NAME
cons – console, clocks, process/process group ids, user, null, reboot, etc.

SYNOPSIS
bind #c /dev
/dev/bintime
/dev/config
/dev/cons
/dev/consctl
/dev/cputime
/dev/drivers
/dev/hostdomain
/dev/hostowner
/dev/kmesg
/dev/kprint
/dev/null
/dev/osversion
/dev/pgrpid
/dev/pid
/dev/ppid
/dev/random
/dev/reboot
/dev/swap
/dev/sysname
/dev/sysstat
/dev/time
/dev/user
/dev/zero

DESCRIPTION
The console device serves a one–level directory giving access to the console and miscellaneous information.

Reading the cons file returns characters typed on the keyboard. Normally, characters are buffered to enable erase and kill processing. A control–U, ^U, typed at the keyboard kills the current input line (removes all characters from the buffer of characters not yet read via cons), and a backspace erases the previous non–kill, non–erase character from the input buffer. Killing and erasing only delete characters back to, but not including, the last newline. Characters typed at the keyboard actually produce 21–bit runes (see utf(6)), but the runes are translated into the variable–length UTF encoding (see utf(6)) before putting them into the buffer. A read(2) of length greater than zero causes the process to wait until a newline or a ^D ends the buffer, and then returns as much of the buffer as the argument to read allows, but only up to one complete line. A terminating ^D is not put into the buffer. If part of the line remains, the next read will return bytes from that remainder and not part of any new line that has been typed since.

If the string rawon has been written to the consctl file and the file is still open, cons is in raw mode: characters are not echoed as they are typed, backspace, ^U and ^D are not treated specially, and characters are available to read as soon as they are typed. Ordinary mode is reentered when rawoff is written to consctl or this file is closed.

A write (see read(2)) to cons causes the characters to be printed on the console screen.

The osversion file contains a textual representation of the operating system's version and parameters. At the moment, it contains one field: the 9P protocol version, currently 2000.

The config file contains a copy of the kernel configuration file used to build the kernel.

The kmesg file holds the last 16 kilobytes of output written to the console by the kernel's print statements or by processes writing to /dev/cons. It is useful for retrieving boot messages once the boot process is over.

The kprint file may be read to receive a copy of the data written to the console by the kernel's print statements or by processes writing to /dev/cons. Only data written after the file is opened is available. If the machine's console is a serial line, the data is sent both to the console and to kprint; if its console is a graphics screen, the data is sent either to the display or to kprint, but not both. (It is advisable not to open kprint on terminals until you have started rio(1).)

The null file throws away anything written to it and always returns zero when read.

The zero file is a read–only file that produces an infinite stream of zero–valued bytes when read.

The drivers file contains, one per line, a listing of the drivers configured in the kernel, in the format
#c cons

The hostdomain file contains the name of the authentication domain that this host belongs to; see authsrv(6). Only the user named in /dev/hostowner may write this.

The hostowner file contains the name of the user that owns the console device files. The hostowner also has group permissions for any local devices.

Reads from random return a stream of random numbers. The numbers are generated by a low priority kernel process that loops incrementing a variable. Each clock tick the variable is sampled and, if it has changed sufficiently, the last few bits are appended to a buffer. This process is inefficient at best producing at most a few hundred bits a second. Therefore, random should be treated as a seed to pseudo–random number generators which can produce a faster rate stream.

Writing the string reboot to reboot causes the system to shutdown and, if possible, restart. Writing the string reboot kernelpath loads the named kernel image and restarts, preserving the kernel configuration in #ec, except that the bootfile variable is set to kernelpath. Only the host owner has the ability to open this file. The named kernel may be a Plan 9 executable or a 32–bit or 64–bit ELF executable. On some architectures (e.g., mips), it may also be a Plan 9 boot image.

Bintime is a binary interface that provides the same information as time (q.v.), in binary form, and also controls clock frequency and clock trim. All integers read or written from bintime are in big endian order. Unlike the other files, reads and writes do not affect the offset. Therefore, there is no need for a seek back to zero between subsequent accesses. A read of bintime returns 24 bytes, three 8 byte numbers, representing nanoseconds since start of epoch, clock ticks, and clock frequency.

A write to bintime is a message with one of 3 formats:
n<8–byte time>   set the nanoseconds since epoch to the given time.
d<8–byte delta><4–byte period>
trim the nanoseconds since epoch by delta over the next period seconds.
f<8–byte freq>    Set the frequency for interpreting clock ticks to be freq ticks per second.

Statistics and Dynamic Status
The rest of the files contain (mostly) read–only strings. Each string has a fixed length: a read(2) of more than that gives a result of that fixed length (the result does not include a terminating zero byte); a read of less than that length leaves the file offset so the rest of the string (but no more) will be read the next time. To reread the file without closing it, seek must be used to reset the offset. When the file contains numeric data each number is formatted in decimal. If the binary number fits in 32 bits, it is formatted as an 11 digit decimal number with leading blanks and one trailing blank; totaling 12 bytes. Otherwise, it is formatted as 21 digit decimal numbers with leading blanks and one trailing blank; totaling 22 bytes.

The cputime file holds six 32–bit numbers, containing the time in milliseconds that the current process has spent in user mode, system calls, real elapsed time, and then the time spent, by exited children and their descendants, in user mode, system calls, and real elapsed time.

The time file holds one 32–bit number representing the seconds since start of epoch and three 64–bit numbers, representing nanoseconds since start of epoch, clock ticks, and clock frequency.

A write of a decimal number to time will set the seconds since epoch.

The sysname file holds the textual name of the machine, e.g. kremvax, if known.

The sysstat file holds 10 numbers: processor number, context switches, interrupts, system calls, page faults, TLB faults, TLB purges, load average, idle time and time spent servicing interrupts. The load average is in units of milli–CPUs and is decayed over time; idle time and interrupt time are percentage units; the others are total counts from boot time. If the machine is a multiprocessor, sysstat holds one line per processor. Writing anything to sysstat resets all of the counts on all processors.

The swap device holds a text block giving memory usage statistics:
n memory
n pagesize
n kernel
n/m    user
n/m    swap
n/m    kernel malloc
n/m    kernel draw

These are total memory (bytes), system page size (bytes), kernel memory (pages), user memory (pages), swap space (pages), kernel malloced data (bytes), and kernel graphics data (bytes). The expression n/m indicates n used out of m available. These numbers are not blank padded.

To turn on swapping, write to swap the textual file descriptor number of a file or device on which to swap. See swap(8).

The other files served by the cons device are all single numbers:
pgrpidprocess group number
pid     process number
ppid    parent's process number

SEE ALSO
draw(3), keyboard(6), authsrv(6), utf(6), swap(8)

SOURCE
/sys/src/9/port/devcons.c

BUGS
For debugging, two control–T's followed by a letter generate console output and manage debugging: ^T^Td toggles whether the console debugger will be run if the system fails. ^T^TD starts the console debugger immediately. ^T^Tk kills the largest process; use with care. ^T^Tp prints data about processes. ^T^Tq prints the run queue for processor 0. ^T^Ts prints the kernel stack. ^T^Tx prints data about kernel memory allocation.

The system can be rebooted by typing ^T^Tr.

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