vcs - Online Linux Manual PageSection : 4
Updated : 2023-02-05
Source : Linux man-pages 6.03
NAMEvcs, vcsa − virtual console memory
DESCRIPTION/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 since Linux 2.6.18) 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]
VERSIONSIntroduced with Linux 1.1.92.
EXAMPLESYou may do a screendump on vt3 by switching to vt1 and typing cat /dev/vcs3 >fooNote that the output does not contain newline characters, so some processing may be required, like in fold −w 81 /dev/vcs3 | lpror (horrors) setterm −dump 3 −file /proc/self/fd/1The /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 attrib;
int ch;
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), SEEK_SET);
(void) read(fd, &s, 2);
ch = s & 0xff;
if (s & mask)
ch |= 0x100;
attrib = ((s & ~mask) >> 8);
printf("ch=%#03x attrib=%#02x\n", ch, attrib);
s ^= 0x1000;
(void) lseek(fd, −2, SEEK_CUR);
(void) write(fd, &s, 2);
exit(EXIT_SUCCESS);
}
SEE ALSOioctl_console(2), tty(4), ttyS(4), gpm(8) 0
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