The XML prolog is the beginning of an XML system that defines programming information. It may contain version info, processing instructions, comments, and doctype. It always occurs at the beginning of the file and may include a byte order mark. Processing instructions and comments can appear anywhere in the system, but only those in the prolog are considered part of the XML declaration. Doctypes are a recommended but uncommon practice.
The XML (Extensible Markup Language) prolog is the portion of any XML system that comes before the root element of the system. This is the part of the XML system where XML programming information is defined. There is no standard for the XML prolog, but it will usually have some of the same traits from system to system. In some cases, it might even be empty because the user doesn’t want to declare any prolog information. Even if empty, it still exists in every XML system.
The XML prolog is sometimes referred to as an XML declaration. It is present in every system, but its use is optional. When used, it typically contains version information, processing instructions, any author comments, and a field for document type definitions, commonly referred to as a doctype. Sometimes it will also contain forwarding information to HyperText Markup Language (HTML) if the user wants to merge XML and HTML.
The XML prolog will always occur in the first part of the file. It will never follow behind another section. The one exception is that a byte order mark may precede the XML prolog when certain types of character encodings are used. In this case, the byte order mark declares the type of character encoding used in the XML system, usually with Unicode (UTF) transformation format. A range of character encodings are available, but UTF-8 and UTF-16 are the most common and recommended encodings; if no byte order mark is specified, computers will read the system as UTF-8 or UTF-16, the default encodings.
The processing instructions define how the application will process the information within the prolog and XML system. Processing instructions can appear anywhere in an XML system. Only the processing instructions in the XML prolog will be considered part of the XML declaration.
Comments, like processing information, can be found anywhere in the XML system, and only comments in the prolog are considered part of the XML prolog. Comments are free text datasets that have no influence on scripting within the system and are usually there to help the programmer remember something or to tell other programmers something. They often include author information, version number, or any other information the author wishes to pass to other script editors. Finally, some authors will include doctypes. This is a recommended but still uncommon practice among authors.
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