man-pages/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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.TH VCS 4 1995-02-19 "Linux" "Linux Programmer's Manual"
.SH NAME
vcs, vcsa \- virtual console memory
.SH DESCRIPTION
\fI/dev/vcs0\fP 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.
.LP
\fI/dev/vcs[1\-63]\fP 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.  \fI/dev/vcsa[0\-63]\fP are the same, but
including attributes, and prefixed with four bytes giving the screen
dimensions and cursor position: \fIlines\fP, \fIcolumns\fP, \fIx\fP, \fIy\fP.
(\fIx\fP = \fIy\fP = 0 at the top left corner of the screen.)
.PP
These replace the screendump
.IR ioctl s
of \fBconsole\fP(4), so the system
administrator can control access using file system permissions.
.PP
The devices for the first eight virtual consoles may be created by:

.nf
    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*
.fi

No
.BR ioctl ()
requests are supported.
.SH EXAMPLE
You may do a screendump on vt3 by switching to vt1 and typing
\fIcat /dev/vcs3 >foo\fP. Note that the output does not contain
newline characters, so some processing may be required, like
in \fIfold \-w 81 /dev/vcs3 | lpr\fP or (horrors)
\fIsetterm \-dump 3 \-file /proc/self/fd/1\fP.
.LP
The \fI/dev/vcsa0\fP 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:

.nf
#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(1);
    }
    (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);
    return 0;
}
.fi
.SH FILES
/dev/vcs[0\-63]
.br
/dev/vcsa[0\-63]
.SH AUTHOR
Andries Brouwer <aeb@cwi.nl>
.SH HISTORY
Introduced with version 1.1.92 of the Linux kernel.
.SH "SEE ALSO"
.BR gpm (8),
.BR console (4),
.BR tty (4),
.BR ttyS (4)