IOCTL_TTY(2)              (2020-06-09)               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 argu-
          ment, 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

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        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.

          TIOCGLCKTRMIOS struct termios *argp
               Gets the locking status of the termios structure of the

          TIOCSLCKTRMIOS const struct termios *argp
               Sets the locking status of the termios structure of the
               terminal.  Only a process with the CAP_SYS_ADMIN capa-
               bility 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 vir-
          tual console changes, for example, by loading a new font).

          The following constants and structure are defined in

          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 func-
               tion returns without doing anything.  When arg is
               nonzero, nobody knows what will happen.

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               (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 mea-
               sured in milliseconds.  HP-UX ignores arg.)

          TCSBRKP   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.

          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 FIONREAD.

          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 pseu-
               doterminal 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

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               2.6.10, only a process with the CAP_SYS_ADMIN capabil-
               ity 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

        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.
               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 capabil-
               ity 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 pro-
               cesses in the current session lose their controlling

        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 fails
               with the error ENOTTY if the terminal is not a master
               pseudoterminal and not our controlling terminal.

        Exclusive mode
          TIOCEXCL  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.)

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          TIOCGEXCL 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.

          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.

        Pseudoterminal ioctls
          TIOCPKT   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 (aq\0aq) 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:

               lb l.  TIOCPKT_FLUSHREAD   The read queue for the ter-
               minal 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 haS/haQ.  TIOCPKT_NOSTOP The start
               and stop characters are not haS/haQ.

               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 event.

               This mode is used by rlogin(1) and rlogind(8) to imple-
               ment a remote-echoed, locally haS/haQ flow-controlled
               remote login.

          TIOCGPKT  const int *argp
               (since Linux 3.8) Return the current packet mode set-
               ting in the integer pointed to by argp.

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

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

          TIOCGPTPEER    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 exam-
               ple, confusion can occur in some cases using ptsname(3)
               with a pathname where a devpts filesystem has been
               mounted in a different mount namespace.)

          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.

          The following bits are used by the above ioctls:

          lb l.  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_DSR DSR (data set ready)

          TIOCMIWAIT     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,

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               TIOCM_RNG, TIOCM_DSR, TIOCM_CD, and TIOCM_CTS.  The
               caller should use TIOCGICOUNT to see which bit has

          TIOCGICOUNT    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
          TIOCGSOFTCAR   int *argp
               ("Get software carrier flag") Get the status of the
               CLOCAL flag in the c_cflag field of the termios struc-

          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 ioctl_console(2).

        Kernel debugging
          #include <linux/tty.h>

          TIOCTTYGSTRUCT 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 appropriately.

               Invalid command parameter.

               Unknown command.

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               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");

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

          This page is part of release 5.10 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

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