Advanced Programming in the UNIX® Environment

ptg10805159

Section 4.24 Device Special Files 137

#include "apue.h"

int

main(void)

{

char *ptr;

size_t size;

if (chdir("/usr/spool/uucppublic") < 0)

err_sys("chdir failed");

ptr = path_alloc(&size); /* our own function */

if (getcwd(ptr, size) == NULL)

err_sys("getcwd failed");

printf("cwd = %s\n", ptr);

exit(0);

}

Figure 4.24Example ofgetcwdfunction

Note thatchdirfollows the symbolic link—as we expect it to, from Figure4.17 — but

when it goes up the directory tree,getcwdhas no idea when it hits the/var/spool

directory that it is pointed to by the symbolic link/usr/spool.This is a characteristic

of symbolic links.

Thegetcwdfunction is useful when we have an application that needs to return to

the location in the file system where it started out.We can save the starting location by

callinggetcwd before we change our working directory.After we complete our

processing, we can pass the pathname obtained fromgetcwdtochdirto return to our

starting location in the file system.

Thefchdirfunction provides us with an easy way to accomplish this task. Instead

of callinggetcwd, we canopenthe current directory and save the file descriptor before

we change to a different location in the file system. When we want to return to where

we started, we can simply pass the file descriptor tofchdir.

4.24 Device Special Files

The two fieldsst_devandst_rdevareoften confused.We’ll need to use these fields

in Section 18.9 when we write thettynamefunction. Therules for their use aresimple.

•Every file system is known by its major and minor device numbers, which are

encoded in the primitive system data typedev_t.The major number identifies

the device driver and sometimes encodes which peripheral boardto

communicate with; the minor number identifies the specific subdevice. Recall

from Figure4.13 that a disk drive often contains several file systems. Each file

system on the same disk drive would usually have the same major number,but

adifferent minor number.