man4/vcs.4

   1 .\" Copyright (c) 1995 James R. Van Zandt <jrv@vanzandt.mv.com>
   2 .\" Sat Feb 18 09:11:07 EST 1995
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  25 .\" Modified, Sun Feb 26 15:08:05 1995, faith@cs.unc.edu
  26 .\" 2007-12-17, Samuel Thibault <samuel.thibault@ens-lyon.org>:
  27 .\"     document the VT_GETHIFONTMASK ioctl
  28 .\" "
  29 .TH VCS 4 2019-03-06 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 vcs, vcsa \- virtual console memory
  32 .SH DESCRIPTION
  33 .I /dev/vcs0
  34 is a character device with major number 7 and minor number
  35 0, usually with mode 0644 and ownership root:tty.
  36 It refers to the memory of the currently
  37 displayed virtual console terminal.
  38 .PP
  39 .I /dev/vcs[1\-63]
  40 are character devices for virtual console
  41 terminals, they have major number 7 and minor number 1 to 63, usually
  42 mode 0644 and ownership root:tty.
  43 .IR /dev/vcsa[0\-63]
  44 are the same, but
  45 using
  46 .IR "unsigned short" s
  47 (in host byte order) that include attributes,
  48 and prefixed with four bytes giving the screen
  49 dimensions and cursor position:
  50 .IR lines ,
  51 .IR columns ,
  52 .IR x ,
  53 .IR y .
  54 .RI ( x
  55 =
  56 .I y
  57 = 0 at the top left corner of the screen.)
  58 .PP
  59 When a 512-character font is loaded,
  60 the 9th bit position can be fetched by applying the
  61 .BR ioctl (2)
  62 .B VT_GETHIFONTMASK
  63 operation
  64 (available in Linux kernels 2.6.18 and above)
  65 on
  66 .IR /dev/tty[1\-63] ;
  67 the value is returned in the
  68 .I "unsigned short"
  69 pointed to by the third
  70 .BR ioctl (2)
  71 argument.
  72 .PP
  73 These devices replace the screendump
  74 .BR ioctl (2)
  75 operations of
  76 .BR ioctl_console (2),
  77 so the system
  78 administrator can control access using filesystem permissions.
  79 .PP
  80 The devices for the first eight virtual consoles may be created by:
  81 .PP
  82 .in +4n
  83 .EX
  84 for x in 0 1 2 3 4 5 6 7 8; do
  85     mknod \-m 644 /dev/vcs$x c 7 $x;
  86     mknod \-m 644 /dev/vcsa$x c 7 $[$x+128];
  87 done
  88 chown root:tty /dev/vcs*
  89 .EE
  90 .in
  91 .PP
  92 No
  93 .BR ioctl (2)
  94 requests are supported.
  95 .SH FILES
  96 .I /dev/vcs[0\-63]
  97 .br
  98 .I /dev/vcsa[0\-63]
  99 .\" .SH AUTHOR
 100 .\" Andries Brouwer <aeb@cwi.nl>
 101 .SH VERSIONS
 102 Introduced with version 1.1.92 of the Linux kernel.
 103 .SH EXAMPLE
 104 You may do a screendump on vt3 by switching to vt1 and typing
 105 .PP
 106 .in +4n
 107 .EX
 108 cat /dev/vcs3 >foo
 109 .EE
 110 .in
 111 .PP
 112 Note that the output does not contain
 113 newline characters, so some processing may be required, like
 114 in
 115 .PP
 116 .in +4n
 117 .EX
 118 fold \-w 81 /dev/vcs3 | lpr
 119 .EE
 120 .in
 121 .PP
 122 or (horrors)
 123 .PP
 124 .in +4n
 125 .EX
 126 setterm \-dump 3 \-file /proc/self/fd/1
 127 .EE
 128 .in
 129 .PP
 130 The
 131 .I /dev/vcsa0
 132 device is used for Braille support.
 133 .PP
 134 This program displays the character and screen attributes under the
 135 cursor of the second virtual console, then changes the background color
 136 there:
 137 .PP
 138 .EX
 139 #include <unistd.h>
 140 #include <stdlib.h>
 141 #include <stdio.h>
 142 #include <fcntl.h>
 143 #include <sys/ioctl.h>
 144 #include <linux/vt.h>
 145
 146 int
 147 main(void)
 148 {
 149     int fd;
 150     char *device = "/dev/vcsa2";
 151     char *console = "/dev/tty2";
 152     struct {unsigned char lines, cols, x, y;} scrn;
 153     unsigned short s;
 154     unsigned short mask;
 155     unsigned char attrib;
 156     int ch;
 157
 158     fd = open(console, O_RDWR);
 159     if (fd < 0) {
 160         perror(console);
 161         exit(EXIT_FAILURE);
 162     }
 163     if (ioctl(fd, VT_GETHIFONTMASK, &mask) < 0) {
 164         perror("VT_GETHIFONTMASK");
 165         exit(EXIT_FAILURE);
 166     }
 167     (void) close(fd);
 168     fd = open(device, O_RDWR);
 169     if (fd < 0) {
 170         perror(device);
 171         exit(EXIT_FAILURE);
 172     }
 173     (void) read(fd, &scrn, 4);
 174     (void) lseek(fd, 4 + 2*(scrn.y*scrn.cols + scrn.x), SEEK_SET);
 175     (void) read(fd, &s, 2);
 176     ch = s & 0xff;
 177     if (s & mask)
 178         ch |= 0x100;
 179     attrib = ((s & ~mask) >> 8);
 180     printf("ch=0x%03x attrib=0x%02x\en", ch, attrib);
 181     s ^= 0x1000;
 182     (void) lseek(fd, \-2, SEEK_CUR);
 183     (void) write(fd, &s, 2);
 184     exit(EXIT_SUCCESS);
 185 }
 186 .EE
 187 .SH SEE ALSO
 188 .BR ioctl_console (2),
 189 .BR tty (4),
 190 .BR ttyS (4),
 191 .BR gpm (8)