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

       ioctl_tty - ioctls for terminals and serial lines

       #include <sys/ioctl.h>
       #include <termios.h>      /* Definition of CLOCAL, and
                                    TC*{FLUSH,ON,OFF} constants */

       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
       TCGETS Argument: struct termios *argp

              Equivalent to tcgetattr(fd, argp).

              Get the current serial port settings.

       TCSETS Argument: const struct termios *argp

              Equivalent to tcsetattr(fd, TCSANOW, argp).

              Set the current serial port settings.

              Argument: const struct termios *argp

              Equivalent to tcsetattr(fd, TCSADRAIN, argp).

              Allow the output buffer to drain, and set the current serial port

              Argument: 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, added in Linux 2.6.20, are just like TCGETS,
       TCSETS, TCSETSW, TCSETSF, except that they take a struct termios2 *
       instead of a struct termios *.  If the structure member c_cflag contains
       the flag BOTHER, then the baud rate is stored in the structure members
       c_ispeed and c_ospeed as integer values.  These ioctls are not supported
       on all architectures.

              TCGETS2    struct termios2 *argp
              TCSETS2    const struct termios2 *argp
              TCSETSW2   const struct termios2 *argp
              TCSETSF2   const struct termios2 *argp

       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.

              Argument: struct termios *argp

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

              Argument: const struct termios *argp

              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).

              Argument: struct winsize *argp

              Get window size.

              Argument: 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 Argument: 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, and 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.)

              Argument: int arg

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

              Argument: void

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

              Argument: void

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

   Software flow control
       TCXONC Argument: int arg

              Equivalent to tcflow(fd, arg).

              See tcflow(3) for the argument values TCOOFF, TCOON, TCIOFF,

   Buffer count and flushing
              Argument: int *argp

              Get the number of bytes in the input buffer.

              Argument: int *argp

              Same as FIONREAD.

              Argument: int *argp

              Get the number of bytes in the output buffer.

       TCFLSH Argument: int arg

              Equivalent to tcflush(fd, arg).

              See tcflush(3) for the argument values TCIFLUSH, TCOFLUSH,

   Faking input
              Argument: const char *argp

              Insert the given byte in the input queue.

   Redirecting console output
              Argument: void

              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, then EBUSY is returned,
              but redirection can be stopped by using this ioctl with fd
              pointing at /dev/console or /dev/tty0.

   Controlling terminal
              Argument: 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.  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.

              Argument: 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
              Argument: pid_t *argp

              When successful, equivalent to *argp = tcgetpgrp(fd).

              Get the process group ID of the foreground process group on this

              Argument: const pid_t *argp

              Equivalent to tcsetpgrp(fd, *argp).

              Set the foreground process group ID of this terminal.

              Argument: pid_t *argp

              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
              Argument: void

              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.)

              Argument: int *argp

              (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.

              Argument: void

              Disable exclusive mode.

   Line discipline
              Argument: int *argp

              Get the line discipline of the terminal.

              Argument: const int *argp

              Set the line discipline of the terminal.

   Pseudoterminal ioctls
              Argument: const int *argp

              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 ('\0') 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
              TIOCPKT_START        Output to the terminal is
              TIOCPKT_DOSTOP       The start and stop
                                   characters are ^S/^Q.
              TIOCPKT_NOSTOP       The start and stop
                                   characters are not ^S/^Q.

              While packet 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

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

              Argument: const int *argp

              (since Linux 3.8) Return the current packet mode setting in the
              integer pointed to by argp.

              Argument: int *argp

              Set (if *argp is nonzero) or remove (if *argp is zero) the lock on
              the pseudoterminal slave device.  (See also unlockpt(3).)

              Argument: int *argp

              (since Linux 3.8) Place the current lock state of the
              pseudoterminal slave device in the location pointed to by argp.

              Argument: int flags

              (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.)

       been implemented under Linux.

   Modem control
              Argument: int *argp

              Get the status of modem bits.

              Argument: const int *argp

              Set the status of modem bits.

              Argument: const int *argp

              Clear the indicated modem bits.

              Argument: const int *argp

              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)

              Argument: int arg

              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.

              Argument: struct serial_icounter_struct *argp

              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
              Argument: int *argp

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

              Argument: 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 ioctl_console(2).

   Kernel debugging
       #include <linux/tty.h>

              Argument: struct tty_struct *argp

              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 to indicate the error.

       EINVAL Invalid command parameter.

              Unknown command.

       ENOTTY Inappropriate fd.

       EPERM  Insufficient permission.

       Check the condition of DTR on the serial port.

       #include <stdio.h>
       #include <unistd.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");

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

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-08-27                       IOCTL_TTY(2)