX-like output handler to keep this a flexible interface) and hence avoid the memory consumption of the result.

The libxml nodes

DOM-like trees, as used and generated by libxml and libxslt, are relatively complex. Most node types follow the given structure except a few variations depending on the node type:

description of a libxml node

Nodes carry a name and the node type indicates the kind of node it represents, the most common ones are:

document nodes
element nodes
text nodes

For the XSLT processing, entity nodes should not be generated (i.e. they should be replaced by their content). Most nodes also contains the following "navigation" information:

the containing document
the parent node
the first children node
the last children node
the previous sibling
the following sibling (next)

Elements nodes carries the list of attributes in the properties, an attribute itself holds the navigation pointers and the children list (the attribute value is not represented as a simple string to allow usage of entities references).

The ns points to the namespace declaration for the namespace associated to the node, nsDef is the linked list of namespace declaration present on element nodes.

Most nodes also carry an _private pointer which can be used by the application to hold specific data on this node.

The XSLT processing steps

There are a few steps which are clearly decoupled at the interface level:

parse the stylesheet and generate a DOM tree
take the stylesheet tree and build a compiled version of it (the compilation phase)
take the input and generate a DOM tree
process the stylesheet against the input tree and generate an output tree
serialize the output tree

A few things should be noted here:

the steps 1/ 3/ and 5/ are optional: the DOM representing the stylesheet and input can be created by other means, not just by parsing serialized XML documents, and similarly the result tree DOM can be made available to other processeswithout being serialized.
the stylesheet obtained at 2/ can be reused by multiple processing 4/ (and this should also work in threaded programs)
the tree provided in 2/ should never be freed using xmlFreeDoc, but by freeing the stylesheet.
the input tree created in step 3/ is not modified except the _private field which may be used for labelling keys if used by the stylesheet. It's not modified at all in step 4/ to allow parallel processing using a shared precompiled stylesheet.

The XSLT stylesheet compilation

This is the second step described. It takes a stylesheet tree, and "compiles" it. This associates to each node a structure stored in the _private field and containing information computed in the stylesheet:

a compiled XSLT stylesheet

One xsltStylesheet structure is generated per document parsed for the stylesheet. XSLT documents allow includes and imports of other documents, imports are stored in the imports list (hence keeping the tree hierarchy of includes which is very important for a proper XSLT processing model) and includes are stored in the doclist list. An imported stylesheet has a parent link to allow browsing of the tree.

The DOM tree associated to the document is stored in doc. It is preprocessed to remove ignorable empty nodes and all the nodes in the XSLT namespace are subject to precomputing. This usually consist of extracting all the context information from the context tree (attributes, namespaces, XPath expressions), and storing them in an xsltStylePreComp structure associated to the _private field of the node.

A couple of notable exceptions to this are XSLT template nodes (more on this later) and attribute value templates. If they are actually templates, the value cannot be computed at compilation time. (Some preprocessing could be done like isolation and preparsing of the XPath subexpressions but it's not done, yet.)

The xsltStylePreComp structure also allows storing of the precompiled form of an XPath expression that can be associated to an XSLT element (more on this later).

The XSLT template compilation

A proper handling of templates lookup is one of the keys of fast XSLT processing. (Given a node in the source document this is the process of finding which templates should be applied to this node.) Libxslt follows the hint suggested in the 5.2 Patterns section of the XSLT Recommendation, i.e. it doesn't evaluate it as an XPath expression but tokenizes it and compiles it as a set of rules to be evaluated on a candidate node. There usually is an indication of the node name in the last step of this evaluation and this is used as a key check for the match. As a result libxslt builds a relatively more complex set of structures for the templates:

The templates related structure

Let's describe a bit more closely what is built. First the xsltStylesheet structure holds a pointer to the template hash table. All the XSLT patterns compiled in this stylesheet are indexed by the value of the the target element (or attribute, pi ...) name, so when a element or an attribute "foo" needs to be processed the lookup is done using the name as a key.

Each of the patterns is compiled into an xsltCompMatch (i.e. an ''XSLT compiled match') structure. It holds the set of rules based on the tokenization of the pattern stored in reverse order (matching is easier this way).

The xsltCompMatch are then stored in the hash table, the clash list is itself sorted by priority of the template to implement "naturally" the XSLT priority rules.

Associated to the compiled pattern is the xsltTemplate itself containing the information required for the processing of the pattern including, of course, a pointer to the list of elements used for building the pattern result.

Last but not least a number of patterns do not fit in the hash table because they are not associated to a name, this is the case for patterns applying to the root, any element, any attributes, text nodes, pi nodes, keys etc. Those are stored independently in the stylesheet structure as separate linked lists of xsltCompMatch.

The processing itself

The processing is defined by the XSLT specification (the basis of the algorithm is explained in the Introduction section). Basically it works by taking the root of the input document as the cureent node and applying the following algorithm:

Finding the template applying to current node. This is a lookup in the template hash table, walking the hash list until the node satisfies all the steps of the pattern, then checking the appropriate global template(s) (i.e. templates applying to a node type) to see if there isn't a higher priority rule to apply
If there is no template, apply the default rule (recurse on the children as the current node)
else walk the content list of the selected templates, for each of them:
- if the node is in the XSLT namespace then the node has a _private field pointing to the preprocessed values, jump to the specific code
- if the node is in an extension namespace, look up the associated behavior
- otherwise copy the node.
The closure is usually done through the XSLT apply-templatesconstruct, which invokes this process recursively starting at step 1, to find the appropriate template for the nodes selected by the 'select' attribute of the apply-templates instruction (default: the children of the node currently being processed)

Note that large parts of the input tree may not be processed by a given stylesheet and that conversely some may be processed multiple times. (This often is the case when a Table of Contents is built).

The module transform.c is the one implementing most of this logic. xsltApplyStylesheet() is the entry point, it allocates an xsltTransformContext containing the following:

a pointer to the stylesheet being processed
a stack of templates
a stack of variables and parameters
an XPath context
the template mode
current document
current input node
current selected node list
the current insertion points in the output document
a couple of hash tables for extension elements and functions

Then a new document gets allocated (HTML or XML depending on the type of output), the user parameters and global variables and parameters are evaluated. Then xsltProcessOneNode() which implements the 1-2-3 algorithm is called on the docuemnt node of the input. Step 1/ is implemented by calling xsltGetTemplate(), step 2/ is implemented by xsltDefaultProcessOneNode() and step 3/ is implemented by xsltApplyOneTemplate().

XPath expression compilation

The XPath support is actually implemented in the libxml module (where it is reused by the XPointer implementation). XPath is a relatively classic expression language. The only uncommon feature is that it is working on XML trees and hence has specific syntax and types to handle them.

XPath expressions are compiled using xmlXPathCompile(). It will take an expression string in input and generate a structure containing the parsed expression tree, for example the expression:

/doc/chapter[title='Introduction']

will be compiled as

Compiled Expression : 10 elements
  SORT
    COLLECT  'child' 'name' 'node' chapter
      COLLECT  'child' 'name' 'node' doc
        ROOT
      PREDICATE
        SORT
          EQUAL =
            COLLECT  'child' 'name' 'node' title
              NODE
            ELEM Object is a string : Introduction
              COLLECT  'child' 'name' 'node' title
                NODE

This can be tested using the testXPath command (in the libxml codebase) using the --tree option.

Again, the KISS approach is used. No optimization is done. This could be an interesting thing to add. Michael Kay describes a lot of possible and interesting optimizations done in Saxon which would be possible at this level. I'm unsure they would provide much gain since the expressions tends to be relatively simple in general and stylesheets are still hand generated. Optimizations at the interpretation sounds likely to be more efficient.

XPath interpretation

The interpreter is implemented by xmlXPathCompiledEval() which is the front-end to xmlXPathCompOpEval() the function implementing the evaluation of the expression tree. This evaluation follows the KISS approach again. It's recursive and calls xmlXPathNodeCollectAndTest() to collect a set of nodes when evaluating a COLLECT node.

An evaluation is done within the framework of an XPath context stored in an xmlXPathContext structure, in the framework of a transformation the context is maintained within the XSLT context. Its content follows the requirements from the XPath specification:

the current document
the current node
a hash table of defined variables (but not used by XSLT, which uses its own stack frame for variables, described below)
a hash table of defined functions
the proximity position (the place of the node in the current node list)
the context size (the size of the current node list)
the array of namespace declarations in scope (there also is a namespace hash table but it is not used in the XSLT transformation).

For the purpose of XSLT an extra pointer has been added allowing to retrieve the XSLT transformation context. When an XPath evaluation is about to be performed, an XPath parser context is allocated containing an XPath object stack (this is actually an XPath evaluation context, this is a relic of the time where there was no separate parsing and evaluation phase in the XPath implementation). Here is an overview of the set of contexts associated to an XPath evaluation within an XSLT transformation:

The set of contexts associated

Clearly this is a bit too complex and confusing and should be refactored at the next set of binary incompatible releases of libxml. For example the xmlXPathCtxt has a lot of unused parts and should probably be merged with xmlXPathParserCtxt.

Description of XPath Objects

An XPath expression manipulates XPath objects. XPath defines the default types boolean, numbers, strings and node sets. XSLT adds the result tree fragment type which is basically an unmodifiable node set.

Implementation-wise, libxml follows again a KISS approach, the xmlXPathObject is a structure containing a type description and the various possibilities. (Using an enum could have gained some bytes.) In the case of node sets (or result tree fragments), it points to a separate xmlNodeSet object which contains the list of pointers to the document nodes:

An Node set object pointing to

The XPath API (and its 'internal' part) includes a number of functions to create, copy, compare, convert or free XPath objects.

XPath functions

All the XPath functions available to the interpreter are registered in the function hash table linked from the XPath context. They all share the same signature:

void xmlXPathFunc (xmlXPathParserContextPtr ctxt, int nargs);

The first argument is the XPath interpretation context, holding the interpretation stack. The second argument defines the number of objects passed on the stack for the function to consume (last argument is on top of the stack).

Basically an XPath function does the following:

check nargs for proper handling of errors or functions with variable numbers of parameters
pop the parameters from the stack using obj = valuePop(ctxt);
do the function specific computation
push the result parameter on the stack using valuePush(ctxt, res);
free up the input parameters with xmlXPathFreeObject(obj);
return

Sometime the work can be done directly by modifying in-situ the top object on the stack ctxt->value.

The XSLT variables stack frame

Not to be confused with XPath object stack, this stack holds the XSLT variables and parameters as they are defined through the recursive calls of call-template, apply-templates and default templates. This is used to define the scope of variables being called.

This part seems to be one needing most work , first it is done in a very inefficient way since the location of the variables and parameters within the stylesheet tree is still done at run time (it really should be done statically at compile time), and I am still unsure that my understanding of the template variables and parameter scope is actually right.

This part of the documentation is still to be written once this part of the code will be stable. TODO

Extension support

There is a separate document explaining how the extension support works.

TODOs

redesign the XSLT stack frame handling. Far too much work is done at execution time. Similarly for the attribute value templates handling, at least the embedded subexpressions ought to be precompiled.

Allow output to be saved to a SAX like output (this notion of SAX like API for output should be added directly to libxml).

Implement and test some of the optimization explained by Michael Kay especially:

static slot allocation on the stack frame
specific boolean interpretation of an XPath expression
some of the sorting optimization
Lazy evaluation of location path. (this may require more changes but sounds really interesting. XT does this too.)
Optimization of an expression tree (This could be done as a completely independent module.)

Error reporting, there is a lot of case where the XSLT specification specify that a given construct is an error are not checked adequately by libxslt. Basically one should do a complete pass on the XSLT spec again and add all tests to the stylesheet compilation. Using the DTD provided in the appendix and making direct checks using the libxml validation API sounds a good idea too (though one should take care of not raising errors for elements/attributes in different namespaces).

Double check all the places where the stylesheet compiled form might be modified at run time (extra removal of blanks nodes, hint on the xsltCompMatch).

Thanks:

Thanks to Michael Sperberg-McQueen for various fixes and clarifications on this document!

Writing extensions

Table of content

Introduction

This document describes the work needed to write extensions to the standard XSLT library for use with libxslt, the XSLT C library developed for the GNOME project.

Before starting reading this document it is highly recommended to get familiar with the libxslt internals.

Note: this documentation is by definition incomplete and I am not good at spelling, grammar, so patches and suggestions are really welcome.

Basics

The XSLT specification provides two ways to extend an XSLT engine:

providing new extension functions which can be called from XPath expressions
providing new extension elements which can be inserted in stylesheets

In both cases the extensions need to be associated to a new namespace, i.e. an URI used as the name for the extension's namespace (there is no need to have a resource there for this to work).

libxslt provides a few extensions itself, either in the libxslt namespace "http://xmlsoft.org/XSLT/namespace" or in namespaces for other well known extensions provided by other XSLT processors like Saxon, Xalan or XT.

Extension modules

Since extensions are bound to a namespace name, usually sets of extensions coming from a given source are using the same namespace name defining in practice a group of extensions providing elements, functions or both. From the libxslt point of view those are considered as an "extension module", and most of the APIs work at a module point of view.

Registration of new functions or elements are bound to the activation of the module. This is currently done by declaring the namespace as an extension by using the attribute extension-element-prefixes on the xsl:stylesheet element.

An extension module is defined by 3 objects:

the namespace name associated
an initialization function
a shutdown function

Registering a module

Currently a libxslt module has to be compiled within the application using libxslt. There is no code to load dynamically shared libraries associated to a namespace (this may be added but is likely to become a portability nightmare).

The current way to register a module is to link the code implementing it with the application and to call a registration function:

int xsltRegisterExtModule(const xmlChar *URI,
                          xsltExtInitFunction initFunc,
                          xsltExtShutdownFunction shutdownFunc);

The associated header is read by:

#include<libxslt/extensions.h>

which also defines the type for the initialization and shutdown functions

Loading a module

Once the module URI has been registered and if the XSLT processor detects that a given stylesheet needs the functionalities of an extended module, this one is initialized.

The xsltExtInitFunction type defines the interface for an initialization function:

/**
 * xsltExtInitFunction:
 * @ctxt:  an XSLT transformation context
 * @URI:  the namespace URI for the extension
 *
 * A function called at initialization time of an XSLT
 * extension module
 *
 * Returns a pointer to the module specific data for this
 * transformation
 */
typedef void *(*xsltExtInitFunction)(xsltTransformContextPtr ctxt,
                                     const xmlChar *URI);

There are 3 things to notice:

The function gets passed the namespace name URI as an argument. This allows a single function to provide the initialization for multiple logical modules.
It also gets passed a transformation context. The initialization is done at run time before any processing occurs on the stylesheet but it will be invoked separately each time for each transformation.
It returns a pointer. This can be used to store module specific information which can be retrieved later when a function or an element from the extension is used. An obvious example is a connection to a database which should be kept and reused along with the transformation. NULL is a perfectly valid return; there is no way to indicate a failure at this level

What this function is expected to do is:

prepare the context for this module (like opening the database connection)
register the extensions specific to this module

Registering an extension function

There is a single call to do this registration:

int xsltRegisterExtFunction(xsltTransformContextPtr ctxt,
                            const xmlChar *name,
                            const xmlChar *URI,
                            xmlXPathEvalFunc function);

The registration is bound to a single transformation instance referred by ctxt, name is the UTF8 encoded name for the NCName of the function, and URI is the namespace name for the extension (no checking is done, a module could register functions or elements from a different namespace, but it is not recommended).

Implementing an extension function

The implementation of the function must have the signature of a libxml XPath function:

/**
 * xmlXPathEvalFunc:
 * @ctxt: an XPath parser context
 * @nargs: the number of arguments passed to the function
 *
 * an XPath evaluation function, the parameters are on the
 * XPath context stack
 */

typedef void (*xmlXPathEvalFunc)(xmlXPathParserContextPtr ctxt,
                                 int nargs);

The context passed to an XPath function is not an XSLT context but an XPath context. However it is possible to find one from the other:

The function xsltXPathGetTransformContext provides this lookup facility:

xsltTransformContextPtr
         xsltXPathGetTransformContext
                          (xmlXPathParserContextPtr ctxt);

The xmlXPathContextPtr associated to an xsltTransformContext is stored in the xpathCtxt field.

The first thing an extension function may want to do is to check the arguments passed on the stack, the nargs parameter will tell how many of them were provided on the XPath expression. The macro valuePop will extract them from the XPath stack:

#include <libxml/xpath.h>
#include <libxml/xpathInternals.h>

xmlXPathObjectPtr obj = valuePop(ctxt);

Note that ctxt is the XPath context not the XSLT one. It is then possible to examine the content of the value. Check the description of XPath objects if necessary. The following is a common sequence checking whether the argument passed is a string and converting it using the built-in XPath string() function if this is not the case:

if (obj->type != XPATH_STRING) {
    valuePush(ctxt, obj);
    xmlXPathStringFunction(ctxt, 1);
    obj = valuePop(ctxt);
}

Most common XPath functions are available directly at the C level and are exported either in <libxml/xpath.h> or in <libxml/xpathInternals.h>.

The extension function may also need to retrieve the data associated to this module instance (the database connection in the previous example) this can be done using the xsltGetExtData:

void * xsltGetExtData(xsltTransformContextPtr ctxt,
                      const xmlChar *URI);

Again the URI to be provided is the one which was used when registering the module.

Once the function finishes, don't forget to:

push the return value on the stack using valuePush(ctxt, obj)
deallocate the parameters passed to the function using xmlXPathFreeObject(obj)

Examples for extension functions

The module libxslt/functions.c contains the sources of the XSLT built-in functions, including document(), key(), generate-id(), etc. as well as a full example module at the end. Here is the test function implementation for the libxslt:test function:

/**
 * xsltExtFunctionTest:
 * @ctxt:  the XPath Parser context
 * @nargs:  the number of arguments
 *
 * function libxslt:test() for testing the extensions support.
 */
static void
xsltExtFunctionTest(xmlXPathParserContextPtr ctxt, int nargs)
{
    xsltTransformContextPtr tctxt;
    void *data;

    tctxt = xsltXPathGetTransformContext(ctxt);
    if (tctxt == NULL) {
        xsltGenericError(xsltGenericErrorContext,
            "xsltExtFunctionTest: failed to get the transformation context\n");
        return;
    }
    data = xsltGetExtData(tctxt, (const xmlChar *) XSLT_DEFAULT_URL);
    if (data == NULL) {
        xsltGenericError(xsltGenericErrorContext,
            "xsltExtFunctionTest: failed to get module data\n");
        return;
    }
#ifdef WITH_XSLT_DEBUG_FUNCTION
    xsltGenericDebug(xsltGenericDebugContext,
                     "libxslt:test() called with %d args\n", nargs);
#endif
}

Registering an extension element

There is a single call to do this registration:

int xsltRegisterExtElement(xsltTransformContextPtr ctxt,
                           const xmlChar *name,
                           const xmlChar *URI,
                           xsltTransformFunction function);

It is similar to the mechanism used to register an extension function, except that the signature of an extension element implementation is different.

The registration is bound to a single transformation instance referred to by ctxt, name is the UTF8 encoded name for the NCName of the element, and URI is the namespace name for the extension (no checking is done, a module could register elements for a different namespace, but it is not recommended).

Implementing an extension element

The implementation of the element must have the signature of an XSLT transformation function:

/** 
 * xsltTransformFunction: 
 * @ctxt: the XSLT transformation context
 * @node: the input node
 * @inst: the stylesheet node 
 * @comp: the compiled information from the stylesheet 
 * 
 * signature of the function associated to elements part of the
 * stylesheet language like xsl:if or xsl:apply-templates.
 */ 
typedef void (*xsltTransformFunction)
                          (xsltTransformContextPtr ctxt,
                           xmlNodePtr node,
                           xmlNodePtr inst,
                           xsltStylePreCompPtr comp);

The first argument is the XSLT transformation context. The second and third arguments are xmlNodePtr i.e. internal memory representation of XML nodes. They are respectively node from the the input document being transformed by the stylesheet and inst the extension element in the stylesheet. The last argument is comp a pointer to a precompiled representation of inst but usually for an extension function this value is NULL by default (it could be added and associated to the instruction in inst->_private).

The same functions are available from a function implementing an extension element as in an extension function, including xsltGetExtData().

The goal of an extension element being usually to enrich the generated output, it is expected that they will grow the currently generated output tree. This can be done by grabbing ctxt->insert which is the current libxml node being generated (Note this can also be the intermediate value tree being built for example to initialize a variable, the processing should be similar). The functions for libxml tree manipulation from <libxml/tree.h> can be employed to extend or modify the tree, but it is required to preserve the insertion node and its ancestors since there are existing pointers to those elements still in use in the XSLT template execution stack.

Example for extension elements

The module libxslt/transform.c contains the sources of the XSLT built-in elements, including xsl:element, xsl:attribute, xsl:if, etc. There is a small but full example in functions.c providing the implementation for the libxslt:test element, it will output a comment in the result tree:

/**
 * xsltExtElementTest:
 * @ctxt:  an XSLT processing context
 * @node:  The current node
 * @inst:  the instruction in the stylesheet
 * @comp:  precomputed information
 *
 * Process a libxslt:test node
 */
static void
xsltExtElementTest(xsltTransformContextPtr ctxt, xmlNodePtr node,
                   xmlNodePtr inst,
                   xsltStylePreCompPtr comp)
{
    xmlNodePtr comment;

    if (ctxt == NULL) {
        xsltGenericError(xsltGenericErrorContext,
                         "xsltExtElementTest: no transformation context\n");
        return;
    }
    if (node == NULL) {
        xsltGenericError(xsltGenericErrorContext,
                         "xsltExtElementTest: no current node\n");
        return;
    }
    if (inst == NULL) {
        xsltGenericError(xsltGenericErrorContext,
                         "xsltExtElementTest: no instruction\n");
        return;
    }
    if (ctxt->insert == NULL) {
        xsltGenericError(xsltGenericErrorContext,
                         "xsltExtElementTest: no insertion point\n");
        return;
    }
    comment =
        xmlNewComment((const xmlChar *)
                      "libxslt:test element test worked");
    xmlAddChild(ctxt->insert, comment);
}

The shutdown of a module

When the XSLT processor ends a transformation, the shutdown function (if it exists) for each of the modules initialized is called. The xsltExtShutdownFunction type defines the interface for a shutdown function:

/**
 * xsltExtShutdownFunction:
 * @ctxt:  an XSLT transformation context
 * @URI:  the namespace URI for the extension
 * @data:  the data associated to this module
 *
 * A function called at shutdown time of an XSLT extension module
 */
typedef void (*xsltExtShutdownFunction) (xsltTransformContextPtr ctxt,
                                         const xmlChar *URI,
                                         void *data);

This is really similar to a module initialization function except a third argument is passed, it's the value that was returned by the initialization function. This allows the routine to deallocate resources from the module for example close the connection to the database to keep the same example.

Future work

Well, some of the pieces missing:

a way to load shared libraries to instantiate new modules
a better detection of extension functions usage and their registration without having to use the extension prefix which ought to be reserved to element extensions.
more examples
implementations of the EXSLT common extension libraries, Thomas Broyer nearly finished implementing them.

Contributions

Bjorn Reese is the author of the number support and worked on the XSLTMark support
William Brack was an early adopted, contributed a number of patches and spent quite some time debugging non-trivial problems in early versions of libxslt
Igor Zlatkovic is now the maintainer of the Windows port, he provides binaries
Thomas Broyer provided a lot of suggestions, and drafted most of the extension API
John Fleck maintains a tutorial for libxslt
Matt Sergeant developed XML::LibXSLT, a perl wrapper for libxml2/libxslt as part of the AxKit XML application server
there is a module for libxml/libxslt support in OpenNSD/AOLServer
Dave Kuhlman provides libxml/libxslt wrappers for Python
Steve Ball, and contributors maintain tcl bindings for libxml2 and libxslt, as well as tkxmllint a GUI for xmllint and tkxsltproc a GUI for xsltproc.
If you want to use libxslt in a Mac OS X/Cocoa or Objective-C framework, Marc Liyanage provides an application TestXSLT for XSLT and XML editing including wrapper classes for the XML parser and XSLT processor.

Daniel Veillard