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 ioctl_console(2), so the system administrator can control access using filesys-
       tem 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

	   fold -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

       ioctl_console(2), tty(4), ttyS(4), gpm(8)

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-05-03								    VCS(4)