Starting

All xmlWriter objects output to either a file (xmlWriterNewToFile()) or to an in-memory variable (xmlWriterNewToString()). You may also tell it to automatically indent the contents, with xmlWriterSetIndent().

The XML prolog (the <?xml version=...) is created with xmlWriterStartDocument(). While not absolutely required to be well-formed XML, it is recommended that all XML documents have an XML prolog.