Linux Man Pages

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 short s (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 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]

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>
 #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%02xn", 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)