6. Devices
- A device driver hides the hardware device’s communication protocols from the operating system and allows the system to interact with the device through a standardized interface.
- Processes can communicate with a device driver via file-like objects.
6.1 Device Types
- A character device represents a hardware device that reads or writes a serial stream of data bytes.
- A block device represents a hardware device that reads or writes data in fixed-size blocks.
6.2 Device Numbers
- Linux identifies devices using two numbers: the major device number and the minor device number.
- The major device number specifies which driver the device corresponds to.
- Minor device numbers distinguish individual devices or components controlled by a single driver.
- The special entry /proc/devices lists major device numbers corresponding to active device drivers currently loaded into the kernel.
[liuchao@localhost ~]$ cat /proc/devices
Character devices:
1 mem
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
6 lp
7 vcs
10 misc
13 input
14 sound
21 sg
29 fb
99 ppdev
116 alsa
128 ptm
136 pts
180 usb
189 usb_device
216 rfcomm
254 pcmcia
Block devices:
1 ramdisk
1 ramdisk
2 fd
8 sd
9 md
22 ide1
65 sd
66 sd
67 sd
68 sd
69 sd
70 sd
71 sd
128 sd
129 sd
130 sd
131 sd
132 sd
133 sd
134 sd
135 sd
253 device-mapper
254 mdp
6.3 Device Entries
- Only superuser processes can create block and character devices using the mknod command or the mknod system call.
- To create a device using the mknod command
- The first argument is the path of the entry in the file system.
- The second argument is b for a block device or c for a character device.
- Provide the major and minor device numbers as the third and fourth arguments.
% mknod ./lp0 c 6 0
- The ls command displays device entries specially.
% ls -l lp0
c(b for block device and c for character device)rw-r----- 1 root root 6, 0(major and minor device numbers) Mar 7 17:03 lp0
- To remove the entry, use rm.
% rm ./lp0
- The /dev Directory
[liuchao@localhost ~]$ ls -all /dev/
total 8
drwxr-xr-x 12 root root 3840 Aug 2 13:22 .
drwxr-xr-x 24 root root 4096 Aug 2 13:20 ..
crw-rw---- 1 root root 14, 12 Aug 2 13:20 adsp
crw------- 1 root root 10, 175 Aug 2 13:20 agpgart
crw------- 1 liuchao root 14, 4 Aug 2 13:20 audio
drwxr-xr-x 3 root root 60 Aug 2 13:20 bus
lrwxrwxrwx 1 root root 3 Aug 2 13:20 cdrom -> hdc
lrwxrwxrwx 1 root root 3 Aug 2 13:20 cdrom-hdc -> hdc
crw------- 1 liuchao root 5, 1 Aug 2 13:20 console
lrwxrwxrwx 1 root root 11 Aug 2 13:20 core -> /proc/kcore
drwxr-xr-x 6 root root 120 Aug 2 13:20 disk
brw-r----- 1 root disk 253, 0 Aug 2 2008 dm-0
brw-r----- 1 root disk 253, 1 Aug 2 2008 dm-1
crw-rw---- 1 root root 14, 9 Aug 2 13:20 dmmidi
crw------- 1 liuchao root 14, 3 Aug 2 13:20 dsp
lrwxrwxrwx 1 root root 13 Aug 2 13:20 fd -> /proc/self/fd
brw-rw---- 1 liuchao floppy 2, 0 Aug 2 13:20 fd0
brw-rw---- 1 liuchao floppy 2, 84 Aug 2 13:20 fd0u1040
brw-rw---- 1 liuchao floppy 2, 88 Aug 2 13:20 fd0u1120
brw-rw---- 1 liuchao floppy 2, 28 Aug 2 13:20 fd0u1440
brw-rw---- 1 liuchao floppy 2, 44 Aug 2 13:20 fd0u1680
brw-rw---- 1 liuchao floppy 2, 60 Aug 2 13:20 fd0u1722
brw-rw---- 1 liuchao floppy 2, 76 Aug 2 13:20 fd0u1743
brw-rw---- 1 liuchao floppy 2, 96 Aug 2 13:20 fd0u1760
brw-rw---- 1 liuchao floppy 2, 116 Aug 2 13:20 fd0u1840
brw-rw---- 1 liuchao floppy 2, 100 Aug 2 13:20 fd0u1920
brw-rw---- 1 liuchao floppy 2, 12 Aug 2 13:20 fd0u360
brw-rw---- 1 liuchao floppy 2, 16 Aug 2 13:20 fd0u720
brw-rw---- 1 liuchao floppy 2, 120 Aug 2 13:20 fd0u800
brw-rw---- 1 liuchao floppy 2, 52 Aug 2 13:20 fd0u820
brw-rw---- 1 liuchao floppy 2, 68 Aug 2 13:20 fd0u830
lrwxrwxrwx 1 root root 3 Aug 2 13:20 floppy-fd0 -> fd0
crw-rw-rw- 1 root root 1, 7 Aug 2 13:20 full
srwx------ 1 liuchao root 0 Aug 2 13:21 gpmctl
brw------- 1 liuchao disk 22, 0 Aug 2 13:20 hdc
crw------- 1 root root 10, 228 Aug 2 13:20 hpet
prw------- 1 root root 0 Aug 2 13:22 initctl
drwxr-xr-x 2 root root 120 Aug 2 13:20 input
crw------- 1 root root 1, 11 Aug 2 13:20 kmsg
srw-rw-rw- 1 root root 0 Aug 2 13:21 log
brw-r----- 1 root disk 7, 0 Aug 2 13:20 loop0
brw-r----- 1 root disk 7, 1 Aug 2 13:20 loop1
brw-r----- 1 root disk 7, 2 Aug 2 13:20 loop2
brw-r----- 1 root disk 7, 3 Aug 2 13:20 loop3
brw-r----- 1 root disk 7, 4 Aug 2 13:20 loop4
brw-r----- 1 root disk 7, 5 Aug 2 13:20 loop5
brw-r----- 1 root disk 7, 6 Aug 2 13:20 loop6
brw-r----- 1 root disk 7, 7 Aug 2 13:20 loop7
crw-rw---- 1 root lp 6, 0 Aug 2 13:20 lp0
lrwxrwxrwx 1 root root 13 Aug 2 13:20 MAKEDEV -> /sbin/MAKEDEV
drwxr-xr-x 2 root root 100 Aug 2 2008 mapper
brw-r----- 1 root disk 9, 0 Aug 2 13:20 md0
crw-r----- 1 root kmem 1, 1 Aug 2 13:20 mem
crw------- 1 liuchao root 14, 2 Aug 2 13:20 midi
crw------- 1 liuchao root 14, 0 Aug 2 13:20 mixer
drwxr-xr-x 2 root root 60 Aug 2 13:20 net
crw-rw-rw- 1 root root 1, 3 Aug 2 13:20 null
crw-rw---- 1 root root 10, 144 Aug 2 13:20 nvram
lrwxrwxrwx 1 root root 3 Aug 2 13:20 par0 -> lp0
crw-rw---- 1 root lp 99, 0 Aug 2 13:20 parport0
crw-rw---- 1 root lp 99, 1 Aug 2 13:20 parport1
crw-rw---- 1 root lp 99, 2 Aug 2 13:20 parport2
crw-rw---- 1 root lp 99, 3 Aug 2 13:20 parport3
crw-r----- 1 root kmem 1, 4 Aug 2 13:20 port
crw------- 1 root root 108, 0 Aug 2 13:20 ppp
crw-rw-rw- 1 root tty 5, 2 Aug 2 13:42 ptmx
drwxr-xr-x 2 root root 0 Aug 2 2008 pts
lrwxrwxrwx 1 root root 4 Aug 2 13:20 ram -> ram1
brw-r----- 1 root disk 1, 0 Aug 2 2008 ram0
brw-r----- 1 root disk 1, 1 Aug 2 2008 ram1
brw-r----- 1 root disk 1, 10 Aug 2 2008 ram10
brw-r----- 1 root disk 1, 11 Aug 2 2008 ram11
brw-r----- 1 root disk 1, 12 Aug 2 2008 ram12
brw-r----- 1 root disk 1, 13 Aug 2 2008 ram13
brw-r----- 1 root disk 1, 14 Aug 2 2008 ram14
brw-r----- 1 root disk 1, 15 Aug 2 2008 ram15
brw-r----- 1 root disk 1, 2 Aug 2 2008 ram2
brw-r----- 1 root disk 1, 3 Aug 2 2008 ram3
brw-r----- 1 root disk 1, 4 Aug 2 2008 ram4
brw-r----- 1 root disk 1, 5 Aug 2 2008 ram5
brw-r----- 1 root disk 1, 6 Aug 2 2008 ram6
brw-r----- 1 root disk 1, 7 Aug 2 2008 ram7
brw-r----- 1 root disk 1, 8 Aug 2 2008 ram8
brw-r----- 1 root disk 1, 9 Aug 2 2008 ram9
lrwxrwxrwx 1 root root 4 Aug 2 13:20 ramdisk -> ram0
crw-rw-rw- 1 root root 1, 8 Aug 2 13:20 random
brw------- 1 root root 253, 0 Aug 2 2008 root
crw-r--r-- 1 root root 10, 135 Aug 2 2008 rtc
brw-r----- 1 root disk 8, 0 Aug 2 2008 sda
brw-r----- 1 root disk 8, 1 Aug 2 13:20 sda1
brw-r----- 1 root disk 8, 2 Aug 2 2008 sda2
crw------- 1 liuchao root 14, 1 Aug 2 13:20 sequencer
crw-rw---- 1 root root 14, 8 Aug 2 13:20 sequencer2
crw------- 1 root root 21, 0 Aug 2 13:20 sg0
drwxrwxrwt 2 root root 40 Aug 2 13:20 shm
drwxr-xr-x 2 root root 180 Aug 2 13:20 snd
lrwxrwxrwx 1 root root 15 Aug 2 13:20 stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 Aug 2 13:20 stdin -> /proc/self/fd/0
lrwxrwxrwx 1 root root 15 Aug 2 13:20 stdout -> /proc/self/fd/1
crw------- 1 root root 4, 0 Aug 2 2008 systty
crw-rw-rw- 1 root tty 5, 0 Aug 2 13:20 tty
crw-rw---- 1 root root 4, 0 Aug 2 2008 tty0
crw------- 1 root root 4, 1 Aug 2 13:21 tty1
crw-rw---- 1 root tty 4, 10 Aug 2 2008 tty10
crw-rw---- 1 root tty 4, 11 Aug 2 2008 tty11
crw-rw---- 1 root tty 4, 12 Aug 2 2008 tty12
crw-rw---- 1 root tty 4, 13 Aug 2 13:20 tty13
crw-rw---- 1 root tty 4, 14 Aug 2 13:20 tty14
crw-rw---- 1 root tty 4, 15 Aug 2 13:20 tty15
crw-rw---- 1 root tty 4, 16 Aug 2 13:20 tty16
crw-rw---- 1 root tty 4, 17 Aug 2 13:20 tty17
crw-rw---- 1 root tty 4, 18 Aug 2 13:20 tty18
crw-rw---- 1 root tty 4, 19 Aug 2 13:20 tty19
crw------- 1 root root 4, 2 Aug 2 13:21 tty2
crw-rw---- 1 root tty 4, 20 Aug 2 13:20 tty20
crw-rw---- 1 root tty 4, 21 Aug 2 13:20 tty21
crw-rw---- 1 root tty 4, 22 Aug 2 13:20 tty22
crw-rw---- 1 root tty 4, 23 Aug 2 13:20 tty23
crw-rw---- 1 root tty 4, 24 Aug 2 13:20 tty24
crw-rw---- 1 root tty 4, 25 Aug 2 13:20 tty25
crw-rw---- 1 root tty 4, 26 Aug 2 13:20 tty26
crw-rw---- 1 root tty 4, 27 Aug 2 13:20 tty27
crw-rw---- 1 root tty 4, 28 Aug 2 13:20 tty28
crw-rw---- 1 root tty 4, 29 Aug 2 13:20 tty29
crw------- 1 root root 4, 3 Aug 2 13:21 tty3
crw-rw---- 1 root tty 4, 30 Aug 2 13:20 tty30
crw-rw---- 1 root tty 4, 31 Aug 2 13:20 tty31
crw-rw---- 1 root tty 4, 32 Aug 2 13:20 tty32
crw-rw---- 1 root tty 4, 33 Aug 2 13:20 tty33
crw-rw---- 1 root tty 4, 34 Aug 2 13:20 tty34
crw-rw---- 1 root tty 4, 35 Aug 2 13:20 tty35
crw-rw---- 1 root tty 4, 36 Aug 2 13:20 tty36
crw-rw---- 1 root tty 4, 37 Aug 2 13:20 tty37
crw-rw---- 1 root tty 4, 38 Aug 2 13:20 tty38
crw-rw---- 1 root tty 4, 39 Aug 2 13:20 tty39
crw------- 1 root root 4, 4 Aug 2 13:21 tty4
crw-rw---- 1 root tty 4, 40 Aug 2 13:20 tty40
crw-rw---- 1 root tty 4, 41 Aug 2 13:20 tty41
crw-rw---- 1 root tty 4, 42 Aug 2 13:20 tty42
crw-rw---- 1 root tty 4, 43 Aug 2 13:20 tty43
crw-rw---- 1 root tty 4, 44 Aug 2 13:20 tty44
crw-rw---- 1 root tty 4, 45 Aug 2 13:20 tty45
crw-rw---- 1 root tty 4, 46 Aug 2 13:20 tty46
crw-rw---- 1 root tty 4, 47 Aug 2 13:20 tty47
crw-rw---- 1 root tty 4, 48 Aug 2 13:20 tty48
crw-rw---- 1 root tty 4, 49 Aug 2 13:20 tty49
crw------- 1 root root 4, 5 Aug 2 13:21 tty5
crw-rw---- 1 root tty 4, 50 Aug 2 13:20 tty50
crw-rw---- 1 root tty 4, 51 Aug 2 13:20 tty51
crw-rw---- 1 root tty 4, 52 Aug 2 13:20 tty52
crw-rw---- 1 root tty 4, 53 Aug 2 13:20 tty53
crw-rw---- 1 root tty 4, 54 Aug 2 13:20 tty54
crw-rw---- 1 root tty 4, 55 Aug 2 13:20 tty55
crw-rw---- 1 root tty 4, 56 Aug 2 13:20 tty56
crw-rw---- 1 root tty 4, 57 Aug 2 13:20 tty57
crw-rw---- 1 root tty 4, 58 Aug 2 13:20 tty58
crw-rw---- 1 root tty 4, 59 Aug 2 13:20 tty59
crw------- 1 root root 4, 6 Aug 2 13:21 tty6
crw-rw---- 1 root tty 4, 60 Aug 2 13:20 tty60
crw-rw---- 1 root tty 4, 61 Aug 2 13:20 tty61
crw-rw---- 1 root tty 4, 62 Aug 2 13:20 tty62
crw-rw---- 1 root tty 4, 63 Aug 2 13:20 tty63
crw-rw---- 1 root root 4, 7 Aug 2 2008 tty7
crw-rw---- 1 root root 4, 8 Aug 2 13:20 tty8
crw-rw---- 1 root tty 4, 9 Aug 2 2008 tty9
crw-rw---- 1 root uucp 4, 64 Aug 2 2008 ttyS0
crw-rw---- 1 root uucp 4, 65 Aug 2 2008 ttyS1
crw-rw---- 1 root uucp 4, 66 Aug 2 2008 ttyS2
crw-rw---- 1 root uucp 4, 67 Aug 2 2008 ttyS3
drwxr-xr-x 4 root root 80 Aug 2 13:33 .udev
cr--r--r-- 1 root root 1, 9 Aug 2 13:20 urandom
crw------- 1 vcsa tty 7, 0 Aug 2 13:20 vcs
crw------- 1 vcsa tty 7, 1 Aug 2 13:21 vcs1
crw------- 1 vcsa tty 7, 2 Aug 2 13:21 vcs2
crw------- 1 vcsa tty 7, 3 Aug 2 13:21 vcs3
crw------- 1 vcsa tty 7, 4 Aug 2 13:21 vcs4
crw------- 1 vcsa tty 7, 5 Aug 2 13:21 vcs5
crw------- 1 vcsa tty 7, 6 Aug 2 13:21 vcs6
crw------- 1 vcsa tty 7, 7 Aug 2 13:22 vcs7
crw------- 1 vcsa tty 7, 8 Aug 2 13:20 vcs8
crw------- 1 vcsa tty 7, 128 Aug 2 13:20 vcsa
crw------- 1 vcsa tty 7, 129 Aug 2 13:21 vcsa1
crw------- 1 vcsa tty 7, 130 Aug 2 13:21 vcsa2
crw------- 1 vcsa tty 7, 131 Aug 2 13:21 vcsa3
crw------- 1 vcsa tty 7, 132 Aug 2 13:21 vcsa4
crw------- 1 vcsa tty 7, 133 Aug 2 13:21 vcsa5
crw------- 1 vcsa tty 7, 134 Aug 2 13:21 vcsa6
crw------- 1 vcsa tty 7, 135 Aug 2 13:22 vcsa7
crw------- 1 vcsa tty 7, 136 Aug 2 13:20 vcsa8
drwx------ 2 root root 80 Aug 2 13:20 VolGroup00
lrwxrwxrwx 1 root root 4 Aug 2 13:20 X0R -> null
crw-rw-rw- 1 root root 1, 5 Aug 2 13:20 zero
- Accessing Devices by Opening Files
int fd = open (“/dev/lp0”, O_WRONLY);
write (fd, buffer, buffer_length);
close (fd);
6.4 Hardware Devices
- Partial Listing of Common Block Devices
Device
Name
Major
Minor
First floppy drive
/dev/fd0
2
0
Second floppy drive
/dev/fd1
2
1
Primary IDE controller, master device
/dev/hda
3
0
Primary IDE controller, master device, first partition
/dev/hda1
3
1
Primary IDE controller, secondary device
/dev/hdb
3
64
Primary IDE controller, secondary device, first partition
/dev/hdb1
3
65
Secondary IDE controller, master device
/dev/hdc
22
0
Secondary IDE controller, secondary device
/dev/hdd
22
64
First SCSI drive
/dev/sda
8
0
First SCSI drive, first partition
/dev/sda1
8
1
Second SCSI disk
/dev/sdb
8
16
Second SCSI disk, first partition
/dev/sdb1
8
17
First SCSI CD-ROM drive
/dev/scd0
11
0
Second SCSI CD-ROM drive
/dev/scd1
11
1
- Some Common Character Devices
Device
Name
Major
Minor
Parallel port 0
/dev/lp0 or /dev/par0
6
0
Parallel port 1
/dev/lp1 or /dev/par1
6
1
First serial port
/dev/ttyS0
4
64
Second serial port
/dev/ttyS1
4
65
IDE tape drive
/dev/ht0
37
0
First SCSI tape drive
/dev/st0
9
0
Second SCSI tape drive
/dev/st1
9
1
System console
/dev/console
5
1
First virtual terminal
/dev/tty1
4
1
Second virtual terminal
/dev/tty2
4
2
Process’s current terminal device
/dev/tty
5
0
Sound card
/dev/audio
14
4
6.5 Special Devices
6.5.1. /dev/null
- Linux discards any data written to /dev/null.
Run a command and discard its standard output, redirect standard output to /dev/null:
% verbose_command > /dev/null
- Reading from /dev/null always results in an end-of-file.
If you copy from /dev/null to another file, the destination will be a zero-length file:
% cp /dev/null empty-file
% ls -l empty-file
-rw-rw---- 1 samuel samuel 0 Mar 8 00:27 empty-file
6.5.2. /dev/zero
- The device entry /dev/zero behaves as if it were an infinitely long file filled with 0 bytes.
- As much data as you’d try to read from /dev/zero, Linux “generates” enough 0 bytes.
% ./hexdump /dev/zero
0x000000 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000010 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000020 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x000030 : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
...
- Memory mapping /dev/zero is an advanced technique for allocating memory.
6.5.3. /dev/full
- The entry /dev/full behaves as if it were a file on a file system that has no more room.
- A write to /dev/full fails and sets errno to ENOSPC.
% cp /etc/fstab /dev/full
cp: /dev/full: No space left on device
6.5.4. Random Number Devices
- The rand function actually generates pseudorandom numbers.
- By measuring the time delay between your input actions, Linux is capable of generating an unpredictable stream of high-quality random numbers.
- You can access this stream by reading from /dev/random and /dev/urandom.
- When Linux exhausts its store of randomness by reading a large number of bytes from /dev/random, Linux blocks the read operation.
Try displaying the contents of /dev/random using the od command
% od -t x1 /dev/random
0000000 2c 9c 7a db 2e 79 3d 65 36 c2 e3 1b 52 75 1e 1a
0000020 d3 6d 1e a7 91 05 2d 4d c3 a6 de 54 29 f4 46 04
0000040 b3 b0 8d 94 21 57 f3 90 61 dd 26 ac 94 c3 b9 3a
0000060 05 a3 02 cb 22 0a bc c9 45 dd a6 59 40 22 53 d4
The output will eventually pause when Linux exhausts its store of randomness. Now try moving your mouse or typing on the keyboard, and watch additional random numbers appear.
- A read from /dev/urandom will never block.
- If Linux runs out of randomness, it uses a cryptographic algorithm to generate pseudorandom bytes from the past sequence of random bytes.
- Function to Generate a Random Number from /dev/random
#include <assert.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
/* Return a random integer between MIN and MAX, inclusive. Obtain
randomness from /dev/random. */
int random_number (int min, int max)
{
/* Store a file descriptor opened to /dev/random in a static variable. That way, we don’t need to open the file every time this function is called. */
static int dev_random_fd = -1;
char* next_random_byte;
int bytes_to_read;
unsigned random_value;
/* Make sure MAX is greater than MIN. */
assert (max > min);
/* If this is the first time this function is called, open a file descriptor to /dev/random. */
if (dev_random_fd == -1) {
dev_random_fd = open (“/dev/random”, O_RDONLY);
assert (dev_random_fd != -1);
}
/* Read enough random bytes to fill an integer variable. */
next_random_byte = (char*) &random_value;
bytes_to_read = sizeof (random_value);
/* Loop until we’ve read enough bytes. Because /dev/random is filled from user-generated actions, the read may block and may only return a single random byte at a time. */
do {
int bytes_read;
bytes_read = read (dev_random_fd, next_random_byte, bytes_to_read);
bytes_to_read -= bytes_read;
next_random_byte += bytes_read;
} while (bytes_to_read > 0);
/* Compute a random number in the correct range. */
return min + (random_value % (max - min + 1));
}
int main(int argc, char * argv)
{
while(1)
{
int random = random_number(10, 20);
printf("Get a random number : %d\n",random);
}
}
- Run the program compiled from the above code.
[liuchao@localhost Ramdom]$ ./random_number
Get a random number : 11
Get a random number : 17
Get a random number : 11
Get a random number : 18
Get a random number : 13
Get a random number : 11
Get a random number : 14
Get a random number : 20
- Type something randomly in the terminal, and the result will be
[liuchao@localhost Ramdom]$ ./random_number
Get a random number : 11
Get a random number : 17
Get a random number : 11
Get a random number : 18
Get a random number : 13
Get a random number : 11
Get a random number : 14
Get a random number : 20
Get a random number : 19
Get a random number : 11
Get a random number : 16
Get a random number : 14
Get a random number : 12
Get a random number : 18
Get a random number : 12
Get a random number : 15
Get a random number : 17
Get a random number : 13
Get a random number : 15
Get a random number : 11
Get a random number : 11
Get a random number : 16
6.5.5. Loopback Devices
- A loopback device enables you to simulate a block device using an ordinary disk file.
- Loopback devices are named /dev/loop0, /dev/loop1, and so on. Each can be used to simulate a single block device at one time.
- Only the superuser can set up a loopback device.
- To construct a virtual file system and mount it with a loopback device, follow these steps:
- Create an empty file to hold the virtual file system.
% dd if=/dev/zero of=/tmp/disk-image count=20480
20480+0 records in
20480+0 records out
% ls -l /tmp/disk-image
-rw-rw---- 1 root r oot 1 0485760 Mar 8 01:56 /tmp/disk-image
- Construct the file to an ext2 file system with the mke2fs command.
% mke2fs -q /tmp/disk-image
mke2fs 1.18, 11-Nov-1999 for EXT2 FS 0.5b, 95/08/09
disk-image is not a block special device.
Proceed anyway? (y,n) y
- Create an empty directory and mount the file system to the directory using a loopback device.
% mkdir /tmp/virtual-fs
% mount -o loop=/dev/loop0 /tmp/disk-image /tmp/virtual-fs
% df -h /tmp/virtual-fs
Filesystem Size Used Avail Use% Mounted on
/tmp/disk-image 9.7M 13k 9.2M 0% /tmp/virtual-fs
- You can use it like any other disk:
% cd /tmp/virtual-fs
% echo ‘Hello, world!’ > test.txt
% ls -l
total 13
drwxr-xr-x 2 root root 12288 Mar 8 02:00 lost+found
-rw-rw---- 1 root root 14 Mar 8 02:12 test.txt
% cat test.txt
Hello, world!
- When you’re done, unmount the virtual file system.
% cd /tmp
% umount /tmp/virtual-fs
- Instead of creating a file system from scratch, you can copy one directly from a device and mount it as a loop device.
% cp /dev/cdrom /tmp/cdrom-image
% mount -o loop=/dev/loop0 /tmp/cdrom-image /mnt/cdrom
6.6 PTYs
- devpts mounted at /dev/pts and associated with none hardware device is a “magic” file system created by the Linux kernel.
- /dev/pts is a special directory that varies with time and reflects the state of the running system.
- Linux creates a PTY for every new terminal window you open and displays a corresponding entry in /dev/pts.
- The PTY device acts like a terminal device which accepts input from the keyboard and displays text output from the programs.
- You can determine the PTY associated with a given terminal window
% ps -o pid,tty,cmd
PID TT CMD
28832 pts/4 bash
29287 pts/4 ps -o pid,tty,cmd
This particular terminal window is running in PTY 4.
- You can read from or write to the PTY device.
- If you read from it, you’ll hijack keyboard input that would otherwise be sent to the program running in the PTY.
- If you write to it, the data will appear in that window.
Open two terminals
In one terminal
[liuchao@localhost ~]$ ps -o pid,tty,cmd
PID TT CMD
2413 pts/1 -bash
3717 pts/1 ps -o pid,tty,cmd
In the other
[liuchao@localhost ~]$ ps -o pid,tty,cmd
PID TT CMD
3163 pts/2 -bash
3742 pts/2 ps -o pid,tty,cmd
In the first terminal
[liuchao@localhost ~]$ echo "hello world" > /dev/pts/2
In the second terminal
[liuchao@localhost ~]$ hello world
6.7 ioctl
- The ioctl system call is an all-purpose interface for controlling hardware devices.
- The first argument is a file descriptor.
- The second argument is a request code that indicates the operation that you want to perform.
- (cdrom-eject.c) Eject a CD-ROM
#include <fcntl.h>
#include <linux/cdrom.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int main (int argc, char* argv[])
{
/* Open a file descriptor to the device specified on the command line. */
int fd = open (argv[1], O_RDONLY);
/* Eject the CD-ROM. */
ioctl (fd, CDROMEJECT);
/* Close the file descriptor. */
close (fd);
return 0;
}