If you ever have a problem with a QNX 4 filesystem, you'll need to understand how it stores data on a disk.
This knowledge will help you recognize and possibly correct damage if you ever have to rebuild a filesystem. The <sys/fs_qnx4.h> header file contains the definitions for the structures that this section describes.
For an overall description of the QNX 4 filesystem, see the Working with Filesystems chapter.