SolrException: Document [null] missing required field: id

I have changed the schema.xml file and added few fields in that like this

<field name="c_url" type="string" indexed="true" stored="true" />
   <field name="c_content_type" type="text" indexed="true" stored="true" />
   <field name="c_title" type="text" indexed="true" stored="true" />
   <field name="c_keywords" type="text" indexed="true" stored="true" multiValued="true" />
   <field name="c_text" type="text" indexed="true" stored="true" />
   <field name="c_timestamp" type="text" indexed="true" stored="true" />
   <field name="c_public" type="text" indexed="true" stored="true" multiValued="true" />
   <field name="c_groups" type="text" indexed="true" stored="true" multiValued="true" />
   <field name="c_sitename" type="text" indexed="true" stored="true" />
   <field name="c_context" type="text" indexed="true" stored="true" />
   <field name="c_modified_date" type="text" indexed="true" stored="true" />

so corresponding to these fields I have created one xml file and added some dummy data into that like this.

    <add><doc>
  <field name="c_url">http://www.google.com/</field>
  <field name="c_content_type">text/html</field>
  <field name="c_title">Testing Data</field>
  <field name="c_keywords">software</field>
  <field name="c_keywords">software_cycle</field>
  <field name="c_text">search</field>
  <field name="c_timestamp">2006-02-13T15:26:37Z</field>
  <field name="c_public">Optimized</field>
  <field name="c_public">Optimized_data</field>
  <field name="c_groups">Standards</field>
  <field name="c_groups">Standards_data</field>
  <field name="c_sitename">Google</field>
  <field name="c_context">Scalability</field>
  <field name="c_modified_date">2010-10-13T15:26:37Z</field>
  </doc></add>

And when I tried to reindex the data into solr like this:-

C:\apache-solr-3.2.0\example\exampledocs>java -Durl=http://localhost:7788/solr/u
pdate -jar post.jar *.xml
SimplePostTool: version 1.3
SimplePostTool: POSTing files to http://localhost:7788/solr/update..
SimplePostTool: POSTing file gb18030-example.xml
SimplePostTool: POSTing file hd.xml
SimplePostTool: POSTing file ipod_other.xml
SimplePostTool: POSTing file ipod_video.xml
SimplePostTool: POSTing file mem.xml
SimplePostTool: POSTing file monitor.xml
SimplePostTool: POSTing file monitor2.xml
SimplePostTool: POSTing file mp500.xml
SimplePostTool: POSTing file sd500.xml
SimplePostTool: POSTing file solr.xml
SimplePostTool: POSTing file text.xml
SimplePostTool: FATAL: Solr returned an error #400 Bad Request

I always get an error after text.xml file and If I remove this text.xml file then I don't get any error.. This is the below error I am getting if I include the text.xml file. Any help will be appreciated.

SEVERE: org.apache.solr.common.SolrException: Document [null] missing required field: id
        at org.apache.solr.update.DocumentBuilder.toDocument(DocumentBuilder.java:336)
        at org.apache.solr.update.processor.RunUpdateProcessor.processAdd(RunUpdateProcessorFactory.java:60)
        at org.apache.solr.handler.XMLLoader.processUpdate(XMLLoader.java:147)
        at org.apache.solr.handler.XMLLoader.load(XMLLoader.java:77)
        at org.apache.solr.handler.ContentStreamHandlerBase.handleRequestBody(ContentStreamHandlerBase.java:67)
        at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:129)
        at org.apache.solr.core.SolrCore.execute(SolrCore.java:1360)
        at org.apache.solr.servlet.SolrDispatchFilter.execute(SolrDispatchFilter.java:356)
        at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:252)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
        at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:864)
        at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:579)
        at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1665)
        at java.lang.Thread.run(Thread.java:662)