vcs(4) — Linux manual page


VCS(4)                    Linux Programmer's Manual                   VCS(4)

NAME         top

       vcs, vcsa - virtual console memory

DESCRIPTION         top

       /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 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];
           chown root:tty /dev/vcs*

       No ioctl(2) requests are supported.

FILES         top


VERSIONS         top

       Introduced with version 1.1.92 of the Linux kernel.

EXAMPLES         top

       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 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) {
           if (ioctl(fd, VT_GETHIFONTMASK, &mask) < 0) {
           (void) close(fd);
           fd = open(device, O_RDWR);
           if (fd < 0) {
           (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=0x%03x attrib=0x%02x\n", ch, attrib);
           s ^= 0x1000;
           (void) lseek(fd, -2, SEEK_CUR);
           (void) write(fd, &s, 2);

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                           VCS(4)

Pages that refer to this page: ioctl_console(2)console_ioctl(4)tty(4)