bionic (4) vcsa.4.gz

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)

COLOPHON

       This  page  is  part  of  release  4.15  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/.