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All SCO OpenServer files have a consistent structure to conceal physical properties of the device storing the file, such as the size of a disk track. It is not necessary, nor even possible, to preallocate space for a file. The size of a file is the number of bytes in it, with the last byte determined by the high-water mark of writes to the file. SCO OpenServer presents each file as a featureless, randomly addressable sequence of bytes arranged as a one-dimensional array of bytes ending with EOF.
The SCO OpenServer file system organizes files and directories into a tree-like structure of directories with files attached anywhere (and possibly multiply) into this hierarchy of directories. Files can be accessed by a ``full pathname'' or ``relative pathname'', have independent protection modes, are automatically allocated and de-allocated, and can be linked across directories.
In the hierarchically arranged directory tree-structure, each directory contains a list of names (character strings) and the associated file index, which implicitly refers to the same device as does the directory. Because directories are themselves files, the naming structure is potentially an arbitrary directed graph. Administrative rules restrict it to have the form of a tree, except that non-directory-files may have several names (entries in various directories).
The same non-directory-file may appear in several directories under possibly different names. This feature is called ``linking''; a directory-entry for a file is sometimes called a ``link''. SCO OpenServer differs from other systems in which linking is permitted in that all links to a file have equal status. That is, a file does not exist within a particular directory; the directory-entry for a file consists merely of its name and a pointer to the information actually describing the file. Thus, a file exists independently of any directory-entry, although in practice a file is removed along with the last link to it.