console(4) [linux man page]

CONSOLE(4)						     Linux Programmer's Manual							CONSOLE(4)

NAME

       console - console terminal and virtual consoles

DESCRIPTION

       A  Linux  system  has  up to 63 virtual consoles (character devices with major number 4 and minor number 1 to 63), usually called /dev/ttyn
       with 1 <= n <= 63.  The current console is also addressed by /dev/console or /dev/tty0, the character device with major number 4 and  minor
       number  0.   The  device  files	/dev/*	are  usually created using the script MAKEDEV, or using mknod(1), usually with mode 0622 and owner
       root.tty.

       Before kernel version 1.1.54 the number of virtual consoles was compiled into the kernel (in tty.h: #define NR_CONSOLES	8)  and  could	be
       changed by editing and recompiling.  Since version 1.1.54 virtual consoles are created on the fly, as soon as they are needed.

       Common ways to start a process on a console are: (a) tell init(1) (in inittab(5)) to start a mingetty(8) (or agetty(8)) on the console; (b)
       ask openvt(1) to start a process on the console; (c) start X--it will find the first unused console, and display its output there.   (There
       is also the ancient doshell(8).)

       Common  ways  to  switch consoles are: (a) use Alt+Fn or Ctrl+Alt+Fn to switch to console n; AltGr+Fn might bring you to console n+12 [here
       Alt and AltGr refer to the left and right Alt keys, respectively]; (b) use Alt+RightArrow or Alt+LeftArrow to cycle through  the  presently
       allocated  consoles; (c) use the program chvt(1).  (The key mapping is user settable, see loadkeys(1); the above mentioned key combinations
       are according to the default settings.)

       The command deallocvt(1) (formerly disalloc) will free the memory taken by the screen buffers for consoles that no longer have any  associ-
       ated process.

   Properties
       Consoles  carry a lot of state.	I hope to document that some other time.  The most important fact is that the consoles simulate vt100 ter-
       minals.	In particular, a console is reset to the initial state by printing the two characters ESC c.  All escape sequences can be found in
       console_codes(4).

FILES

       /dev/console
       /dev/tty*

SEE ALSO

       chvt(1),  deallocvt(1),	init(1),  loadkeys(1),	mknod(1),  openvt(1),  console_codes(4),  console_ioctl(4),  tty(4), ttyS(4), charsets(7),
       agetty(8), mapscrn(8), mingetty(8), resizecons(8), setfont(8)

Linux								    1994-10-31								CONSOLE(4)

VCS(4)							     Linux Programmer's Manual							    VCS(4)

NAME

       vcs, vcsa - virtual console memory

DESCRIPTION

       /dev/vcs0  is  a character device with major number 7 and minor number 0, usually of mode 0644 and owner root.tty.  It refers to the memory
       of the currently displayed virtual console terminal.

       /dev/vcs[1-63] are character devices for virtual console terminals, they have major number 7 and minor number 1 to 63,  usually	mode  0644
       and  owner  root.tty.   /dev/vcsa[0-63]	are the same, but using unsigned shorts (in host byte order) that include attributes, and prefixed
       with four bytes giving the screen dimensions and cursor position: lines, columns, x, y.	(x = y = 0 at the top left corner of the screen.)

       When a 512-character font is loaded, the 9th bit position can be fetched by applying the ioctl(2) VT_GETHIFONTMASK operation (available	in
       Linux kernels 2.6.18 and above) on /dev/tty[1-63]; the value is returned in the unsigned short pointed to by the third ioctl(2) argument.

       These  devices  replace	the screendump ioctl(2) operations of console(4), so the system administrator can control access using file system
       permissions.

       The devices for the first eight virtual consoles may be created by:

	   for x in 0 1 2 3 4 5 6 7 8; do
	       mknod -m 644 /dev/vcs$x c 7 $x;
	       mknod -m 644 /dev/vcsa$x c 7 $[$x+128];
	   done
	   chown root:tty /dev/vcs*

       No ioctl(2) requests are supported.

FILES

       /dev/vcs[0-63]
       /dev/vcsa[0-63]

VERSIONS

       Introduced with version 1.1.92 of the Linux kernel.

EXAMPLE

       You may do a screendump on vt3 by switching to vt1 and typing

	   cat /dev/vcs3 >foo

       Note that the output does not contain newline characters, so some processing may be required, like in

	   old -w 81 /dev/vcs3 | lpr

       or (horrors)

	   xetterm -dump 3 -file /proc/self/fd/1

       The /dev/vcsa0 device is used for Braille support.

       This program displays the character and screen attributes under the cursor of the second virtual console, then changes the background color
       there:

       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>
       #include <fcntl.h>
       #include <sys/ioctl.h>
       #include <linux/vt.h>

       int
       main(void)
       {
	   int fd;
	   char *device = "/dev/vcsa2";
	   char *console = "/dev/tty2";
	   struct {unsigned char lines, cols, x, y;} scrn;
	   unsigned short s;
	   unsigned short mask;
	   unsigned char ch, attrib;

	   fd = open(console, O_RDWR);
	   if (fd < 0) {
	       perror(console);
	       exit(EXIT_FAILURE);
	   }
	   if (ioctl(fd, VT_GETHIFONTMASK, &mask) < 0) {
	       perror("VT_GETHIFONTMASK");
	       exit(EXIT_FAILURE);
	   }
	   (void) close(fd);
	   fd = open(device, O_RDWR);
	   if (fd < 0) {
	       perror(device);
	       exit(EXIT_FAILURE);
	   }
	   (void) read(fd, &scrn, 4);
	   (void) lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), 0);
	   (void) read(fd, &s, 2);
	   ch = s & 0xff;
	   if (attrib & mask)
	       ch |= 0x100;
	   attrib = ((s & ~mask) >> 8);
	   printf("ch='%c' attrib=0x%02x
", ch, attrib);
	   attrib ^= 0x10;
	   (void) lseek(fd, -1, 1);
	   (void) write(fd, &attrib, 1);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO

       console(4), tty(4), ttyS(4), gpm(8)

COLOPHON

       This  page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2007-12-17								    VCS(4)