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 R ioctl s 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.

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

Introduced with version 1.1.92 of the Linux kernel.

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.

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>

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

    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, &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);
    exit(EXIT_SUCCESS);
}

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