VCS

vcs, vcsa ? virtual console memory

/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 including 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.)

These replace the screendump ioctls 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() requests are supported.

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) setterm -dump 3 -file /proc/self/fd/1.

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 <stdio.h>

#include <fcntl.h>

void main()

{

int fd;

struct {char lines, cols, x, y;} scrn;

char ch, attrib;

fd = open("/dev/vcsa2", O_RDWR);

(void)read(fd, &scrn, 4);

(void)lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), 0);

(void)read(fd, &ch, 1);

(void)read(fd, &attrib, 1);

printf("ch=’%c’ attrib=0x%02x\n", ch, attrib);

attrib ^= 0x10;

(void)lseek(fd, -1, 1);

(void)write(fd, &attrib, 1);

}

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

Andries Brouwer <aeb@cwi.nl>

Introduced with version 1.1.92 of the Linux kernel.

console(4), tty(4), ttys(4), selection(1)