vcs (4)

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 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 in Linux kernels 2.6.18 and above) 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 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)

xetterm -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 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='%c' attrib=0x%02x\n", ch, attrib); attrib ^= 0x10; (void) lseek(fd, -1, 1); (void) write(fd, &attrib, 1); exit(EXIT_SUCCESS); }

SEE ALSO

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

Information

Source
Linux kernel
OS/version
Linux
Source updated
May 3, 2017
Page created
February 9, 2018
Page generated
March 5, 2018