Advanced Programming in the UNIX® Environment

ptg10805159

Section 4.14 File Systems 113

4.14 File Systems

To appreciate the concept of links to a file, we need a conceptual understanding of the

structure of the UNIX file system. Understanding the difference between an i-node and

adirectory entry that points to an i-node is also useful.

Various implementations of the UNIX file system areinuse today.Solaris, for

example, supports several types of disk file systems: the traditional BSD-derived UNIX

file system (calledUFS), a file system (calledPCFS) to read and write DOS-formatted

diskettes, and a file system (called HSFS) to read CD file systems. We saw one

difference between file system types in Figure2.20. UFSis based on the Berkeley fast

file system, which we describe in this section.

Each file system type has its own characteristic features — and some of these features can be

confusing. For example, most UNIX file systems support case-sensitive filenames. Thus, if

you create one file namedfile.txtand another namedfile.TXT,then two distinct files are

created. On Mac OS X, however,theHFSfile system is case-preserving with case-insensitive

comparisons. Thus, if you createfile.txt,when you try to createfile.TXT,you will

overwritefile.txt.However,only the name used when the file was created is stored in the

file system (the case-preserving aspect). In fact, any permutation of uppercase and lowercase

letters in the sequencef,i,l,e,.,t,x,twill match when searching for the file (the case-

insensitive comparison aspect). As a consequence, besidesfile.txtandfile.TXT,wecan

access the file with the namesFile.txt,fILE.tXt,andFiLe.TxT.

We can think of a disk drive being divided into one or morepartitions. Each

partition can contain a file system, as shown in Figure4.13. The i-nodes arefixed-length

entries that contain most of the information about a file.

disk drive partition partition partition

file system cylinder group 0 cylinder group 1 ... cylinder groupn

super

block

copy

cg

info

i-node

map

block

bitmap i-nodes data blocks

i-node i-node ... i-node

boot block(s)

super block

Figure 4.13 Disk drive, partitions, and a file system