An Operating Systems Vade Mecum

224 File Structures Chapter 6159234678101112disk Adisk Bdisk CFigure 6.15 Subdirectories crossing disk boundariesthe file descriptor associated with the given file name (for example, file descriptor2). Every access to a file descriptor is first checked against the table; if the filedescriptor is listed there, the file descriptor for the root of the mounted device isused instead. As a result, the contents of the file mentioned in the Mount call areno longer accessible.For example, if a process mounts disk B onto file 2 and then mounts disk C onto file 8,the virtual file structure looks like Figure 6.16. File 2 has been replaced by file 5, whichis a directory leading to other files. The old contents of file 2 are hidden. Mounting disksis usually performed by a startup process during initialization. However, disks can bemounted later; this facility is especially useful for dismountable units like floppies.Before a dismountable disk is physically removed, the file manager must be informed by12 (5)3 46 78 (9)10 11 12Figure 6.16 Result of mounting disks