.\" Copyright (c) 1995 James R. Van Zandt <jrv@vanzandt.mv.com>

.\" Sat Feb 18 09:11:07 EST 1995

.\"

.\" %%%LICENSE_START(GPLv2+_DOC_FULL)

.\" This is free documentation; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

.\" published by the Free Software Foundation; either version 2 of

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.\"

.\" Modified, Sun Feb 26 15:08:05 1995, faith@cs.unc.edu

.\" 2007-12-17, Samuel Thibault <samuel.thibault@ens-lyon.org>:

.\"     document the VT_GETHIFONTMASK ioctl

.\" "

.TH VCS 4 2017-05-03 "Linux" "Linux Programmer's Manual"

.SH NAME

vcs, vcsa \- virtual console memory

.SH DESCRIPTION

.I /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.

.PP

.I /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.

.IR /dev/vcsa[0\-63]

are the same, but

using

.IR "unsigned short" s

(in host byte order) that include attributes,

and prefixed with four bytes giving the screen

dimensions and cursor position:

.IR lines ,

.IR columns ,

.IR x ,

.IR y .

.RI ( x

=

.I y

= 0 at the top left corner of the screen.)

.PP

When a 512-character font is loaded,

the 9th bit position can be fetched by applying the

.BR ioctl (2)

.B VT_GETHIFONTMASK

operation

(available in Linux kernels 2.6.18 and above)

on

.IR /dev/tty[1\-63] ;

the value is returned in the

.I "unsigned short"

pointed to by the third

.BR ioctl (2)

argument.

.PP

These devices replace the screendump

.BR ioctl (2)

operations of

.BR ioctl_console (2),

so the system

administrator can control access using filesystem permissions.

.PP

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

.PP

.in +4n

.EX

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*

.EE

.in

.PP

No

.BR ioctl (2)

requests are supported.

.SH FILES

.I /dev/vcs[0\-63]

.br

.I /dev/vcsa[0\-63]

.\" .SH AUTHOR

.\" Andries Brouwer <aeb@cwi.nl>

.SH VERSIONS

Introduced with version 1.1.92 of the Linux kernel.

.SH EXAMPLE

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

.PP

    cat /dev/vcs3 >foo

.PP

Note that the output does not contain

newline characters, so some processing may be required, like

in

.PP

    fold \-w 81 /dev/vcs3 | lpr

.PP

or (horrors)

.PP

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

.PP

The

.I /dev/vcsa0

device is used for Braille support.

.PP

This program displays the character and screen attributes under the

cursor of the second virtual console, then changes the background color

there:

.PP

.EX

#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=\(aq%c\(aq attrib=0x%02x\\n", ch, attrib);

    attrib ^= 0x10;

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

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

    exit(EXIT_SUCCESS);

}

.EE

.SH SEE ALSO

.BR ioctl_console (2),

.BR tty (4),

.BR ttyS (4),

.BR gpm (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.