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

Get and Set Terminal Attributes

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

Locking the termios structure

termios structure of a terminal can be locked. The lock is itself a termios structure, with nonzero bits or fields indicating a locked value.
TIOCGLCKTRMIOS struct termios *argp
Gets the locking status of the
termios structure of the terminal.
TIOCSLCKTRMIOS const struct termios *argp
Sets the locking status of the
termios structure of the terminal. Only root (more precisely: 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>.

TIOCGWINSZ     struct winsize *argp
Get window size.
TIOCSWINSZ     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        int arg
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.)

TCSBRKP 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.
TIOCSBRK       void
Turn break on, that is, start sending zero bits.
TIOCCBRK       void
Turn break off, that is, stop sending zero bits.

Software flow control

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

Buffer count and flushing

FIONREAD      int *argp
Get the number of bytes in the input buffer.
TIOCINQ        int *argp
Same as
TIOCOUTQ    int *argp
Get the number of bytes in the output buffer.
TCFLSH int arg
Equivalent to
tcflush(fd, arg).
See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH, TCIOFLUSH.

Faking input

TIOCSTI       const char *argp
Insert the given byte in the input queue.

Redirecting console output

TIOCCONS      void
Redirect output that would have gone to
/dev/console or /dev/tty0 to the given terminal. If that was a pseudo-terminal 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 root (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

TIOCSCTTY     int arg
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. If this terminal is already the controlling terminal of a different session group then the ioctl fails with EPERM, unless the caller is root (more precisely: 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.
TIOCNOTTY       void
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

TIOCGPGRP     pid_t *argp
When successful, equivalent to
*argp = tcgetpgrp(fd).
Get the process group ID of the foreground process group on this terminal.
TIOCSPGRP      const pid_t *argp
Equivalent to
tcsetpgrp(fd, *argp).
Set the foreground process group ID of this terminal.
TIOCGSID       pid_t *argp
Get the session ID of the given terminal.
This will fail with ENOTTY in case the terminal is not a master pseudo-terminal and not our controlling terminal. Strange.

Exclusive mode

TIOCEXCL      void
Put the terminal into exclusive mode.
No further open(2) operations on the terminal are permitted. (They will fail with EBUSY, except for root, that is, a process with the CAP_SYS_ADMIN capability.)
TIOCNXCL       void
Disable exclusive mode.

Line discipline

TIOCGETD      int *argp
Get the line discipline of the terminal.
TIOCSETD       const int *argp
Set the line discipline of the terminal.

Pseudo-terminal ioctls

TIOCPKT       const int *argp
Enable (when
*argp is nonzero) or disable packet mode. Can be applied to the master side of a pseudo-terminal 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 (aq aq) followed by data written on the slave side of the pseudo-terminal. 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.

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      int *argp
get the status of modem bits.
TIOCMSET       const int *argp
set the status of modem bits.
TIOCMBIC       const int *argp
clear the indicated modem bits.
TIOCMBIS       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

("Get software carrier flag")
Get the status of the CLOCAL flag in the c_cflag field of the termios structure.
TIOCSSOFTCAR   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 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 console_ioctl(4).

Kernel debugging

#include <linux/tty.h>
TIOCTTYGSTRUCT      struct tty_struct *argp
Get the
tty_struct corresponding to fd.


The ioctl() 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 not set");
         puts("TIOCM_DTR is set");


ioctl(2), termios(3), console_ioctl(4), pty(7)


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