tty ioctl - ioctls for terminals and serial lines


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

Locking the termios structure

The termios structure of a terminal can be locked. The lock is itself a termios structure, with non-zero bits or fields indicating a locked value.
TIOCGLCKTRMIOS  struct termios *
 Gets the locking status of the termios structure of the terminal.
TIOCSLCKTRMIOS  const struct termios *
 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 *
 Get window size.
TIOCSWINSZ      const struct winsize *
 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 non-zero, nobody knows what will happen.

(SVr4, UnixWare, Solaris, Linux treat tcsendbreak(fd,arg) with non-zero 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 non-zero) as a time interval measured in milliseconds. HP-UX ignores arg.)

 So-called "POSIX version" of TCSBRK. It treats non-zero 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

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

Buffer count and flushing

FIONREAD        int *
 Get the number of bytes in the input buffer.
 Same as FIONREAD.
TIOCOUTQ        int *
 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

TIOCSTI const char *
 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
 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.
 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 *
 When successful, equivalent to *argp = tcgetpgrp(fd). Get the process group ID of the foreground process group on this terminal.
TIOCSPGRP       const pid_t *
 Equivalent to tcsetpgrp(fd, *argp). Set the foreground process group ID of this terminal.
TIOCGSID        pid_t *
 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 *
 Get the line discipline of the terminal.
TIOCSETD        const int *
 Set the line discipline of the terminal.

Pseudo-terminal ioctls

TIOCPKT const int *
 Enable (when *argp is non-zero) 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 non-zero control byte, or has a single byte containing zero (' ') 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 *
 get the status of modem bits.
TIOCMSET        const int *
 set the status of modem bits.
TIOCMBIC        const int *
 clear the indicated modem bits.
TIOCMBIS        const int *
 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 *
 ("Set software carrier flag") Set the CLOCAL flag in the termios structure when *argp is non-zero, 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 *
 Get the tty_struct corresponding to fd.


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


EINVAL Invalid command parameter.
 Unknown command.
ENOTTY Inappropriate fd.
EPERM Insufficient permission.


Check the condition of DTR on the serial port.

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

int main(void) { 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), pty(7)


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