man4/vcs.4

   1 .\" Copyright (c) 1995 James R. Van Zandt <jrv@vanzandt.mv.com>
   2 .\" Sat Feb 18 09:11:07 EST 1995
   3 .\"
   4 .\" SPDX-License-Identifier: GPL-2.0-or-later
   5 .\"
   6 .\" Modified, Sun Feb 26 15:08:05 1995, faith@cs.unc.edu
   7 .\" 2007-12-17, Samuel Thibault <samuel.thibault@ens-lyon.org>:
   8 .\"     document the VT_GETHIFONTMASK ioctl
   9 .\" "
  10 .TH vcs 4 (date) "Linux man-pages (unreleased)"
  11 .SH NAME
  12 vcs, vcsa \- virtual console memory
  13 .SH DESCRIPTION
  14 .I /dev/vcs0
  15 is a character device with major number 7 and minor number
  16 0, usually with mode 0644 and ownership root:tty.
  17 It refers to the memory of the currently
  18 displayed virtual console terminal.
  19 .PP
  20 .I /dev/vcs[1\-63]
  21 are character devices for virtual console
  22 terminals, they have major number 7 and minor number 1 to 63, usually
  23 mode 0644 and ownership root:tty.
  24 .I /dev/vcsa[0\-63]
  25 are the same, but
  26 using
  27 .IR "unsigned short" s
  28 (in host byte order) that include attributes,
  29 and prefixed with four bytes giving the screen
  30 dimensions and cursor position:
  31 .IR lines ,
  32 .IR columns ,
  33 .IR x ,
  34 .IR y .
  35 .RI ( x
  36 =
  37 .I y
  38 = 0 at the top left corner of the screen.)
  39 .PP
  40 When a 512-character font is loaded,
  41 the 9th bit position can be fetched by applying the
  42 .BR ioctl (2)
  43 .B VT_GETHIFONTMASK
  44 operation
  45 (available in Linux kernels 2.6.18 and above)
  46 on
  47 .IR /dev/tty[1\-63] ;
  48 the value is returned in the
  49 .I "unsigned short"
  50 pointed to by the third
  51 .BR ioctl (2)
  52 argument.
  53 .PP
  54 These devices replace the screendump
  55 .BR ioctl (2)
  56 operations of
  57 .BR ioctl_console (2),
  58 so the system
  59 administrator can control access using filesystem permissions.
  60 .PP
  61 The devices for the first eight virtual consoles may be created by:
  62 .PP
  63 .in +4n
  64 .EX
  65 for x in 0 1 2 3 4 5 6 7 8; do
  66     mknod \-m 644 /dev/vcs$x c 7 $x;
  67     mknod \-m 644 /dev/vcsa$x c 7 $[$x+128];
  68 done
  69 chown root:tty /dev/vcs*
  70 .EE
  71 .in
  72 .PP
  73 No
  74 .BR ioctl (2)
  75 requests are supported.
  76 .SH FILES
  77 .I /dev/vcs[0\-63]
  78 .br
  79 .I /dev/vcsa[0\-63]
  80 .\" .SH AUTHOR
  81 .\" Andries Brouwer <aeb@cwi.nl>
  82 .SH VERSIONS
  83 Introduced with version 1.1.92 of the Linux kernel.
  84 .SH EXAMPLES
  85 You may do a screendump on vt3 by switching to vt1 and typing
  86 .PP
  87 .in +4n
  88 .EX
  89 cat /dev/vcs3 >foo
  90 .EE
  91 .in
  92 .PP
  93 Note that the output does not contain
  94 newline characters, so some processing may be required, like
  95 in
  96 .PP
  97 .in +4n
  98 .EX
  99 fold \-w 81 /dev/vcs3 | lpr
 100 .EE
 101 .in
 102 .PP
 103 or (horrors)
 104 .PP
 105 .in +4n
 106 .EX
 107 setterm \-dump 3 \-file /proc/self/fd/1
 108 .EE
 109 .in
 110 .PP
 111 The
 112 .I /dev/vcsa0
 113 device is used for Braille support.
 114 .PP
 115 This program displays the character and screen attributes under the
 116 cursor of the second virtual console, then changes the background color
 117 there:
 118 .PP
 119 .EX
 120 #include <unistd.h>
 121 #include <stdlib.h>
 122 #include <stdio.h>
 123 #include <fcntl.h>
 124 #include <sys/ioctl.h>
 125 #include <linux/vt.h>
 126
 127 int
 128 main(void)
 129 {
 130     int fd;
 131     char *device = "/dev/vcsa2";
 132     char *console = "/dev/tty2";
 133     struct {unsigned char lines, cols, x, y;} scrn;
 134     unsigned short s;
 135     unsigned short mask;
 136     unsigned char attrib;
 137     int ch;
 138
 139     fd = open(console, O_RDWR);
 140     if (fd < 0) {
 141         perror(console);
 142         exit(EXIT_FAILURE);
 143     }
 144     if (ioctl(fd, VT_GETHIFONTMASK, &mask) < 0) {
 145         perror("VT_GETHIFONTMASK");
 146         exit(EXIT_FAILURE);
 147     }
 148     (void) close(fd);
 149     fd = open(device, O_RDWR);
 150     if (fd < 0) {
 151         perror(device);
 152         exit(EXIT_FAILURE);
 153     }
 154     (void) read(fd, &scrn, 4);
 155     (void) lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), SEEK_SET);
 156     (void) read(fd, &s, 2);
 157     ch = s & 0xff;
 158     if (s & mask)
 159         ch |= 0x100;
 160     attrib = ((s & \(timask) >> 8);
 161     printf("ch=%#03x attrib=%#02x\en", ch, attrib);
 162     s \(ha= 0x1000;
 163     (void) lseek(fd, \-2, SEEK_CUR);
 164     (void) write(fd, &s, 2);
 165     exit(EXIT_SUCCESS);
 166 }
 167 .EE
 168 .SH SEE ALSO
 169 .BR ioctl_console (2),
 170 .BR tty (4),
 171 .BR ttyS (4),
 172 .BR gpm (8)