# The XML C parser and toolkit of Gnome

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Table of Contents:

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

### License(s)

1. Licensing Terms for libxml

   libxml2 is released under the MIT License; see the file Copyright in the distribution for the precise wording

2. Can I embed libxml2 in a proprietary application ?

   Yes. The MIT License allows you to keep proprietary the changes you made to libxml, but it would be graceful to send-back bug fixes and improvements as patches for possible incorporation in the main development tree.

### Installation

1. Do Not Use libxml1, use libxml2

2. Where can I get libxml ?

   The original distribution comes from xmlsoft.org or gnome.org

   Most Linux and BSD distributions include libxml, this is probably the safer way for end-users to use libxml.

   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 constrained by backward compatibility issues with existing applications, 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. The libxml packages provided on xmlsoft.org provide libxml.so.0

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

   The most generic solution is to re-fetch 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 packages (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 libxml2 ?

   As most UNIX libraries libxml2 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 a similar utility to update your list of installed shared libs.

2. What other libraries are needed to compile/install libxml2 ?

   Libxml2 does not require 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 libxml2 will detect and use the following libs:

   - libz : a highly portable and available widely compression library.
   - iconv: a powerful character encoding conversion library. It is included by default in recent glibc libraries, so it doesn't need to be installed specifically on Linux. It now seems a 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

   Sometimes 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 fail due to limitations in make. Try using GNU-make instead.

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

   The configure script (and other Makefiles) are generated. Use the autogen.sh script to regenerate the configure script 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. Troubles compiling or linking programs using libxml2

   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 `xml2-config` which is installed as part of libxml2 usual install process which provides those flags. Use

   `xml2-config --cflags`

   to get the compilation flags and

   `xml2-config --libs`

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

   `CFLAGS=`xml2-config --cflags``

   `LIBS=`xml2-config --libs``

2. I want to install my own copy of libxml2 in my home directory and link my programs against it, but it doesn't work

   There are many different ways to accomplish this. Here is one way to do this under Linux. Suppose your home directory is `/home/user.` Then:

   - Create a subdirectory, let's call it `myxml`
   - unpack the libxml2 distribution into that subdirectory
   - chdir into the unpacked distribution (`/home/user/myxml/libxml2` )
   - configure the library using the "`--prefix`" switch, specifying an installation subdirectory in `/home/user/myxml`, e.g.

     `./configure --prefix /home/user/myxml/xmlinst` {other configuration options}

   - now run `make` followed by `make install`
   - At this point, the installation subdirectory contains the complete "private" include files, library files and binary program files (e.g. xmllint), located in

     `/home/user/myxml/xmlinst/lib, /home/user/myxml/xmlinst/include` and `/home/user/myxml/xmlinst/bin`

     respectively.
   - In order to use this "private" library, you should first add it to the beginning of your default PATH (so that your own private program files such as xmllint will be used instead of the normal system ones). To do this, the Bash command would be

     `export PATH=/home/user/myxml/xmlinst/bin:$PATH`

   - Now suppose you have a program `test1.c` that you would like to compile with your "private" library. Simply compile it using the command

     `gcc `xml2-config --cflags --libs` -o test test.c`

     Note that, because your PATH has been set with `/home/user/myxml/xmlinst/bin` at the beginning, the xml2-config program which you just installed will be used instead of the system default one, and this will automatically get the correct libraries linked with your program.

3. xmlDocDump() generates output on one line.

   Libxml2 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 libxml2 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 affect other parts of the content of your document. See xmlKeepBlanksDefault () and xmlSaveFormatFile ()

4. Extra nodes in the document:

   For an 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 pnode;
       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 which 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 cases where you are certain there is no mixed-content in the document.

5. I get compilation errors of existing code like when accessing root or child 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.

6. 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

7. Random crashes in threaded applications

   Read and follow all advices on the thread safety page, and make 100% sure you never call xmlCleanupParser() while the library or an XML document might still be in use by another thread.

8. 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 please send patches.

9. Where can I get more examples and information than provided on the web page?

   Ideally a libxml2 book would be nice. I have no such plan ... But you can:

   - check more deeply the existing generated doc
   - have a look at the set of examples.
   - look for examples of use for libxml2 function using the Gnome code or by asking on Google.
   - Browse the libxml2 source , I try to write code as clean and documented as possible, so looking at it may be helpful. In particular the code of xmllint.c and of the various testXXX.c test programs should provide good examples of how to do things with the library.

10. What about C++ ?

    libxml2 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 which may fulfill your needs:

    - by Ari Johnson <ari@btigate.com>:

      Website: https://libxmlplusplus.github.io/libxmlplusplus/

      Download: https://download.gnome.org/sources/libxml++/

11. How to validate a document a posteriori ?

    It is possible to validate documents which had not been validated at initial parsing time or documents which 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);

12. So what is this funky "xmlChar" used all the time?

    It is a null terminated sequence of utf-8 characters. And only utf-8! You need to convert strings encoded in different ways to utf-8 before passing them to the API. This can be accomplished with the iconv library for instance.

13. etc ...

Daniel Veillard