man4/vcs.4

.\" Copyright (c) 1995 James R. Van Zandt <jrv@vanzandt.mv.com>
.\" Sat Feb 18 09:11:07 EST 1995
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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 2020-11-01 "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 with mode 0644 and ownership 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 ownership 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 EXAMPLES
You may do a screendump on vt3 by switching to vt1 and typing
.PP
.in +4n
.EX
cat /dev/vcs3 >foo
.EE
.in
.PP
Note that the output does not contain
newline characters, so some processing may be required, like
in
.PP
.in +4n
.EX
fold \-w 81 /dev/vcs3 | lpr
.EE
.in
.PP
or (horrors)
.PP
.in +4n
.EX
setterm \-dump 3 \-file /proc/self/fd/1
.EE
.in
.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 attrib;
    int ch;

    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), SEEK_SET);
    (void) read(fd, &s, 2);
    ch = s & 0xff;
    if (s & mask)
        ch |= 0x100;
    attrib = ((s & \(timask) >> 8);
    printf("ch=%#03x attrib=%#02x\en", ch, attrib);
    s \(ha= 0x1000;
    (void) lseek(fd, \-2, SEEK_CUR);
    (void) write(fd, &s, 2);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR ioctl_console (2),
.BR tty (4),
.BR ttyS (4),
.BR gpm (8)