TTY(4)

✨ TTY(4) — general terminal interface ✨

okay so bestie, this is the manual page for tty — the general terminal interface. It's giving kernel interfaces, it's giving serial ports, it's giving your terminal's whole personality. Let's get into it. 💅

---

📜 NAME 📜

tty — general terminal interface

---

🎀 SYNOPSIS 🎀

#include <sys/ioctl.h>

---

💖 DESCRIPTION 💖

This section describes the interface to the terminal drivers in the system.

🌸 Terminal Special Files 🌸

Each hardware terminal port on the system usually has a terminal special device file associated with it in the directory /dev/ (for example, /dev/tty03). When a user logs into the system on one of these hardware terminal ports, the system has already opened the associated device and prepared the line for normal interactive use (see getty(8)). There is also a special case of a terminal file that connects not to a hardware terminal port, but to another program on the other side. These special terminal devices are called ptys and provide the mechanism necessary to give users the same interface to the system when logging in over a network (using telnet(1) for example). Even in these cases the details of how the terminal file was opened and set up is already handled by special software in the system. Thus, users do not normally need to worry about the details of how these lines are opened or used. Also, these lines are often used for dialing out of a system (through an out-calling modem), but again the system provides programs that hide the details of accessing these terminal special files (see tip(1)).

When an interactive user logs in, the system prepares the line to behave in a certain way (called a line discipline), the particular details of which is described in stty(1) at the command level, and in termios(4) at the programming level. A user may be concerned with changing settings associated with his particular login terminal and should refer to the preceding man pages for the common cases. The remainder of this man page is concerned with describing details of using and controlling terminal devices at a low level, such as that possibly required by a program wishing to provide features similar to those provided by the system.

🦋 Terminal File Operations 🦋

All of the following operations are invoked using the ioctl(2) system call. Refer to that man page for a description of the request and argp parameters. In addition to the ioctl requests defined here, the specific line discipline in effect will define other requests specific to it (actually termios(4) defines them as function calls, not ioctl requests.) The following section lists the available ioctl requests. The name of the request, a description of its purpose, and the typed argp parameter (if any) are listed. For example, the first entry says

TIOCSPGRP int *tpgrp

and would be called on the terminal associated with file descriptor zero by the following code fragment:

int pgrp;

pgrp = getpgrp();
ioctl(0, TIOCSPGRP, &pgrp);

👑 Terminal File Request Descriptions 👑

Now the main event, bestie. Every request, every vibe. No cap. ✨

• TIOCSETD int *ldisc — This call is obsolete but left for compatibility. Before FreeBSD 8.0, it would change to the new line discipline pointed to by ldisc.
• TIOCGETD int *ldisc — Return the current line discipline in the integer pointed to by ldisc.
• TIOCSBRK void — Set the terminal hardware into BREAK condition.
• TIOCCBRK void — Clear the terminal hardware BREAK condition.
• TIOCSDTR void — Assert data terminal ready (DTR).
• TIOCCDTR void — Clear data terminal ready (DTR).
• TIOCGPGRP int *tpgrp — Return the current process group with which the terminal is associated in the integer pointed to by tpgrp. This is the underlying call that implements the termios(4) tcgetattr() call.
• TIOCSPGRP int *tpgrp — Associate the terminal with the process group (as an integer) pointed to by tpgrp. This is the underlying call that implements the termios(4) tcsetattr() call.
• TIOCGETA struct termios *term — Place the current value of the termios state associated with the device in the termios structure pointed to by term. This is the underlying call that implements the termios(4) tcgetattr() call.
• TIOCSETA struct termios *term — Set the termios state associated with the device immediately. This is the underlying call that implements the termios(4) tcsetattr() call with the TCSANOW option.
• TIOCSETAW struct termios *term — First wait for any output to complete, then set the termios state associated with the device. This is the underlying call that implements the termios(4) tcsetattr() call with the TCSADRAIN option.
• TIOCSETAF struct termios *term — First wait for any output to complete, clear any pending input, then set the termios state associated with the device. This is the underlying call that implements the termios(4) tcsetattr() call with the TCSAFLUSH option.
• TIOCOUTQ int *num — Place the current number of characters in the output queue in the integer pointed to by num.
• TIOCSTI char *cp — Simulate typed input. Pretend as if the terminal received the character pointed to by cp.
• TIOCNOTTY void — In the past, when a process that did not have a controlling terminal (see The Controlling Terminal in termios(4)) first opened a terminal device, it acquired that terminal as its controlling terminal. For some programs this was a hazard as they did not want a controlling terminal in the first place, and this provides a mechanism to disassociate the controlling terminal from the calling process. It must be called by opening the file /dev/tty and calling TIOCNOTTY on that file descriptor.

The current system does not allocate a controlling terminal to a process on an open() call: there is a specific ioctl called TIOCSCTTY to make a terminal the controlling terminal. In addition, a program can fork() and call the setsid() system call which will place the process into its own session — which has the effect of disassociating it from the controlling terminal. This is the new and preferred method for programs to lose their controlling terminal.

However, environmental restrictions may prohibit the process from being able to fork() and call the setsid() system call to disassociate it from the controlling terminal. In this case, it must use TIOCNOTTY.

• TIOCSTOP void — Stop output on the terminal (like typing ^S at the keyboard).
• TIOCSTART void — Start output on the terminal (like typing ^Q at the keyboard).
• TIOCSCTTY void — Make the terminal the controlling terminal for the process (the process must not currently have a controlling terminal).
• TIOCDRAIN void — Wait until all output is drained, or until the drain wait timeout expires.
• TIOCGDRAINWAIT int *timeout — Return the current drain wait timeout in seconds.
• TIOCSDRAINWAIT int *timeout — Set the drain wait timeout in seconds. A value of zero disables timeouts. The default drain wait timeout is controlled by the tunable sysctl(8) OID kern.tty_drainwait.
• TIOCEXCL void — Set exclusive use on the terminal. No further opens are permitted except by root. Of course, this means that programs that are run by root (or setuid) will not obey the exclusive setting — which limits the usefulness of this feature.
• TIOCNXCL void — Clear exclusive use of the terminal. Further opens are permitted.
• TIOCFLUSH int *what — If the value of the int pointed to by what contains the FREAD bit as defined in <sys/file.h>, then all characters in the input queue are cleared. If it contains the FWRITE bit, then all characters in the output queue are cleared. If the value of the integer is zero, then it behaves as if both the FREAD and FWRITE bits were set (i.e., clears both queues).
• TIOCGWINSZ struct winsize *ws — Put the window size information associated with the terminal in the winsize structure pointed to by ws. The window size structure contains the number of rows and columns (and pixels if appropriate) of the devices attached to the terminal. It is set by user software and is the means by which most full-screen oriented programs determine the screen size. The winsize structure is defined in <sys/ioctl.h>.
• TIOCSWINSZ struct winsize *ws — Set the window size associated with the terminal to be the value in the winsize structure pointed to by ws (see above).
• TIOCCONS int *on — If on points to a non-zero integer, redirect kernel console output (kernel printf's) to this terminal. If on points to a zero integer, redirect kernel console output back to the normal console. This is usually used on workstations to redirect kernel messages to a particular window.
• TIOCMSET int *state — The integer pointed to by state contains bits that correspond to modem state. Following is a list of defined variables and the modem state they represent:
  • TIOCM_LE — Line Enable.
  • TIOCM_DTR — Data Terminal Ready.
  • TIOCM_RTS — Request To Send.
  • TIOCM_ST — Secondary Transmit.
  • TIOCM_SR — Secondary Receive.
  • TIOCM_CTS — Clear To Send.
  • TIOCM_CAR — Carrier Detect.
  • TIOCM_CD — Carrier Detect (synonym).
  • TIOCM_RNG — Ring Indication.
  • TIOCM_RI — Ring Indication (synonym).
  • TIOCM_DSR — Data Set Ready.

This call sets the terminal modem state to that represented by state. Not all terminals may support this.

• TIOCMGET int *state — Return the current state of the terminal modem lines as represented above in the integer pointed to by state.
• TIOCMBIS int *state — The bits in the integer pointed to by state represent modem state as described above, however the state is OR-ed in with the current state.
• TIOCMBIC int *state — okay so state points at an integer, and the bits set in it describe modem state exactly like we just talked about ✨. But here's the twist bestie: every bit that's on in state gets cleared on the terminal. So like, you're not setting anything, you're un-setting it 💅. Iconic little reverse card behavior, fr fr.

---

🫶 IMPLEMENTATION NOTES 🫶

So if you ever wanna know how many bytes have gone in and out across every terminal device combined, the vibes are literally just sitting there in two read-only sysctl(8) variables:

• kern.tty_nin — total input bytes through all terminal devices 📥
• kern.tty_nout — total output bytes through all terminal devices 📤

That's it, that's the whole note. Read-only, so don't try to write to them, they will not be taking suggestions 🙈.

---

🌈 SEE ALSO 🌈

Go be nosy, bestie, these are the besties-in-arms of this page 💖:

• stty(1)
• ioctl(2)
• ng_tty(4)
• pty(4)
• termios(4)
• getty(8)

---

And that's the whole page, you made it to the end 🎀🦋 This lil' beauty ships from Debian, dated November 27, 2019, and lives its life as TTY(4). Go forth and ioctl, you absolute legend 👑💅