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man:vcs

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

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) argu-
     ment.
     These devices replace the screendump ioctl(2) operations of  ioctl_con-
     sole(2),  so the system administrator can control access using filesys-
     tem 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 pro-
     cessing 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 4.16 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/.

Linux 2017-05-03 VCS(4)

/data/webs/external/dokuwiki/data/pages/man/vcs.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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