Scroll to navigation

vcs(4) Device Drivers Manual vcs(4)

NAME

vcs, vcsa - virtual console memory

DESCRIPTION

/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.

/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. /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 since Linux 2.6.18) 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 ioctl_console(2), 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 Linux 1.1.92.

EXAMPLES

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>
#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 & ~mask) >> 8);
printf("ch=%#03x attrib=%#02x\n", ch, attrib);
s ^= 0x1000;
(void) lseek(fd, -2, SEEK_CUR);
(void) write(fd, &s, 2);
exit(EXIT_SUCCESS); }

SEE ALSO

ioctl_console(2), tty(4), ttyS(4), gpm(8)

2023-02-05 Linux man-pages 6.03