tty ioctl − ioctls for terminals and serial lines

#include <termios.h>

int ioctl(int fd, int cmd, ...);

The ioctl() call for terminals and serial ports accepts many possible command arguments. Most require a third argument, of varying type, here called argp or arg.

Use of ioctl makes for non-portable programs. Use the POSIX interface described in termios(3) whenever possible.

Get and Set Terminal Attributes

TCGETS

Equivalent to tcgetattr(fd, argp).
Get the current serial port settings.

const struct termios *argp

Equivalent to tcsetattr(fd, TCSANOW, argp).
Set the current serial port settings.

const struct termios *argp

Equivalent to tcsetattr(fd, TCSADRAIN, argp).
Allow the output buffer to drain, and set the current serial port settings.

const struct termios *argp

Equivalent to tcsetattr(fd, TCSAFLUSH, argp).
Allow the output buffer to drain, discard pending input, and set the current serial port settings.

The following four ioctls are just like TCGETS, TCSETS, TCSETSW, TCSETSF, except that they take a struct termio * instead of a struct termios *.

TCGETA

const struct termio *argp

const struct termio *argp

Locking the termios structure

The termios structure of a tty can be locked. The lock is itself a termios structure, with nonzero bits or fields indicating a locked value.

TIOCGLCKTRMIOS

Gets the locking status of the termios structure of the terminal.

const struct termios *argp

Sets the locking status of the termios structure of the terminal. Only root can do this.

Get and Set Window Size

Window sizes are kept in the kernel, but not used by the kernel (except in the case of virtual consoles, where the kernel will update the window size when the size of the virtual console changes, e.g. by loading a new font).

TIOCGWINSZ

Get window size.

const struct winsize *argp

Set window size.

The struct used by these ioctls is defined as

struct winsize {
        unsigned short ws_row;
        unsigned short ws_col;
        unsigned short ws_xpixel;   /* unused */
        unsigned short ws_ypixel;   /* unused */
};

When the window size changes, a SIGWINCH signal is sent to the foreground process group.

Sending a Break

TCSBRK

Equivalent to tcsendbreak(fd, arg).
If the terminal is using asynchronous serial data transmission, and arg is zero, then send a break (a stream of zero bits) for between 0.25 and 0.5 seconds. If the terminal is not using asynchronous serial data transmission, then either a break is sent, or the function returns without doing anything. When arg is nonzero, nobody knows what will happen.

(SVR4, UnixWare, Solaris, Linux treat tcsendbreak(fd,arg) with nonzero arg like tcdrain(fd). SunOS treats arg as a multiplier, and sends a stream of bits arg times as long as done for zero arg. DG-UX and AIX treat arg (when nonzero) as a timeinterval measured in milliseconds. HP-UX ignores arg.)

int arg

So-called "POSIX version" of TCSBRK. It treats nonzero arg as a timeinterval measured in deciseconds, and does nothing when the driver does not support breaks.

void

Turn break on, that is, start sending zero bits.

void

Turn break off, that is, stop sending zero bits.

Software flow control

TCXONC

Equivalent to tcflow(fd, arg).
See tcflow(3) for the argument values TCOOFF, TCOON, TCIOFF, TCION.

Buffer count and flushing

FIONREAD

Get the number of bytes in the input buffer.

int *argp

Same as FIONREAD.

int *argp

Get the number of bytes in the output buffer.

int arg

Equivalent to tcflush(fd, arg).
See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH, TCIOFLUSH.

Faking input

TIOCSTI

Insert the given byte in the input queue.

Redirecting console output

TIOCCONS

Redirect output that would have gone to /dev/console or /dev/tty0 to the given tty. If that was a pty master, send it to the slave. Anybody can do this as long as the output was not redirected yet. If it was redirected already EBUSY is returned, but root may stop redirection by using this ioctl with fd pointing at /dev/console or /dev/tty0.

Controlling tty

TIOCSCTTY

Make the given tty the controlling tty of the current process. The current process must be a session leader and not have a controlling tty already. If this tty is already the controlling tty of a different session group then the ioctl fails with EPERM, unless the caller is root and arg equals 1, in which case the tty is stolen, and all processes that had it as controlling tty lose it.

void

If the given tty was the controlling tty of the current process, give up this controlling tty. If the process was session leader, then send SIGHUP and SIGCONT to the foreground process group and all processes in the current session lose their controlling tty.

Process group and session ID

TIOCGPGRP

When successful, equivalent to *argp = tcgetpgrp(fd).
Get the process group ID of the foreground proces group on this tty.

const pid_t *argp

Equivalent to tcsetpgrp(fd, *argp).
Set the foreground process group id of this tty.

pid_t *argp

Get the session ID of the given tty. This will fail with ENOTTY in case the tty is not a master pty and not our controlling tty. Strange.

Exclusive mode

TIOCEXCL

Put the tty into exclusive mode. No further open(2) operations on the terminal are permitted. (They will fail with EBUSY, except for root.)

void

Disable exclusive mode.

Line discipline

TIOCGETD

Get the line discipline of the tty.

const int *argp

Set the line discipline of the tty.

Pseudo-tty ioctls

TIOCPKT

Enable (when *argp is nonzero) or disable packet mode. Can be applied to the master side of a pseudotty only (and will return ENOTTY otherwise). In packet mode, each subsequent read(2) will return a packet that either contains a single nonzero control byte, or has a single zero byte followed by data written on the slave side of the pty. If the first byte is not TIOCPKT_DATA (0), it is an OR of one or more of the following bits:

TIOCPKT_FLUSHREAD   The read queue for the terminal is flushed.
TIOCPKT_FLUSHWRITE  The write queue for the terminal is flushed.
TIOCPKT_STOP        Output to the terminal is stopped.
TIOCPKT_START       Output to the terminal is restarted.
TIOCPKT_DOSTOP      t_stopc is ‘^S’ and t_startc is ‘^Q’.
TIOCPKT_NOSTOP      the start and stop characters are not ‘^S/^Q’.

While this mode is in use, the presence of control status information to be read from the master side may be detected by a select(2) for exceptional conditions.

This mode is used by rlogin(1) and rlogind(8) to implement a remote-echoed, locally ‘^S/^Q’ flow-controlled remote login.

The BSD ioctls TIOCSTOP, TIOCSTART, TIOCUCNTL, TIOCREMOTE have not been implemented under Linux.

Modem control

TIOCMGET

get the status of modem bits.

const int *argp

set the status of modem bits.

const int *argp

clear the indicated modem bits.

const int *argp

set the indicated modem bits.

Bits used by these four ioctls:

TIOCM_LE        DSR (data set ready/line enable)
TIOCM_DTR       DTR (data terminal ready)
TIOCM_RTS       RTS (request to send)
TIOCM_ST        Secondary TXD (transmit)
TIOCM_SR        Secondary RXD (receive)
TIOCM_CTS       CTS (clear to send)
TIOCM_CAR       DCD (data carrier detect)
TIOCM_CD         see TIOCM_CAR
TIOCM_RNG       RNG (ring)
TIOCM_RI         see TIOCM_RNG
TIOCM_DSR       DSR (data set ready)

Marking a line as local

TIOCGSOFTCAR

("Get software carrier flag") Get the status of the CLOCAL flag in the c_cflag field of the termios structure.

const int *argp

("Set software carrier flag") Set the CLOCAL flag in the termios structure when *argp is nonzero, and clear it otherwise.

If the CLOCAL flag for a line is off, the hardware carrier detect (DCD) signal is significant, and an open(2) of the corresponding tty will block until DCD is asserted, unless the O_NONBLOCK flag is given. If CLOCAL is set, the line behaves as if DCD is always asserted. The software carrier flag is usually turned on for local devices, and is off for lines with modems.

Linux specific

For the TIOCLINUX ioctl, see console_ioctl(4).

Kernel debugging

#include <linux/tty.h>

TIOCTTYGSTRUCT

Get the tty_struct corresponding to fd.

The ioctl() system call returns 0 on success. On error it returns −1 and sets errno appropriately.

ENOIOCTLCMD

Unknown command.

EINVAL

Invalid command parameter.

EPERM

Insufficient permission.

ENOTTY

Inappropriate fd.

Check the condition of DTR on the serial port.

#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>

main() {
    int fd, serial;

   fd = open("/dev/ttyS0", O_RDONLY);
    ioctl(fd, TIOCMGET, &serial);
    if (serial & TIOCM_DTR)
        puts("TIOCM_DTR is not set");
    else
        puts("TIOCM_DTR is set");
    close(fd);
}

ioctl(2), termios(3), console_ioctl(4)