ch05: Files and Directories

ch05: Files and Directories

Ju, Hong TaekComputer Network Lab.

Keimyung [email protected]

Rm: 1228, Tel: 580-5234

Objectives Learn about file systems and directories Experiment with directory traversal Explore UNIX i-node implementation Use functions for accessing directories Understand hard links and symbolic links

5.1 Unix File System Navigation A file system is a collection of file and

attributes Operating systems organizes physical disks into

file systems to provide high-level logical access to the actual bytes of files

A directory is a file containing directory entries that associated a file name with the physical location of a file on disk

root directory is at the top of the file system

File system, directory entry, absolute path name

5.1 The current working directory Each process has an associated directory,

called the current working directory Path names do not begins with / are called

relative path names

#include <unistd.h>int chdir(const char *path);char *getcwd(char *buf, size_t size);

A more flexible approach uses the pathconf function to determine the real value for the maximum path length at run time sysconf: system wide limit pathconf: take a path name and a limit designator fpathconf: take a file descriptor and a limit designator

#include <unistd.h>long fpathconf(int filedes, int name);long pathconf(const char *path, int name);long sysconf(int name);

5.2 Directory Access Directory require specialized functions whose correspondin

g names end with “dir” opendir, closedir, readdir They should not be accessed with the ordinary open, close

and read functions

The DIR type represents a directory stream which is an ordered sequence of all of the directory entries in a particular directory

#include <dirent.h>DIR *opendir(const char *filename);struct dirent *readdir(DIR *dirp);void rewinddir(DIR *dirp);int closedir(DIR *dirp);

5.2.1 Accessing file status information Retrieving file status information

stat is given the name of a file. fstat is used for open files. lstat does the same thing as stat except that if the file

is a symbolic link, it gives information about the link, rather than the file it is linked to.

#include <sys/stat.h>int lstat(const char *restrict path, struct stat *restrict buf);int stat(const char *restrict path, struct stat *restrict buf);int fstat(int fildes, struct stat *buf);

The contents of the struct statdev_t st_dev; /* device ID of device containing file */ino_t st_ino; /* file serial number */mode_t st_mode; /* file mode */nlink_t st_nlink; /* number of hard links */uid_t st_uid; /* user ID of file */gid_t st_gid; /* group ID of file */off_t st_size; /* file size in bytes (regular files) */ /* path size (symbolic links) */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last data modification */time_t st_ctime; /* time of last file status change */

Macros to test the st_mode fieldfor the file type

5.3 Unix File System Implementation Structure of a typical UNIX file system

5.3.1 Unix file implementation i-node

directory entry contains only a name and an index into a table giving information about a file.

The table and the index are both referred to as an i-node.

5.4 Hard links and Symbolic links A link is an association between a filename and an i-node UNIX has two types of links: hard and symbolic (also called soft) Directory entries are called hard links because they directly link file

names to i-nodes Each i-node contains a count of the number of hard links to the i-n

ode. When a file is created, a new directory entry is created an a new i-

node is assigned. Additional hard links can be created within newname oldname

or with the link system call A new hard link to an existing file creates a new directory entry but

assigns no other additional disk space A new hard link increments the link count in the i-node A hard link can be removed with the rm command or the unlink sys

tem call These decrement the link count

The i-node and associated disk space are freed when the count is decremented to 0.

A directory entry, i-node, and data block for a simple file.

Two hard links to the same file for the previous figure

Copy

1. open("/dirA/name1");2. read3. close4. modify memory image file file5. rename("/dirA/name1","dirA/name1.bak");6. open("/dirA/name1");7. write8. close

Symbolic links A symbolic link is a special type of file that contai

ns the name of another file A reference to the name of a symbolic link cause

s the operating system to use the name stored in the file, rather than the name itself

Symbolic lines are created with the commandln -s newname oldname

Symbolic links do not affect the link count in the i-node.

Unlink hard links, symbolic links can span filesystems