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IOCTL_TTY(2) Linux Programmer's Manual IOCTL_TTY(2)


ioctl_tty - ioctls for terminals and serial lines


#include <termios.h>

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


The ioctl(2) 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 nonportable programs. Use the POSIX interface described in termios(3) whenever possible.

Get and set terminal attributes

Equivalent to tcgetattr(fd, argp).
Get the current serial port settings.
Equivalent to tcsetattr(fd, TCSANOW, argp).
Set the current serial port settings.
Equivalent to tcsetattr(fd, TCSADRAIN, argp).
Allow the output buffer to drain, and set the current serial port settings.
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 struct termio *argp
TCSETA const struct termio *argp
TCSETAW const struct termio *argp
TCSETAF const struct termio *argp

Locking the termios structure

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

Gets the locking status of the termios structure of the terminal.
Sets the locking status of the termios structure of the terminal. Only a process with the CAP_SYS_ADMIN capability 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, for example, by loading a new font).

The following constants and structure are defined in <sys/ioctl.h>.

Get window size.
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

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 time interval measured in milliseconds. HP-UX ignores 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.
Turn break on, that is, start sending zero bits.
Turn break off, that is, stop sending zero bits.

Software flow control

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

Buffer count and flushing

Get the number of bytes in the input buffer.
Get the number of bytes in the output buffer.
Equivalent to tcflush(fd, arg).
See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH, TCIOFLUSH.

Faking input

Insert the given byte in the input queue.

Redirecting console output

Redirect output that would have gone to /dev/console or /dev/tty0 to the given terminal. If that was a pseudoterminal master, send it to the slave. In Linux before version 2.6.10, anybody can do this as long as the output was not redirected yet; since version 2.6.10, only a process with the CAP_SYS_ADMIN capability may do this. If output was redirected already EBUSY is returned, but redirection can be stopped by using this ioctl with fd pointing at /dev/console or /dev/tty0.

Controlling terminal

Make the given terminal the controlling terminal of the calling process. The calling process must be a session leader and not have a controlling terminal already. For this case, arg should be specified as zero.
If this terminal is already the controlling terminal of a different session group, then the ioctl fails with EPERM, unless the caller has the CAP_SYS_ADMIN capability and arg equals 1, in which case the terminal is stolen, and all processes that had it as controlling terminal lose it.
If the given terminal was the controlling terminal of the calling process, give up this controlling terminal. 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 terminal.

Process group and session ID

When successful, equivalent to *argp = tcgetpgrp(fd).
Get the process group ID of the foreground process group on this terminal.
Equivalent to tcsetpgrp(fd, *argp).
Set the foreground process group ID of this terminal.
Get the session ID of the given terminal. This fails with the error ENOTTY if the terminal is not a master pseudoterminal and not our controlling terminal. Strange.

Exclusive mode

Put the terminal into exclusive mode. No further open(2) operations on the terminal are permitted. (They fail with EBUSY, except for a process with the CAP_SYS_ADMIN capability.)
(since Linux 3.8) If the terminal is currently in exclusive mode, place a nonzero value in the location pointed to by argp; otherwise, place zero in *argp.
Disable exclusive mode.

Line discipline

Get the line discipline of the terminal.
Set the line discipline of the terminal.

Pseudoterminal ioctls

Enable (when *argp is nonzero) or disable packet mode. Can be applied to the master side of a pseudoterminal 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 byte containing zero (' ') followed by data written on the slave side of the pseudoterminal. 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      The start and stop characters are ^S/^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 or a poll(2) for the POLLPRI event.
This mode is used by rlogin(1) and rlogind(8) to implement a remote-echoed, locally ^S/^Q flow-controlled remote login.
(since Linux 3.8) Return the current packet mode setting in the integer pointed to by argp.
Set (if *argp is nonzero) or remove (if *argp is zero) the pseudoterminal slave device. (See also unlockpt(3).)
(since Linux 3.8) Place the current lock state of the pseudoterminal slave device in the location pointed to by argp.
(since Linux 4.13) Given a file descriptor in fd that refers to a pseudoterminal master, open (with the given open(2)-style flags) and return a new file descriptor that refers to the peer pseudoterminal slave device. This operation can be performed regardless of whether the pathname of the slave device is accessible through the calling process's mount namespace.
Security-conscious programs interacting with namespaces may wish to use this operation rather than open(2) with the pathname returned by ptsname(3), and similar library functions that have insecure APIs. (For example, confusion can occur in some cases using ptsname(3) with a pathname where a devpts filesystem has been mounted in a different mount namespace.)

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

Modem control

Get the status of modem bits.
Set the status of modem bits.
Clear the indicated modem bits.
Set the indicated modem bits.

The following bits are used by the above 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)
Wait for any of the 4 modem bits (DCD, RI, DSR, CTS) to change. The bits of interest are specified as a bit mask in arg, by ORing together any of the bit values, TIOCM_RNG, TIOCM_DSR, TIOCM_CD, and TIOCM_CTS. The caller should use TIOCGICOUNT to see which bit has changed.
Get counts of input serial line interrupts (DCD, RI, DSR, CTS). The counts are written to the serial_icounter_struct structure pointed to by argp.
Note: both 1->0 and 0->1 transitions are counted, except for RI, where only 0->1 transitions are counted.

Marking a line as local

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


For the TIOCLINUX ioctl, see ioctl_console(2).

Kernel debugging

#include <linux/tty.h>

Get the tty_struct corresponding to fd. This command was removed in Linux 2.5.67.


The ioctl(2) system call returns 0 on success. On error, it returns -1 and sets errno appropriately.


Invalid command parameter.
Unknown command.
Inappropriate fd.
Insufficient permission.


Check the condition of DTR on the serial port.

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

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


ldattach(1), ioctl(2), ioctl_console(2), termios(3), pty(7)


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2017-09-15 Linux