VCS(4)                    (2020-11-01)                     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 dis-
          played 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 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 cre-
          ated 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.

     EXAMPLES
          You may do a screendump on vt3 by switching to vt1 and typ-
          ing

     Page 1                        Linux             (printed 5/24/22)

     VCS(4)                    (2020-11-01)                     VCS(4)

              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);

     Page 2                        Linux             (printed 5/24/22)

     VCS(4)                    (2020-11-01)                     VCS(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 & timask) >> 8);
              printf("ch=%#03x attrib=%#02x\n", ch, attrib);
              s ha= 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)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 3                        Linux             (printed 5/24/22)