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VCS

Section: Linux Programmer's Manual (4)
Updated: 1995-02-19
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NAME

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 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 ioctls 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.  

FILES

/dev/vcs[0-63]
/dev/vcsa[0-63]  

AUTHOR

Andries Brouwer <aeb@cwi.nl>  

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) 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(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);
    exit(EXIT_SUCCESS);
}
 

SEE ALSO

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


 

Index

NAME
DESCRIPTION
FILES
AUTHOR
VERSIONS
EXAMPLE
SEE ALSO