Manpages - vcs.4

vcs, vcsa - virtual console memory

/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 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 *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.

/dev/vcs[0-63]
/dev/vcsa[0-63]

Introduced with version 1.1.92 of the Linux kernel.

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);
  }

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

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