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categories: XML,June,2009,JurgenK

Dealing with DTDs

(This page comes from the XML Gawk tutorial.)

The declaration of a document type in the header of an XML file is an optional part of the data, not a mandatory one. If such a declaration is present, the reference to the DTD will not be resolved and its contents will not be parsed. However, the presence of the declaration will be reported by gawk. When the declaration starts, the variable XMLSTARTDOCT contains the name of the root element's tag; and later, when the declaration ends, the variable XMLENDDOCT is set to 1. In between, the array variable XMLATTR will be populated with the values of the public identifier of the DTD (if any) and the value of the system's identifier of the DTD (if any). Other parts of the declaration (elements, attributes and entities) will not be reported.

     @load xml
       version    = XMLATTR["VERSION"        ]
       encoding   = XMLATTR["ENCODING"       ]
       standalone = XMLATTR["STANDALONE"     ]
       root       = XMLSTARTDOCT
       pub_id     = XMLATTR["PUBLIC"         ]
       sys_id     = XMLATTR["SYSTEM"         ]
       intsubset  = XMLATTR["INTERNAL_SUBSET"]
       print FILENAME
       print "  version    '" version    "'"
       print "  encoding   '" encoding   "'"
       print "  standalone '" standalone "'"
       print "  root   id '" root   "'"
       print "  public id '" pub_id "'"
       print "  system id '" sys_id "'"
       print "  intsubset '" intsubset "'"
       print ""
       version = encoding = standalone = ""
       root = pub_id = sys_id = intsubset ""

Most users can safely ignore these variables if they are only interested in the data itself. But some users may take advantage of these variables for checking requirements of the XML data. If your data base consists of thousands of XML file of diverse origins, the public identifier of their DTDs will help you gain an oversight over the kind of data you have to handle and over potential version conflicts. The script shown above will assist you in analyzing your data files. It searches for the variables mentioned above and evaluates their content. At the start of the DTD, the tag name of the root element is stored; the identifiers are also stored and finally, those values are printed along with the name of the file which was analyzed. After each DTD, the remembered values are set to an empty string until the DTD of the next file arrives.

In the following, you can see an example output of the script shown above. Obviously, the first entry is a DocBook file (English version 4.2) containing a book element which has to be validated against a local copy of the DTD at CERN in Switzerland. The second file is a chapter element of DocBook (English version 4.1.2) to be validated against a DTD on the Internet. Finally, the third entry is a file describing a project of the GanttProject application. There is only a tag name for the root element specified, a DTD does not seem to exist.

       version    ''
       encoding   ''
       standalone ''
       root   id  'book'
       public id  '-//OASIS//DTD DocBook XML V4.2//EN'
       system id  '/afs/'
       intsubset  ''
       version    ''
       encoding   ''
       standalone ''
       root   id  'chapter'
       public id  '-//OASIS//DTD DocBook XML V4.1.2//EN'
       system id  ''
       intsubset  ''
       version    '1.0'
       encoding   'UTF-8'
       standalone ''
       root   id  ''
       public id  ''
       system id  ''
       intsubset  ''

You may wish to make changes to this script if you need it in daily work. For example, the script currently reports nothing for files which have no DTD declaration in them. You can easily change this by appending an action for the END rule which reports in case all the variables root, pub_id and sys_id are empty. As it is, the script parses the entire XML file, although the DTD is always positioned at the top, before the root element. Parsing the root element is unnecessary and you can improve the speed of the script significantly if you tell it to stop parsing when the first element (the root element) comes in.

  XMLSTARTELEM { nextfile } 


Jurgen Kahrs


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