.HLP File Format

Introduction

A .hlp file is a compiled help file for WinHelp, the hypertext help viewer of Microsoft Windows. WinHelp shipped with Windows 3.0 in 1990 and stayed the standard way to write Windows help until Windows XP. Authors wrote topics in Rich Text Format, listed them in a project (.hpj) file and ran a help compiler, which packed the text, pictures, links and index into one compressed file for reading. Microsoft phased WinHelp out from Windows Vista, which came without it, offered the viewer as a separate download up to Windows 8.1, and removed it from Windows 10.

The format is reverse-engineered. Microsoft never published it. What is known comes from Pete Davis’s articles in Dr. Dobb’s Journal in 1993, Manfred Winterhoff’s HELPDECO notes, and the winhlp32 program in the Wine project. These pages record what those sources say and what has been checked against real files; fields whose meaning nobody has worked out are marked as unknown.

A .hlp file dropped on any page opens in the explorer. It shows the tree of internal files, the readable text of every topic with its links, the keywords, the pictures and the fonts.

Structure

A help file is a small file system. After a 16-byte header come the internal files, one after another in any order, with a directory that names them. Each internal file has a 9-byte header and then its content. Space freed by the compiler is kept on a free list.

Versions

The |SYSTEM internal file holds a version number, Minor, set by the compiler. The layout changed with the compiler, but one reader can handle them all once it has read this number. The reference pages note each difference where it matters.

HELPDECO’s notes also list Minor 27 for some Multimedia Viewer files (.mvb), which use the same container with records of their own.

Links