FAQ

# The XML C library for Gnome

## FAQ

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Table of Content:

- Licence(s)
- Installation
- Compilation
- Developer corner

### Licence(s)

1. Licensing Terms for libxml

   libxml is released under the MIT Licence, see the file Copyright in the distribution for the precise wording

2. Can I embed libxml in a proprietary application ?

   Yes. The MIT Licence allows you to also keep proprietary the changes you made to libxml, but it would be graceful to provide back bugfixes and improvements as patches for possible incorporation in the main development tree

### Installation

1. Unless you are forced to because your application links with a Gnome library requiring it, Do Not Use libxml1, use libxml2
2. Where can I get libxml ?

   The original distribution comes from rpmfind.net or gnome.org

   Most linux and Bsd distribution includes libxml, this is probably the safer way for end-users

   David Doolin provides precompiled Windows versions at http://www.ce.berkeley.edu/~doolin/code/libxmlwin32/

3. I see libxml and libxml2 releases, which one should I install ?
   - If you are not concerned by any existing backward compatibility with existing application, install libxml2 only
   - If you are not doing development, you can safely install both. usually the packages libxml and libxml2 are compatible (this is not the case for development packages)
   - If you are a developer and your system provides separate packaging for shared libraries and the development components, it is possible to install libxml and libxml2, and also libxml-devel and libxml2-devel too for libxml2 >= 2.3.0
   - If you are developing a new application, please develop against libxml2(-devel)
4. I can't install the libxml package it conflicts with libxml0

   You probably have an old libxml0 package used to provide the shared library for libxml.so.0, you can probably safely remove it. Anyway the libxml packages provided on rpmfind.net provides libxml.so.0

5. I can't install the libxml(2) RPM package due to failed dependancies

   The most generic solution is to refetch the latest src.rpm , and rebuild it locally with

   `rpm --rebuild libxml(2)-xxx.src.rpm`

   if everything goes well it will generate two binary rpm (one providing the shared libs and xmllint, and the other one, the -devel package providing includes, static libraries and scripts needed to build applications with libxml(2)) that you can install locally.

### Compilation

1. What is the process to compile libxml ?

   As most UNIX libraries libxml follows the "standard":

   `gunzip -c xxx.tar.gz | tar xvf -`

   `cd libxml-xxxx`

   `./configure --help`

   to see the options, then the compilation/installation proper

   `./configure [possible options]`

   `make`

   `make install`

   At that point you may have to rerun ldconfig or similar utility to update your list of installed shared libs.

2. What other libraries are needed to compile/install libxml ?

   Libxml does not requires any other library, the normal C ANSI API should be sufficient (please report any violation to this rule you may find).

   However if found at configuration time libxml will detect and use the following libs:

   - libz : a highly portable and available widely compression library
   - iconv: a powerful character encoding conversion library. It's included by default on recent glibc libraries, so it doesn't need to be installed specifically on linux. It seems it's now part of the official UNIX specification. Here is one implementation of the library which source can be found here.
3. make check fails on some platforms

   Sometime the regression tests results don't completely match the value produced by the parser, and the makefile uses diff to print the delta. On some platforms the diff return breaks the compilation process, if the diff is small this is probably not a serious problem.

   Sometimes (especially on Solaris) make checks fails due to limitations in make. Try using GNU-make instead.

4. I use the CVS version and there is no configure script

   The configure (and other Makefiles) are generated. Use the autogen.sh script to regenerate the configure and Makefiles, like:

   `./autogen.sh --prefix=/usr --disable-shared`

5. I have troubles when running make tests with gcc-3.0

   It seems the initial release of gcc-3.0 has a problem with the optimizer which miscompiles the URI module. Please use another compiler

### Developer corner

1. xmlDocDump() generates output on one line

   libxml will not invent spaces in the content of a document since all spaces in the content of a document are significant. If you build a tree from the API and want indentation:

   1. the correct way is to generate those yourself too
   2. the dangerous way is to ask libxml to add those blanks to your content modifying the content of your document in the process. The result may not be what you expect. There is NO way to guarantee that such a modification won't impact other part of the content of your document. See xmlKeepBlanksDefault () and xmlSaveFormatFile ()
2. Extra nodes in the document:

   For a XML file as below:

       <?xml version="1.0"?>
       <PLAN xmlns="http://www.argus.ca/autotest/1.0/">
       <NODE CommFlag="0"/>
       <NODE CommFlag="1"/>
       </PLAN>

   after parsing it with the function pxmlDoc=xmlParseFile(...);

   I want to the get the content of the first node (node with the CommFlag="0")

   so I did it as following;

       xmlNodePtr pode;
       pnode=pxmlDoc->children->children;

   but it does not work. If I change it to

       pnode=pxmlDoc->children->children->next;

   then it works. Can someone explain it to me.

   In XML all characters in the content of the document are significant including blanks and formatting line breaks.

   The extra nodes you are wondering about are just that, text nodes with the formatting spaces wich are part of the document but that people tend to forget. There is a function xmlKeepBlanksDefault () to remove those at parse time, but that's an heuristic, and its use should be limited to case where you are sure there is no mixed-content in the document.

3. I get compilation errors of existing code like when accessing root or childs fields of nodes

   You are compiling code developed for libxml version 1 and using a libxml2 development environment. Either switch back to libxml v1 devel or even better fix the code to compile with libxml2 (or both) by following the instructions.

4. I get compilation errors about non existing xmlRootNode or xmlChildrenNode fields

   The source code you are using has been upgraded to be able to compile with both libxml and libxml2, but you need to install a more recent version: libxml(-devel) >= 1.8.8 or libxml2(-devel) >= 2.1.0

5. XPath implementation looks seriously broken

   XPath implementation prior to 2.3.0 was really incomplete, upgrade to a recent version, there is no known bug in the current version.

6. The example provided in the web page does not compile

   It's hard to maintain the documentation in sync with the code <grin/> ...

   Check the previous points 1/ and 2/ raised before, and send patches.

7. Where can I get more examples and informations than in the web page

   Ideally a libxml book would be nice. I have no such plan ... But you can:

   - check more deeply the existing generated doc
   - looks for examples of use for libxml function using the Gnome code for example the following will query the full Gnome CVs base for the use of the xmlAddChild() function:

     http://cvs.gnome.org/lxr/search?string=xmlAddChild

     This may be slow, a large hardware donation to the gnome project could cure this :-)

   - Browse the libxml source , I try to write code as clean and documented as possible, so looking at it may be helpful. Especially the code of xmllint.c and of the various testXXX.c tests programs should provide good example on how to do things with the library.
8. What about C++ ?

   libxml is written in pure C in order to allow easy reuse on a number of platforms, including embedded systems. I don't intend to convert to C++.

   There is however a C++ wrapper provided by Ari Johnson <ari@btigate.com> which may fullfill your needs:

   Website: http://lusis.org/~ari/xml++/

   Download: http://lusis.org/~ari/xml++/libxml++.tar.gz

9. How to validate a document a posteriori ?

   It is possible to validate documents which had not been validated at initial parsing time or documents who have been built from scratch using the API. Use the xmlValidateDtd() function. It is also possible to simply add a Dtd to an existing document:

       xmlDocPtr doc; /* your existing document */
               xmlDtdPtr dtd = xmlParseDTD(NULL, filename_of_dtd); /* parse the DTD */
               dtd->name = xmlStrDup((xmlChar*)"root_name"); /* use the given root */

               doc->intSubset = dtd;
               if (doc->children == NULL) xmlAddChild((xmlNodePtr)doc, (xmlNodePtr)dtd);
               else xmlAddPrevSibling(doc->children, (xmlNodePtr)dtd);

10. etc ...

Daniel Veillard