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Designed for C and C++ programmers who are new to XML development, this article gives an overview of tools to assemble in preparation for XML development. Tool tables outline generic XML tools like IDEs and schema designers, parsers, XSLT tools, SOAP and XML-RPC libraries, and other libraries either usable from or actually written in C and/or C++. The article includes advice for installing open-source libraries on Windows, Unix, and Linux, plus a brief glossary of key XML terms.
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It seems as if everywhere you look there is some new XML-related tool being released in source code form written in Java. Despite Java's apparent dominance in the XML arena, many C/C++ programmers do XML development, and there are a large assortment of XML tools for the C and C++ programmer. We'll confront XML library issues like validation, schemas, and API models. Next, we'll look at a collection of generic XML tools like IDEs and schema designers. Finally, we'll conclude with a list and discussion of libraries either usable from or actually written in C and/or C++.In this article, I'll skip arguments for using XML; I'll assume that you already have good reasons for wanting to ramp up on this technology. Also, I'll leave the more detailed explanations of XML to the background sources in . Suffice it to say that XML is a standard for data exchange (not just a file format). The data may be exchanged in the form of application files in XML format, or over network connections that exist only for a moment before being discarded at the conclusion of an online transaction or at the close of a network connection.Also, this isn't a comparative review that rates tools. My goal is to explain the types of tools you'll probably need and to point you to likely candidates. You'll still need to research, test, and compare tool features against your project needs to assemble your ultimate toolbox.Two sets of tools
To incorporate XML in your own software projects, you're going to want to have two sets of tools in your bag of tricks. The first set is a dialect designer (or more properly "schema designer"). The second set of tools includes software libraries that will add parsing and XML-generation features to your application.Designing your own XML dialect
An XML dialect is just a particular set of XML tags along with some rules for how the tags fit together. The two dominant ways for specifying or defining an XML dialect currently are through a Document Type Definition (DTD) or an XML Schema. I'll refer to both of these collectively as a schema.Your project's domain may already have a particular schema designed for you. If not, you can hack up your own schema using a plain text editor. A more refined approach is to use an actual dialect designer that can check syntax. (An incorrect schema won't help when you try to use it later to validate your XML data.)Design tools
Nowadays, most programmers' text editors -- particularly the ones found in IDEs -- have decent macro and template support for features like syntax highlighting and autocompletion of a partially typed word or phrase. Therefore I am omitting from this discussion any XML editors that do only syntax highlighting and autocompletion. Microsoft Word or an Emacs macro can do that, so "XML editor" ought to mean something more.The tools shown in Table 1 fall into three categories:
The IDE (integrated development environment, the Swiss Army knife approach)The schema-sensitive XML editor (reinforces allowable tag structure and attributes found in a schema or DTD)Schema designer (for writing your own Schema or DTD)Because all of the tools in Table 1 are running applications, they are suitable for all XML developers -- not just those using C++ (unlike the tools listed in the other tables in this article).Table 1. Dialect design tools for various platformsProductVendorDescriptionLicensePlatformTIBCO/ExtensibilityXML Schema/DTD designer and IDEcommercialJava, Win32Active StateXML editor and IDEcommercialLinux, Win32, othersAltovaIDE for XML editing and schema designcommercialWin32MicrosoftXML editorfreeWin32Morphon XMLLunatech ResearchSchema-sensitive XML data editorcommercialJavaUniversity of EdinburghSchema-sensitive XML data editornoncommercialWin32, Linux, UnixIBM alphaWorksSchema-sensitive XML data editorfree trial/commercialMac, Unix, Win32PierlouSchema-sensitive XML data editornoncommercialJavaPhillip LenssenAlternative to Microsoft's XML NotepadnoncommercialWin32SoftquadDTD sensitive XML editorcommercialWin32MerlotChannelpointVisual XML supports plug-ins for DTDsnoncommercialJavaElCel TechnologiesCommand-line XML validation toolnoncommercialWin32ElCel TechnologiesProduces canonical XML by merging XML data with DTDnoncommercialWin32C and C++ tools
The rest of this article serves up the meat and potatoes of adding XML functionality for the C/C++ programmer through software libraries. You'll see in the next sections of this article that many more command-line utilities can be found included as test and/or sample programs in the software libraries. As an example, Transformiix can be used as a library, a Perl module, or a command-line tool.Up until now I've tried to be frugal in my use of XML-related terms. Before you read further, though, if you are not familiar with XML basics, you might want to scan the terms defined in the
sidebar. The terms will assist you both in following the rest of this article and in sorting out the features of the tools and libraries mentioned as you explore them in greater depth on your own.ParsersOnce you have a DTD or schema and the XML document to go with it, you'll need a parser to read and interpret the XML document.
outlines parser libraries for C/C++ developers. Before you start poring over the grid in the table, though, you'll need a little bit of background.ValidationXML parsers come in two forms: validating and nonvalidating. Which one do you need? If you are working without a formal DTD or schema, validation features won't be important to you. If you have or are planning to use a DTD or schema, you will probably prefer a validating parser. (In that case, I suggest that you also learn how to read and write a DTD/schema by hand so that when validation issues arise you will be able to deal with the errors. Sometimes the error is in the DTD/schema, so you may be debugging the DTD/schema files as well as your XML data.)Parser API models
Two common API models exist for interfacing software with an XML parser:
the document model and the event model. The document API model parses XML data to
produce an object. The object abstracts the document's contents into a tree
structure. The application operates on this tree-structure object. The event API
model uses a callback mechanism to notify the application of the structure of the
XML data. The events/callbacks usually occur at the time of parsing.API standards: DOM and SAX
The generic parser API models have been further refined into specific API standards.
The W3C has recommended DOM (levels 1 and 2, with level 3 in draft) as a standardized document
API model. While not a W3C project, SAX has taken its place as the de facto standard event API model.W3C standards
As you compare features in parsers and other XML tools, look for support for W3C recommendations and emerging specifications, such as namespaces, XPath, XLink, XInclude, and XInfoset. Keep in mind that XML technologies are maturing rapidly and that support for the first level of a specification, such as the DOM, may lack important functionality introduced in level two of that specification. If functionality in the most current form of a specification is important to your project, choose your tools accordingly.Reading the table
In Table 2, the Event column specifies parsers that support a push or event model API, like SAX. The Doc column specifies parsers that support a pull or document model API, like DOM. As before, the table lists both commercial and noncommercial tools (see the sidebar
for details about the software licenses.)Table 2. Parsers for C/C++ developersLibrarySupplierEventDocHighlightsLicenseJames Clark/expat teamnative & SAX-Very fast push model parser with a native API and SAX wrappers.LGPL (free)GnomeSAXDOMV SAX & DOM does DTD validationLGPL (free)MicrosoftSAXDOMThe Microsoft XML library for Win32EULA (free)Apache Software FoundationSAXDOMDoes SAX plus DOM levels 1 and 2; DTD incremental XML SchemaApache (free)XTLVivid CreationsSAXDOMSTL-based XML toolkit with SAX and DOMcommercialUniversity of Edinburgh-nativeValidating namespace-aware XML parser in CGPL (free)IBM alphaWorksSAXDOMIBM-sponsored variant of XercesApache (free)OracleSAXDOMOracle-sponsored XML toolkit for C++noncommercialExtreme! Lab-nativeIndiana University-sponsored lightweight XML toolkit for C++noncommercialPhiDaNi Software-nativeParser generator, generates C-source parsercommercialOpen-source top three
The three most popular of the open-source XML libraries are expat, libxml, and Xerces. All three are cross-platform, and each serves as a basis for an XSLT library implementation, which give you a growth path once you've satisfied your basic XML needs.Expat is an open-source event-oriented XML parsing library originated by James Clark.
He has transferred the project to a small team on SourceForge. A SAX wrapper is available.
The expat parser can found in a number of projects, such as the open-source browser Mozilla,
the XSLT processor Transformiix, and the RDF tool repat.Libxml offers a dual-mode API for both SAX and DOM-like operations. It supports validating
against a DTD and is used in Gnome's XSLT processor, libxslt. Libxml was rewritten and
released as libxml(2) but may also be referred to as libxml2. Users of this library should make sure they have the current version.Xerces is a very solid, well-documented library that serves as the basis for the IBM
alphaWorks XML4C library. Xerces is also used in the Apache XSLT processor, Xalan.
Xerces supports DOM, SAX, and validating against a DTD. The latest versions reads and
interprets parts of the W3C XML Schema Recommendation (with complete XML Schema support
targeted for the end of 2001).Compiling and linking one of the top three libraries into your project is painless. Most packages include thorough instructions for each platform. Here some sample installation instructions.Building an open-source library on Windows
Building libxml on Windows from scratch is an easy four steps:
Download the source tarball.Use a program like Winzip to unzip the contents to a directory. Be sure you instruct your unzipping utility to preserve path names for any subdirectories that libxml may need.Locate the libxml2.dsw file in the ./win32/dsp
subfolder and open it from MS Developer Studio.Select Build All from the top menu in DevStudio. This builds all the sample and test programs
along with the libxml DLLs needed to run them.You can use the above steps to build Xerces on Windows. The only difference is to look for the samples.dsw workspace file in the ./c/samples/Projects/Win32/VC6 subfolder.Expat has started including DSP project makefiles. Look in the lib and xmlwf subfolders.Building an open-source library on Unix
For projects running on Linux or Unix, in most cases you can untar the source code to an empty directory, set some options, and type "make" to build a shared library. Solaris users: Don't forget to use the GNU untar utility. The following code worked for me from the bash shell under Slackware Linux:tar -x &xerces-c-src1_5_1.tar
cd xerces-c-src1_5_1
export XERCESCROOT=/home/mine/xerces-c-src1_5_1
chmod 377 runConfigure
./runConfigure -p linux
cd ../samples
chmod 377 runConfigure
./runConfigure -p linux
gmakeXML and COM
MSXML, Microsoft's proprietary XML offering for the Windows family of operating
systems, is implemented as a collection of scriptable COM objects, so it plays well
in other language environments and is well documented. The library supports DOM along
with a native document-oriented interface. SAX events are also supported.As an alternative to MSXML, the Apache XML Project's Xerces library comes with a
COM wrapper that will make it act as a drop-in replacement for MSXML in many cases.
Vivid Creations offers COM wrappers for the SAX and DOM APIs to its XTL library
that also serve as substitutes for MSXML.XML transforms: XSLT and XQuery
The transform is the next step up the XML evolutionary ladder from merely processing XML data at the
element and attribute level. An XML transform operates on incoming XML data to produce XML output. A transform can reorganize tag structure, add/remove tags and attributes, and filter to zoom in on select fragments of XML data.The XQuery documentation refers to the transform process as a query but the meaning is the same.XSLT and XQuery are XML dialects for specifying how to perform such actions on random XML data. You can write a script file with the changes expressed in XML as XSTL or XQuery instead of loading some XML data into a DOM and having to programmatically manipulate the DOM version to produce a desired result. This more generalized approach leads to greater flexibility and reduced development time. Now your Web developers who are not C/C++ programmers can write their own transforms as XML, which may free a C++ programmer for more complex work.Table 3. C/C++ Transform/Query librariesLibrarySupplierHighlightsLicenseGnomebuilt on top of libxmlLGPL or X11-like (free)ApacheBuilt on top of the Xerces parserApache (free)TransformiixMITREXSLT processor built on expatnoncommercialOliver GerardinXSLT compiler, produces C codenoncommercialGinger AllianceXSL enginenoncommercialMessaging: XML-RPC and SOAP tools
For the purpose of this article, messaging refers to having two software agents communicate with one another. Such messaging is sometimes called message-oriented middleware. (This is not messaging like AOL, MSN, or ICQ, okay? There is an XML-based instant-messaging protocol effort underway called Jabber. I've included a link in
for your curiosity, but, again, that's not what I'm talking about here.)Using XML for messaging has become popular enough to have produced these two alternatives: XML-RPC and SOAP. The most appealing feature of these protocols is that clients, servers, and peers can differ dramatically in terms of the developer's choice of tools for implementation. It's as though all developers get to use their favorite language, development kit, or software library and still work together.(As a side note, Gregor Purdy has written an excellent critique of XML-RPC in the form of a proposed alternative (see ).Table 4 includes a few libraries for use in the message-oriented middleware category. This is not an exhaustive list of the resources in this category, and there are new tools evolving quickly, but it's a good start.Table 4. C/C++ Messaging librariesLibrarySupplierHighlightsLicensePlatformSimon FellOpen Source SOAP effortnoncommercialLinux, Unix, Win32SQL DataC++ SOAP client toolkitcommercialWin32mozilla.orgScriptable XPCOM componentnoncommercialnumerousFirst PeerXML-RPC library in CnoncommercialLinux, Unix, Win32mozilla.orgScriptable XPCOM componentnoncommercialnumerousEpinionsXML-RPC library in CnoncommercialLinux, Unix, Win32In parting
These tools ought to give you a good start on your XML toolbox. If you want to suggest other C/C++ tools for XML that you have tried or to make any other comment, join the discussion attached to this article (use the link in Resources or click the Discuss icon at the top or bottom of the article page). XML terminologyThese XML terms may come in handy as you read about the libraries discussed in this article:Document model: Technique for parsing and manipulating XML data
this is also called a "pull" model. See the DOM API standard as an example.DOM: The Document Object Model is a specific tree-structured programming model of an XML document described as a standard by the W3C. The DOM standard is currently divided into three levels. DOM 1.0 refers to DOM Level 1.0 DOM Level two is the most current spec that has been approved by the W3C as a R DOM Level 3 is in draft at the time of this writing.DTD: Document Type Definition. An XML file that defines XML elements and XML attributes for those elements and that specifies rules for how XML tags may be nested and what data an element may contain.Event model: Technique for parsing XML data using c this is also
called a "push" model. See the SAX API standard as an example.Namespaces: Means of unambiguously identifying XML tags from different
DTDs or schemas so they can be mixed in the same XML document. RDF is highly depende the
XML 1.0 token "xmlns" can be used to define a namespace within an XML document.RDF: Resource Description Framework, a compact XML dialect for associating XML attribute data with information that usually resides elsewhere. Your driver's license would be analogous to an RDF XML file that describes you.SAX: Simple API for XML is a standard programming interface for XML parser
SAX uses an event-oriented programming model. SAX is a de facto standard first developed
by David Megginson and now maintained by the XML-dev mailing list.SOAP: Simple Object Access Protocol is a network protocol similar to
XML-RPC (see XML-RPC). Using SOAP, an application can create a remote object, invoke methods on that
object, and retrieve results.Validation: Verifying that a well formed XML document is correct with respect
to a DTD or schema.Well formed: An XML document whose tags and data are consistent with XML 1.0 syntax.W3C: The Worldwide Web Consortium, which has become the key standards body
for most of the XML-related technologies. The W3C calls a finally approved specification a Recommendation
(rather than a standard).XML 1.0: The first standard for XML syntax blessed by the W3C; establishes
basic rules for XML data, such as all tags must be closed with a slash (/)
like this: &example/& or followed by a closing tag like this:
&example&close the tag on your way out&/example&.XML-RPC: XML Remote Procedure Call. XML-RPC is a standard XML dialect for
invoking methods and servi as you can guess, XML-RPC uses XML for the messaging between
client and server.XML Schema: XML Schema, a W3C Recommendation, works much like a DTD to
define the structure of an XML document but with more flexibility. XML Schema uses XML 1.0 syntax to specify
the schema, in contrast to the older SGML syntax used for DTDs.XQuery: Similar in some of its functionality to XSLT but designed more
toward acting as a query language for XML data -- analogous to using SQL in a relational database. Less mature
than XSLT as a specification, XQuery may become the SQL of the next decade.XSLT: Extensible Style Language Transforms, an XML dialect for transforming
XML content. You apply an XSLT file to some XML input data to produce the desired XML output data.
ResourcesVisit , the host with the most in XML specifications.Catch up on XML basics with
or Doug Tidwell's
here on developerWorks and other offerings in the
area.Look at Howard Katz's .Review Gregor Purdy's excellent .Gain an understanding of XSLT in Michael Kay's technical overview, .Find out about that other kind of messaging (instant messaging) at .Get a handle on the XML developer skillset -- become an IBM Certified Developer in
and related technologies.: The W3C core XML 1.0 specification.: The W3C Document Object Model level 1 API Recommendation.: The W3C Document Object Model level 2 API Recommendation.: The Simple API for XML event model de facto standard.: W3C Recommendation for handling XML namespaces.: All about the W3C XML Schema Recommendation
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