This section outlines the formats of various files. The C structure declarations for the file formats are given where applicable. Usually, the headers containing these structure declarations can be found in the directories /usr/include or /usr/include/sys. For inclusion in C language programs, however, the syntax #include <filename.h> or #include <sys/filename.h> should be used.

Because the operating system now allows the existence of multiple file system types, there are several instances of multiple manual pages with the same name. These pages all display the name of the FSType to which they pertain, in the form name_ fstype at the top of the page. For example, fs_ufs(4).