vcs man page

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_ioctl(4), so the system administrator can control access using filesystem 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\n", ch, attrib);
    attrib ^= 0x10;
    (void) lseek(fd, -1, 1);
    (void) write(fd, &attrib, 1);
    exit(EXIT_SUCCESS);
}

See Also

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

Colophon

This page is part of release 4.08 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/.

Referenced By

console_ioctl(4), lcdvc(1).

vcsa(4) is an alias of vcs(4).

2016-07-17 Linux Linux Programmer's Manual