File-on-File Mounting File System

The File-on-File Mounting (FFM) file system allows regular, character, or block-special files to be mounted over regular files, and, for the most part, is only used by the SVR4-compatible system calls fattach and fdetach of a STREAMS-based pipe (or FIFO). With FFM, a FIFO, which normally has no file system object associated with it, is given a name in the file system space. As a result, a process that is unrelated to the process that created the FIFO can then access the FIFO.

In addition to programs using FFM through the fattach system call, users can mount one regular file on top of another using the mount command. Mounting a file on top of another file does not destroy the contents of the covered file; it simply associates the name of the covered file with the mounted file, making the contents of the covered file temporarily unavailable. The covered file can be accessed after the file mounted on top of it is unmounted, either by a reboot or by a call to fdetach, or by entering the umount command. Note that the contents of the covered file are still available to any process which had the file open at the time of the call to fattach or when a user issued a mount command that covered the file.